version 1.3, 2002/06/02 10:31:29
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version 1.28, 2005/01/26 21:24:15
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\input texinfo @c -*-texinfo-*- |
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@comment %**start of header |
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@setfilename vmgen.info |
@include version.texi |
@include version.texi |
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@settitle Vmgen (Gforth @value{VERSION}) |
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@c @syncodeindex pg cp |
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@comment %**end of header |
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@copying |
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This manual is for Vmgen |
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(version @value{VERSION}, @value{UPDATED}), |
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the virtual machine interpreter generator |
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Copyright @copyright{} 2002,2003,2005 Free Software Foundation, Inc. |
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@quotation |
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Permission is granted to copy, distribute and/or modify this document |
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under the terms of the GNU Free Documentation License, Version 1.1 or |
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any later version published by the Free Software Foundation; with no |
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Invariant Sections, with the Front-Cover texts being ``A GNU Manual,'' |
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and with the Back-Cover Texts as in (a) below. A copy of the |
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license is included in the section entitled ``GNU Free Documentation |
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License.'' |
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(a) The FSF's Back-Cover Text is: ``You have freedom to copy and modify |
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this GNU Manual, like GNU software. Copies published by the Free |
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Software Foundation raise funds for GNU development.'' |
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@end quotation |
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@end copying |
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@dircategory Software development |
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@direntry |
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* Vmgen: (vmgen). Virtual machine interpreter generator |
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@end direntry |
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@titlepage |
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@title Vmgen |
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@subtitle for Gforth version @value{VERSION}, @value{UPDATED} |
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@author M. Anton Ertl (@email{anton@@mips.complang.tuwien.ac.at}) |
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@page |
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@vskip 0pt plus 1filll |
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@insertcopying |
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@end titlepage |
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@contents |
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@ifnottex |
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@node Top, Introduction, (dir), (dir) |
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@top Vmgen |
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@insertcopying |
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@end ifnottex |
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@menu |
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* Introduction:: What can Vmgen do for you? |
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* Why interpreters?:: Advantages and disadvantages |
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* Concepts:: VM interpreter background |
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* Invoking Vmgen:: |
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* Example:: |
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* Input File Format:: |
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* Error messages:: reported by Vmgen |
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* Using the generated code:: |
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* Hints:: VM archictecture, efficiency |
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* The future:: |
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* Changes:: from earlier versions |
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* Contact:: Bug reporting etc. |
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* Copying This Manual:: Manual License |
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* Index:: |
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@detailmenu |
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--- The Detailed Node Listing --- |
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Concepts |
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* Front end and VM interpreter:: Modularizing an interpretive system |
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* Data handling:: Stacks, registers, immediate arguments |
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* Dispatch:: From one VM instruction to the next |
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Example |
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* Example overview:: |
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* Using profiling to create superinstructions:: |
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Input File Format |
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* Input File Grammar:: |
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* Simple instructions:: |
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* Superinstructions:: |
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* Store Optimization:: |
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* Register Machines:: How to define register VM instructions |
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Input File Grammar |
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* Eval escapes:: what follows \E |
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Simple instructions |
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* C Code Macros:: Macros recognized by Vmgen |
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* C Code restrictions:: Vmgen makes assumptions about C code |
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* Stack growth direction:: is configurable per stack |
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Using the generated code |
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* VM engine:: Executing VM code |
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* VM instruction table:: |
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* VM code generation:: Creating VM code (in the front-end) |
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* Peephole optimization:: Creating VM superinstructions |
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* VM disassembler:: for debugging the front end |
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* VM profiler:: for finding worthwhile superinstructions |
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Hints |
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* Floating point:: and stacks |
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Copying This Manual |
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* GNU Free Documentation License:: License for copying this manual. |
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@end detailmenu |
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@end menu |
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@c @ifnottex |
@c @ifnottex |
This file documents vmgen (Gforth @value{VERSION}). |
@c This file documents Vmgen (Gforth @value{VERSION}). |
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@c ************************************************************ |
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@node Introduction, Why interpreters?, Top, Top |
@chapter Introduction |
@chapter Introduction |
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Vmgen is a tool for writing efficient interpreters. It takes a simple |
Vmgen is a tool for writing efficient interpreters. It takes a simple |
Line 12 it). The run-time efficiency of the res
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Line 132 it). The run-time efficiency of the res
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within a factor of 10 of machine code produced by an optimizing |
within a factor of 10 of machine code produced by an optimizing |
compiler. |
compiler. |
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The interpreter design strategy supported by vmgen is to divide the |
The interpreter design strategy supported by Vmgen is to divide the |
interpreter into two parts: |
interpreter into two parts: |
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@itemize @bullet |
@itemize @bullet |
Line 29 machine code.
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Line 149 machine code.
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@end itemize |
@end itemize |
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Such a division is usually used in interpreters, for modularity as well |
Such a division is usually used in interpreters, for modularity as well |
as for efficiency reasons. The virtual machine code is typically passed |
as for efficiency. The virtual machine code is typically passed between |
between front end and virtual machine interpreter in memory, like in a |
front end and virtual machine interpreter in memory, like in a |
load-and-go compiler; this avoids the complexity and time cost of |
load-and-go compiler; this avoids the complexity and time cost of |
writing the code to a file and reading it again. |
writing the code to a file and reading it again. |
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Line 39 A @emph{virtual machine} (VM) represents
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Line 159 A @emph{virtual machine} (VM) represents
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machine code. Control flow occurs through VM branch instructions, like |
machine code. Control flow occurs through VM branch instructions, like |
in a real machine. |
in a real machine. |
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In this setup, vmgen can generate most of the code dealing with virtual |
@cindex functionality features overview |
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In this setup, Vmgen can generate most of the code dealing with virtual |
machine instructions from a simple description of the virtual machine |
machine instructions from a simple description of the virtual machine |
instructions (@pxref...), in particular: |
instructions (@pxref{Input File Format}), in particular: |
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@table @emph |
@table @strong |
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@item VM instruction execution |
@item VM instruction execution |
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Line 59 typically provide other means for debugg
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Line 180 typically provide other means for debugg
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source level. |
source level. |
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@item VM code profiling |
@item VM code profiling |
Useful for optimizing the VM insterpreter with superinstructions |
Useful for optimizing the VM interpreter with superinstructions |
(@pxref...). |
(@pxref{VM profiler}). |
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@end table |
@end table |
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VMgen supports efficient interpreters though various optimizations, in |
To create parts of the interpretive system that do not deal with VM |
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instructions, you have to use other tools (e.g., @command{bison}) and/or |
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hand-code them. |
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@cindex efficiency features overview |
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@noindent |
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Vmgen supports efficient interpreters though various optimizations, in |
particular |
particular |
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@itemize |
@itemize @bullet |
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@item Threaded code |
@item Threaded code |
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Line 81 Replicating VM (super)instructions for b
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Line 208 Replicating VM (super)instructions for b
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@end itemize |
@end itemize |
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As a result, vmgen-based interpreters are only about an order of |
@cindex speed for JVM |
magintude slower than native code from an optimizing C compiler on small |
As a result, Vmgen-based interpreters are only about an order of |
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magnitude slower than native code from an optimizing C compiler on small |
benchmarks; on large benchmarks, which spend more time in the run-time |
benchmarks; on large benchmarks, which spend more time in the run-time |
system, the slowdown is often less (e.g., the slowdown of a |
system, the slowdown is often less (e.g., the slowdown of a |
Vmgen-generated JVM interpreter over the best JVM JIT compiler we |
Vmgen-generated JVM interpreter over the best JVM JIT compiler we |
Line 91 and all other interpreters we looked at
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Line 219 and all other interpreters we looked at
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interpreter). |
interpreter). |
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VMs are usually designed as stack machines (passing data between VM |
VMs are usually designed as stack machines (passing data between VM |
instructions on a stack), and vmgen supports such designs especially |
instructions on a stack), and Vmgen supports such designs especially |
well; however, you can also use vmgen for implementing a register VM and |
well; however, you can also use Vmgen for implementing a register VM |
still benefit from most of the advantages offered by vmgen. |
(@pxref{Register Machines}) and still benefit from most of the advantages |
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offered by Vmgen. |
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There are many potential uses of the instruction descriptions that are |
There are many potential uses of the instruction descriptions that are |
not implemented at the moment, but we are open for feature requests, and |
not implemented at the moment, but we are open for feature requests, and |
we will implement new features if someone asks for them; so the feature |
we will consider new features if someone asks for them; so the feature |
list above is not exhaustive. |
list above is not exhaustive. |
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@c ********************************************************************* |
@c ********************************************************************* |
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@node Why interpreters?, Concepts, Introduction, Top |
@chapter Why interpreters? |
@chapter Why interpreters? |
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@cindex interpreters, advantages |
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@cindex advantages of interpreters |
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@cindex advantages of vmgen |
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Interpreters are a popular language implementation technique because |
Interpreters are a popular language implementation technique because |
they combine all three of the following advantages: |
they combine all three of the following advantages: |
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@itemize |
@itemize @bullet |
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@item Ease of implementation |
@item Ease of implementation |
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Line 116 they combine all three of the following
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Line 249 they combine all three of the following
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@end itemize |
@end itemize |
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Vmgen makes it even easier to implement interpreters. |
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@cindex speed of interpreters |
The main disadvantage of interpreters is their run-time speed. However, |
The main disadvantage of interpreters is their run-time speed. However, |
there are huge differences between different interpreters in this area: |
there are huge differences between different interpreters in this area: |
the slowdown over optimized C code on programs consisting of simple |
the slowdown over optimized C code on programs consisting of simple |
Line 125 slowdown for programs executing complex
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Line 261 slowdown for programs executing complex
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time spent in libraries for executing complex operations is the same in |
time spent in libraries for executing complex operations is the same in |
all implementation strategies). |
all implementation strategies). |
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Vmgen makes it even easier to implement interpreters. It also supports |
Vmgen supports techniques for building efficient interpreters. |
techniques for building efficient interpreters. |
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@c ******************************************************************** |
@c ******************************************************************** |
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@node Concepts, Invoking Vmgen, Why interpreters?, Top |
@chapter Concepts |
@chapter Concepts |
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@menu |
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* Front end and VM interpreter:: Modularizing an interpretive system |
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* Data handling:: Stacks, registers, immediate arguments |
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* Dispatch:: From one VM instruction to the next |
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@end menu |
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@c -------------------------------------------------------------------- |
@c -------------------------------------------------------------------- |
@section Front-end and virtual machine interpreter |
@node Front end and VM interpreter, Data handling, Concepts, Concepts |
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@section Front end and VM interpreter |
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@cindex modularization of interpreters |
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@cindex front-end |
@cindex front-end |
Interpretive systems are typically divided into a @emph{front end} that |
Interpretive systems are typically divided into a @emph{front end} that |
Line 143 representation of the program.
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Line 286 representation of the program.
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@cindex virtual machine |
@cindex virtual machine |
@cindex VM |
@cindex VM |
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@cindex VM instruction |
@cindex instruction, VM |
@cindex instruction, VM |
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@cindex VM branch instruction |
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@cindex branch instruction, VM |
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@cindex VM register |
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@cindex register, VM |
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@cindex opcode, VM instruction |
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@cindex immediate argument, VM instruction |
For efficient interpreters the intermediate representation of choice is |
For efficient interpreters the intermediate representation of choice is |
virtual machine code (rather than, e.g., an abstract syntax tree). |
virtual machine code (rather than, e.g., an abstract syntax tree). |
@emph{Virtual machine} (VM) code consists of VM instructions arranged |
@emph{Virtual machine} (VM) code consists of VM instructions arranged |
sequentially in memory; they are executed in sequence by the VM |
sequentially in memory; they are executed in sequence by the VM |
interpreter, except for VM branch instructions, which implement control |
interpreter, but VM branch instructions can change the control flow and |
structures. The conceptual similarity to real machine code results in |
are used for implementing control structures. The conceptual similarity |
the name @emph{virtual machine}. |
to real machine code results in the name @emph{virtual machine}. |
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Various terms similar to terms for real machines are used; e.g., there |
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are @emph{VM registers} (like the instruction pointer and stack |
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pointer(s)), and the VM instruction consists of an @emph{opcode} and |
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@emph{immediate arguments}. |
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In this framework, vmgen supports building the VM interpreter and any |
In this framework, Vmgen supports building the VM interpreter and any |
other component dealing with VM instructions. It does not have any |
other component dealing with VM instructions. It does not have any |
support for the front end, apart from VM code generation support. The |
support for the front end, apart from VM code generation support. The |
front end can be implemented with classical compiler front-end |
front end can be implemented with classical compiler front-end |
Line 163 interpreter, but some systems also suppo
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Line 317 interpreter, but some systems also suppo
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as an image file, or in a full-blown linkable file format (e.g., JVM). |
as an image file, or in a full-blown linkable file format (e.g., JVM). |
Vmgen currently has no special support for such features, but the |
Vmgen currently has no special support for such features, but the |
information in the instruction descriptions can be helpful, and we are |
information in the instruction descriptions can be helpful, and we are |
open for feature requests and suggestions. |
open to feature requests and suggestions. |
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@c -------------------------------------------------------------------- |
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@node Data handling, Dispatch, Front end and VM interpreter, Concepts |
@section Data handling |
@section Data handling |
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@cindex stack machine |
@cindex stack machine |
@cindex register machine |
@cindex register machine |
Most VMs use one or more stacks for passing temporary data between VM |
Most VMs use one or more stacks for passing temporary data between VM |
instructions. Another option is to use a register machine architecture |
instructions. Another option is to use a register machine architecture |
for the virtual machine; however, this option is either slower or |
for the virtual machine; we believe that using a stack architecture is |
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usually both simpler and faster. |
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however, this option is slower or |
significantly more complex to implement than a stack machine architecture. |
significantly more complex to implement than a stack machine architecture. |
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Vmgen has special support and optimizations for stack VMs, making their |
Vmgen has special support and optimizations for stack VMs, making their |
implementation easy and efficient. |
implementation easy and efficient. |
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You can also implement a register VM with vmgen (@pxref{Register |
You can also implement a register VM with Vmgen (@pxref{Register |
Machines}), and you will still profit from most vmgen features. |
Machines}), and you will still profit from most Vmgen features. |
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@cindex stack item size |
@cindex stack item size |
@cindex size, stack items |
@cindex size, stack items |
Line 192 the data on the stack.
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Line 351 the data on the stack.
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@cindex immediate arguments |
@cindex immediate arguments |
Another source of data is immediate arguments VM instructions (in the VM |
Another source of data is immediate arguments VM instructions (in the VM |
instruction stream). The VM instruction stream is handled similar to a |
instruction stream). The VM instruction stream is handled similar to a |
stack in vmgen. |
stack in Vmgen. |
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@cindex garbage collection |
@cindex garbage collection |
@cindex reference counting |
@cindex reference counting |
Vmgen has no built-in support for nor restrictions against @emph{garbage |
Vmgen has no built-in support for, nor restrictions against |
collection}. If you need garbage collection, you need to provide it in |
@emph{garbage collection}. If you need garbage collection, you need to |
your run-time libraries. Using @emph{reference counting} is probably |
provide it in your run-time libraries. Using @emph{reference counting} |
harder, but might be possible (contact us if you are interested). |
is probably harder, but might be possible (contact us if you are |
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interested). |
@c reference counting might be possible by including counting code in |
@c reference counting might be possible by including counting code in |
@c the conversion macros. |
@c the conversion macros. |
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@c -------------------------------------------------------------------- |
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@node Dispatch, , Data handling, Concepts |
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@section Dispatch |
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@cindex Dispatch of VM instructions |
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@cindex main interpreter loop |
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Understanding this section is probably not necessary for using Vmgen, |
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but it may help. You may want to skip it now, and read it if you find statements about dispatch methods confusing. |
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After executing one VM instruction, the VM interpreter has to dispatch |
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the next VM instruction (Vmgen calls the dispatch routine @samp{NEXT}). |
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Vmgen supports two methods of dispatch: |
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@table @strong |
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@item switch dispatch |
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@cindex switch dispatch |
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In this method the VM interpreter contains a giant @code{switch} |
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statement, with one @code{case} for each VM instruction. The VM |
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instruction opcodes are represented by integers (e.g., produced by an |
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@code{enum}) in the VM code, and dispatch occurs by loading the next |
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opcode, @code{switch}ing on it, and continuing at the appropriate |
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@code{case}; after executing the VM instruction, the VM interpreter |
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jumps back to the dispatch code. |
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@item threaded code |
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@cindex threaded code |
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This method represents a VM instruction opcode by the address of the |
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start of the machine code fragment for executing the VM instruction. |
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Dispatch consists of loading this address, jumping to it, and |
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incrementing the VM instruction pointer. Typically the threaded-code |
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dispatch code is appended directly to the code for executing the VM |
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instruction. Threaded code cannot be implemented in ANSI C, but it can |
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be implemented using GNU C's labels-as-values extension (@pxref{Labels |
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as Values, , Labels as Values, gcc.info, GNU C Manual}). |
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@c call threading |
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@end table |
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Threaded code can be twice as fast as switch dispatch, depending on the |
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interpreter, the benchmark, and the machine. |
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@c ************************************************************* |
@c ************************************************************* |
@chapter Invoking vmgen |
@node Invoking Vmgen, Example, Concepts, Top |
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@chapter Invoking Vmgen |
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@cindex Invoking Vmgen |
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The usual way to invoke vmgen is as follows: |
The usual way to invoke Vmgen is as follows: |
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@example |
@example |
vmgen @var{infile} |
vmgen @var{inputfile} |
@end example |
@end example |
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Here @var{infile} is the VM instruction description file, which usually |
Here @var{inputfile} is the VM instruction description file, which |
ends in @file{.vmg}. The output filenames are made by taking the |
usually ends in @file{.vmg}. The output filenames are made by taking |
basename of @file{infile} (i.e., the output files will be created in the |
the basename of @file{inputfile} (i.e., the output files will be created |
current working directory) and replacing @file{.vmg} with @file{-vm.i}, |
in the current working directory) and replacing @file{.vmg} with |
@file{-disasm.i}, @file{-gen.i}, @file{-labels.i}, @file{-profile.i}, |
@file{-vm.i}, @file{-disasm.i}, @file{-gen.i}, @file{-labels.i}, |
and @file{-peephole.i}. E.g., @command{bison hack/foo.vmg} will create |
@file{-profile.i}, and @file{-peephole.i}. E.g., @command{vmgen |
@file{foo-vm.i} etc. |
hack/foo.vmg} will create @file{foo-vm.i}, @file{foo-disasm.i}, |
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@file{foo-gen.i}, @file{foo-labels.i}, @file{foo-profile.i} and |
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@file{foo-peephole.i}. |
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The command-line options supported by vmgen are |
The command-line options supported by Vmgen are |
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@table @option |
@table @option |
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Line 239 Print version and exit
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Line 445 Print version and exit
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@c env vars GFORTHDIR GFORTHDATADIR |
@c env vars GFORTHDIR GFORTHDATADIR |
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@c **************************************************************** |
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@node Example, Input File Format, Invoking Vmgen, Top |
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@chapter Example |
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@cindex example of a Vmgen-based interpreter |
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@menu |
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* Example overview:: |
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* Using profiling to create superinstructions:: |
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@end menu |
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@c -------------------------------------------------------------------- |
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@node Example overview, Using profiling to create superinstructions, Example, Example |
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@section Example overview |
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@cindex example overview |
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@cindex @file{vmgen-ex} |
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@cindex @file{vmgen-ex2} |
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There are two versions of the same example for using Vmgen: |
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@file{vmgen-ex} and @file{vmgen-ex2} (you can also see Gforth as |
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example, but it uses additional (undocumented) features, and also |
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differs in some other respects). The example implements @emph{mini}, a |
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tiny Modula-2-like language with a small JavaVM-like virtual machine. |
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The difference between the examples is that @file{vmgen-ex} uses many |
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casts, and @file{vmgen-ex2} tries to avoids most casts and uses unions |
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instead. In the rest of this manual we usually mention just files in |
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@file{vmgen-ex}; if you want to use unions, use the equivalent file in |
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@file{vmgen-ex2}. |
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@cindex unions example |
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@cindex casts example |
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The files provided with each example are: |
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@cindex example files |
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@example |
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Makefile |
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README |
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disasm.c wrapper file |
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engine.c wrapper file |
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peephole.c wrapper file |
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profile.c wrapper file |
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mini-inst.vmg simple VM instructions |
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mini-super.vmg superinstructions (empty at first) |
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mini.h common declarations |
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mini.l scanner |
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mini.y front end (parser, VM code generator) |
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support.c main() and other support functions |
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fib.mini example mini program |
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simple.mini example mini program |
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test.mini example mini program (tests everything) |
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test.out test.mini output |
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stat.awk script for aggregating profile information |
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peephole-blacklist list of instructions not allowed in superinstructions |
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seq2rule.awk script for creating superinstructions |
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@end example |
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For your own interpreter, you would typically copy the following files |
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and change little, if anything: |
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@cindex wrapper files |
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@example |
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disasm.c wrapper file |
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engine.c wrapper file |
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peephole.c wrapper file |
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profile.c wrapper file |
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stat.awk script for aggregating profile information |
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seq2rule.awk script for creating superinstructions |
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@end example |
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@noindent |
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You would typically change much in or replace the following files: |
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@example |
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Makefile |
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mini-inst.vmg simple VM instructions |
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mini.h common declarations |
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mini.l scanner |
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mini.y front end (parser, VM code generator) |
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support.c main() and other support functions |
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peephole-blacklist list of instructions not allowed in superinstructions |
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@end example |
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You can build the example by @code{cd}ing into the example's directory, |
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and then typing @code{make}; you can check that it works with @code{make |
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check}. You can run run mini programs like this: |
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@example |
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./mini fib.mini |
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@end example |
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To learn about the options, type @code{./mini -h}. |
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@c -------------------------------------------------------------------- |
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@node Using profiling to create superinstructions, , Example overview, Example |
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@section Using profiling to create superinstructions |
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@cindex profiling example |
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@cindex superinstructions example |
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I have not added rules for this in the @file{Makefile} (there are many |
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options for selecting superinstructions, and I did not want to hardcode |
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one into the @file{Makefile}), but there are some supporting scripts, and |
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here's an example: |
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|
|
Suppose you want to use @file{fib.mini} and @file{test.mini} as training |
|
programs, you get the profiles like this: |
|
|
|
@example |
|
make fib.prof test.prof #takes a few seconds |
|
@end example |
|
|
|
You can aggregate these profiles with @file{stat.awk}: |
|
|
|
@example |
|
awk -f stat.awk fib.prof test.prof |
|
@end example |
|
|
|
The result contains lines like: |
|
|
|
@example |
|
2 16 36910041 loadlocal lit |
|
@end example |
|
|
|
This means that the sequence @code{loadlocal lit} statically occurs a |
|
total of 16 times in 2 profiles, with a dynamic execution count of |
|
36910041. |
|
|
|
The numbers can be used in various ways to select superinstructions. |
|
E.g., if you just want to select all sequences with a dynamic |
|
execution count exceeding 10000, you would use the following pipeline: |
|
|
|
@example |
|
awk -f stat.awk fib.prof test.prof| |
|
awk '$3>=10000'| #select sequences |
|
fgrep -v -f peephole-blacklist| #eliminate wrong instructions |
|
awk -f seq2rule.awk| #transform sequences into superinstruction rules |
|
sort -k 3 >mini-super.vmg #sort sequences |
|
@end example |
|
|
|
The file @file{peephole-blacklist} contains all instructions that |
|
directly access a stack or stack pointer (for mini: @code{call}, |
|
@code{return}); the sort step is necessary to ensure that prefixes |
|
precede larger superinstructions. |
|
|
|
Now you can create a version of mini with superinstructions by just |
|
saying @samp{make} |
|
|
|
|
@c *************************************************************** |
@c *************************************************************** |
|
@node Input File Format, Error messages, Example, Top |
@chapter Input File Format |
@chapter Input File Format |
|
@cindex input file format |
|
@cindex format, input file |
|
|
Vmgen takes as input a file containing specifications of virtual machine |
Vmgen takes as input a file containing specifications of virtual machine |
instructions. This file usually has a name ending in @file{.vmg}. |
instructions. This file usually has a name ending in @file{.vmg}. |
|
|
The examples are taken from the example in @file{vmgen-ex}. |
Most examples are taken from the example in @file{vmgen-ex}. |
|
|
|
@menu |
|
* Input File Grammar:: |
|
* Simple instructions:: |
|
* Superinstructions:: |
|
* Store Optimization:: |
|
* Register Machines:: How to define register VM instructions |
|
@end menu |
|
|
|
@c -------------------------------------------------------------------- |
|
@node Input File Grammar, Simple instructions, Input File Format, Input File Format |
@section Input File Grammar |
@section Input File Grammar |
|
@cindex grammar, input file |
|
@cindex input file grammar |
|
|
The grammar is in EBNF format, with @code{@var{a}|@var{b}} meaning |
The grammar is in EBNF format, with @code{@var{a}|@var{b}} meaning |
``@var{a} or @var{b}'', @code{@{@var{c}@}} meaning 0 or more repetitions |
``@var{a} or @var{b}'', @code{@{@var{c}@}} meaning 0 or more repetitions |
of @var{c} and @code{[@var{d}]} meaning 0 or 1 repetitions of @var{d}. |
of @var{c} and @code{[@var{d}]} meaning 0 or 1 repetitions of @var{d}. |
|
|
|
@cindex free-format, not |
|
@cindex newlines, significance in syntax |
Vmgen input is not free-format, so you have to take care where you put |
Vmgen input is not free-format, so you have to take care where you put |
spaces and especially newlines; it's not as bad as makefiles, though: |
newlines (and, in a few cases, white space). |
any sequence of spaces and tabs is equivalent to a single space. |
|
|
|
@example |
@example |
description: {instruction|comment|eval-escape} |
description: @{instruction|comment|eval-escape|c-escape@} |
|
|
instruction: simple-inst|superinst |
instruction: simple-inst|superinst |
|
|
simple-inst: ident " (" stack-effect " )" newline c-code newline newline |
simple-inst: ident '(' stack-effect ')' newline c-code newline newline |
|
|
|
stack-effect: @{ident@} '--' @{ident@} |
|
|
stack-effect: {ident} " --" {ident} |
super-inst: ident '=' ident @{ident@} |
|
|
super-inst: ident " =" ident {ident} |
comment: '\ ' text newline |
|
|
comment: "\ " text newline |
eval-escape: '\E ' text newline |
|
|
eval-escape: "\e " text newline |
c-escape: '\C ' text newline |
@end example |
@end example |
@c \+ \- \g \f \c |
@c \+ \- \g \f \c |
|
|
Note that the @code{\}s in this grammar are meant literally, not as |
Note that the @code{\}s in this grammar are meant literally, not as |
C-style encodings for no-printable characters. |
C-style encodings for non-printable characters. |
|
|
|
There are two ways to delimit the C code in @code{simple-inst}: |
|
|
|
@itemize @bullet |
|
|
The C code in @code{simple-inst} must not contain empty lines (because |
@item |
vmgen would mistake that as the end of the simple-inst. The text in |
If you start it with a @samp{@{} at the start of a line (i.e., not even |
@code{comment} and @code{eval-escape} must not contain a newline. |
white space before it), you have to end it with a @samp{@}} at the start |
@code{Ident} must conform to the usual conventions of C identifiers |
of a line (followed by a newline). In this case you may have empty |
(otherwise the C compiler would choke on the vmgen output). |
lines within the C code (typically used between variable definitions and |
|
statements). |
|
|
|
@item |
|
You do not start it with @samp{@{}. Then the C code ends at the first |
|
empty line, so you cannot have empty lines within this code. |
|
|
|
@end itemize |
|
|
|
The text in @code{comment}, @code{eval-escape} and @code{c-escape} must |
|
not contain a newline. @code{Ident} must conform to the usual |
|
conventions of C identifiers (otherwise the C compiler would choke on |
|
the Vmgen output), except that idents in @code{stack-effect} may have a |
|
stack prefix (for stack prefix syntax, @pxref{Eval escapes}). |
|
|
|
@cindex C escape |
|
@cindex @code{\C} |
|
@cindex conditional compilation of Vmgen output |
|
The @code{c-escape} passes the text through to each output file (without |
|
the @samp{\C}). This is useful mainly for conditional compilation |
|
(i.e., you write @samp{\C #if ...} etc.). |
|
|
|
@cindex sync lines |
|
@cindex @code{#line} |
|
In addition to the syntax given in the grammer, Vmgen also processes |
|
sync lines (lines starting with @samp{#line}), as produced by @samp{m4 |
|
-s} (@pxref{Invoking m4, , Invoking m4, m4.info, GNU m4}) and similar |
|
tools. This allows associating C compiler error messages with the |
|
original source of the C code. |
|
|
Vmgen understands a few extensions beyond the grammar given here, but |
Vmgen understands a few extensions beyond the grammar given here, but |
these extensions are only useful for building Gforth. You can find a |
these extensions are only useful for building Gforth. You can find a |
description of the format used for Gforth in @file{prim}. |
description of the format used for Gforth in @file{prim}. |
|
|
@subsection |
@menu |
|
* Eval escapes:: what follows \E |
|
@end menu |
|
|
|
@node Eval escapes, , Input File Grammar, Input File Grammar |
|
@subsection Eval escapes |
|
@cindex escape to Forth |
|
@cindex eval escape |
|
@cindex @code{\E} |
|
|
@c woanders? |
@c woanders? |
The text in @code{eval-escape} is Forth code that is evaluated when |
The text in @code{eval-escape} is Forth code that is evaluated when |
vmgen reads the line. If you do not know (and do not want to learn) |
Vmgen reads the line. You will normally use this feature to define |
Forth, you can build the text according to the following grammar; these |
stacks and types. |
rules are normally all Forth you need for using vmgen: |
|
|
If you do not know (and do not want to learn) Forth, you can build the |
|
text according to the following grammar; these rules are normally all |
|
Forth you need for using Vmgen: |
|
|
@example |
@example |
text: stack-decl|type-prefix-decl|stack-prefix-decl |
text: stack-decl|type-prefix-decl|stack-prefix-decl|set-flag |
|
|
stack-decl: "stack " ident ident ident |
stack-decl: 'stack ' ident ident ident |
type-prefix-decl: |
type-prefix-decl: |
's" ' string '" ' ("single"|"double") ident "type-prefix" ident |
's" ' string '" ' ('single'|'double') ident 'type-prefix' ident |
stack-prefix-decl: ident "stack-prefix" string |
stack-prefix-decl: ident 'stack-prefix' string |
|
set-flag: ('store-optimization'|'include-skipped-insts') ('on'|'off') |
@end example |
@end example |
|
|
Note that the syntax of this code is not checked thoroughly (there are |
Note that the syntax of this code is not checked thoroughly (there are |
many other Forth program fragments that could be written there). |
many other Forth program fragments that could be written in an |
|
eval-escape). |
|
|
|
A stack prefix can contain letters, digits, or @samp{:}, and may start |
|
with an @samp{#}; e.g., in Gforth the return stack has the stack prefix |
|
@samp{R:}. This restriction is not checked during the stack prefix |
|
definition, but it is enforced by the parsing rules for stack items |
|
later. |
|
|
If you know Forth, the stack effects of the non-standard words involved |
If you know Forth, the stack effects of the non-standard words involved |
are: |
are: |
|
@findex stack |
|
@findex type-prefix |
|
@findex single |
|
@findex double |
|
@findex stack-prefix |
|
@findex store-optimization |
@example |
@example |
stack ( "name" "pointer" "type" -- ) |
stack ( "name" "pointer" "type" -- ) |
( name execution: -- stack ) |
( name execution: -- stack ) |
type-prefix ( addr u xt1 xt2 n stack "prefix" -- ) |
type-prefix ( addr u item-size stack "prefix" -- ) |
single ( -- xt1 xt2 n ) |
single ( -- item-size ) |
double ( -- xt1 xt2 n ) |
double ( -- item-size ) |
stack-prefix ( stack "prefix" -- ) |
stack-prefix ( stack "prefix" -- ) |
|
store-optimization ( -- addr ) |
|
include-skipped-insts ( -- addr ) |
@end example |
@end example |
|
|
|
An @var{item-size} takes three cells on the stack. |
|
|
|
@c -------------------------------------------------------------------- |
|
@node Simple instructions, Superinstructions, Input File Grammar, Input File Format |
@section Simple instructions |
@section Simple instructions |
|
@cindex simple VM instruction |
|
@cindex instruction, simple VM |
|
|
We will use the following simple VM instruction description as example: |
We will use the following simple VM instruction description as example: |
|
|
Line 332 its stack effect (@code{i1 i2 -- i}). T
|
Line 768 its stack effect (@code{i1 i2 -- i}). T
|
just plain C code. |
just plain C code. |
|
|
@cindex stack effect |
@cindex stack effect |
|
@cindex effect, stack |
The stack effect specifies that @code{sub} pulls two integers from the |
The stack effect specifies that @code{sub} pulls two integers from the |
data stack and puts them in the C variable @code{i1} and @code{i2} (with |
data stack and puts them in the C variables @code{i1} and @code{i2} |
the rightmost item (@code{i2}) taken from the top of stack) and later |
(with the rightmost item (@code{i2}) taken from the top of stack; |
pushes one integer (@code{i)) on the data stack (the rightmost item is |
intuition: if you push @code{i1}, then @code{i2} on the stack, the |
on the top afterwards). |
resulting stack picture is @code{i1 i2}) and later pushes one integer |
|
(@code{i}) on the data stack (the rightmost item is on the top |
|
afterwards). |
|
|
|
@cindex prefix, type |
|
@cindex type prefix |
|
@cindex default stack of a type prefix |
How do we know the type and stack of the stack items? Vmgen uses |
How do we know the type and stack of the stack items? Vmgen uses |
prefixes, similar to Fortran; in contrast to Fortran, you have to |
prefixes, similar to Fortran; in contrast to Fortran, you have to |
define the prefix first: |
define the prefix first: |
Line 351 This defines the prefix @code{i} to refe
|
Line 793 This defines the prefix @code{i} to refe
|
@code{data-stack}. It also specifies that this type takes one stack |
@code{data-stack}. It also specifies that this type takes one stack |
item (@code{single}). The type prefix is part of the variable name. |
item (@code{single}). The type prefix is part of the variable name. |
|
|
|
@cindex stack definition |
|
@cindex defining a stack |
Before we can use @code{data-stack} in this way, we have to define it: |
Before we can use @code{data-stack} in this way, we have to define it: |
|
|
@example |
@example |
Line 358 Before we can use @code{data-stack} in t
|
Line 802 Before we can use @code{data-stack} in t
|
@end example |
@end example |
@c !! use something other than Cell |
@c !! use something other than Cell |
|
|
|
@cindex stack basic type |
|
@cindex basic type of a stack |
|
@cindex type of a stack, basic |
This line defines the stack @code{data-stack}, which uses the stack |
This line defines the stack @code{data-stack}, which uses the stack |
pointer @code{sp}, and each item has the basic type @code{Cell}; other |
pointer @code{sp}, and each item has the basic type @code{Cell}; other |
types have to fit into one or two @code{Cell}s (depending on whether the |
types have to fit into one or two @code{Cell}s (depending on whether the |
type is @code{single} or @code{double} wide), and are converted from and |
type is @code{single} or @code{double} wide), and are cast from and to |
to Cells on accessing the @code{data-stack) with conversion macros |
Cells on accessing the @code{data-stack} with type cast macros |
(@pxref{Conversion macros}). Stacks grow towards lower addresses in |
(@pxref{VM engine}). By default, stacks grow towards lower addresses in |
vmgen. |
Vmgen-erated interpreters (@pxref{Stack growth direction}). |
|
|
|
@cindex stack prefix |
|
@cindex prefix, stack |
We can override the default stack of a stack item by using a stack |
We can override the default stack of a stack item by using a stack |
prefix. E.g., consider the following instruction: |
prefix. E.g., consider the following instruction: |
|
|
Line 374 lit ( #i -- i )
|
Line 823 lit ( #i -- i )
|
@end example |
@end example |
|
|
The VM instruction @code{lit} takes the item @code{i} from the |
The VM instruction @code{lit} takes the item @code{i} from the |
instruction stream (indicated by the prefix @code{#}, and pushes it on |
instruction stream (indicated by the prefix @code{#}), and pushes it on |
the (default) data stack. The stack prefix is not part of the variable |
the (default) data stack. The stack prefix is not part of the variable |
name. Stack prefixes are defined like this: |
name. Stack prefixes are defined like this: |
|
|
Line 382 name. Stack prefixes are defined like t
|
Line 831 name. Stack prefixes are defined like t
|
\E inst-stream stack-prefix # |
\E inst-stream stack-prefix # |
@end example |
@end example |
|
|
This definition defines that the stack prefix @code{#} to specifies the |
This definition defines that the stack prefix @code{#} specifies the |
``stack'' @code{inst-stream}. Since the instruction stream behaves a |
``stack'' @code{inst-stream}. Since the instruction stream behaves a |
little differently than an ordinary stack, it is predefined, and you do |
little differently than an ordinary stack, it is predefined, and you do |
not need to define it. |
not need to define it. |
|
|
|
@cindex instruction stream |
The instruction stream contains instructions and their immediate |
The instruction stream contains instructions and their immediate |
arguments, so specifying that an argument comes from the instruction |
arguments, so specifying that an argument comes from the instruction |
stream indicates an immediate argument. Of course, instruction stream |
stream indicates an immediate argument. Of course, instruction stream |
Line 394 arguments can only appear to the left of
|
Line 844 arguments can only appear to the left of
|
If there are multiple instruction stream arguments, the leftmost is the |
If there are multiple instruction stream arguments, the leftmost is the |
first one (just as the intuition suggests). |
first one (just as the intuition suggests). |
|
|
|
@menu |
|
* C Code Macros:: Macros recognized by Vmgen |
|
* C Code restrictions:: Vmgen makes assumptions about C code |
|
* Stack growth direction:: is configurable per stack |
|
@end menu |
|
|
|
@c -------------------------------------------------------------------- |
|
@node C Code Macros, C Code restrictions, Simple instructions, Simple instructions |
|
@subsection C Code Macros |
|
@cindex macros recognized by Vmgen |
|
@cindex basic block, VM level |
|
|
|
Vmgen recognizes the following strings in the C code part of simple |
|
instructions: |
|
|
|
@table @code |
|
|
|
@item SET_IP |
|
@findex SET_IP |
|
As far as Vmgen is concerned, a VM instruction containing this ends a VM |
|
basic block (used in profiling to delimit profiled sequences). On the C |
|
level, this also sets the instruction pointer. |
|
|
|
@item SUPER_END |
|
@findex SUPER_END |
|
This ends a basic block (for profiling), even if the instruction |
|
contains no @code{SET_IP}. |
|
|
|
@item INST_TAIL; |
|
@findex INST_TAIL; |
|
Vmgen replaces @samp{INST_TAIL;} with code for ending a VM instruction and |
|
dispatching the next VM instruction. Even without a @samp{INST_TAIL;} this |
|
happens automatically when control reaches the end of the C code. If |
|
you want to have this in the middle of the C code, you need to use |
|
@samp{INST_TAIL;}. A typical example is a conditional VM branch: |
|
|
|
@example |
|
if (branch_condition) @{ |
|
SET_IP(target); INST_TAIL; |
|
@} |
|
/* implicit tail follows here */ |
|
@end example |
|
|
|
In this example, @samp{INST_TAIL;} is not strictly necessary, because there |
|
is another one implicitly after the if-statement, but using it improves |
|
branch prediction accuracy slightly and allows other optimizations. |
|
|
|
@item SUPER_CONTINUE |
|
@findex SUPER_CONTINUE |
|
This indicates that the implicit tail at the end of the VM instruction |
|
dispatches the sequentially next VM instruction even if there is a |
|
@code{SET_IP} in the VM instruction. This enables an optimization that |
|
is not yet implemented in the vmgen-ex code (but in Gforth). The |
|
typical application is in conditional VM branches: |
|
|
|
@example |
|
if (branch_condition) @{ |
|
SET_IP(target); INST_TAIL; /* now this INST_TAIL is necessary */ |
|
@} |
|
SUPER_CONTINUE; |
|
@end example |
|
|
|
@item VM_JUMP |
|
@findex VM_JUMP |
|
@code{VM_JUMP(target)} is equivalent to @code{goto *(target)}, but |
|
allows Vmgen to do dynamic superinstructions and replication. You |
|
still need to say @code{SUPER_END}. Also, the goto only happens at |
|
the end (wherever the VM_JUMP is). Essentially, this just suppresses |
|
much of the ordinary dispatch mechanism. |
|
|
|
@end table |
|
|
|
Note that Vmgen is not smart about C-level tokenization, comments, |
|
strings, or conditional compilation, so it will interpret even a |
|
commented-out SUPER_END as ending a basic block (or, e.g., |
|
@samp{RESET_IP;} as @samp{SET_IP;}). Conversely, Vmgen requires the literal |
|
presence of these strings; Vmgen will not see them if they are hiding in |
|
a C preprocessor macro. |
|
|
|
|
|
@c -------------------------------------------------------------------- |
|
@node C Code restrictions, Stack growth direction, C Code Macros, Simple instructions |
|
@subsection C Code restrictions |
|
@cindex C code restrictions |
|
@cindex restrictions on C code |
|
@cindex assumptions about C code |
|
|
|
@cindex accessing stack (pointer) |
|
@cindex stack pointer, access |
|
@cindex instruction pointer, access |
|
Vmgen generates code and performs some optimizations under the |
|
assumption that the user-supplied C code does not access the stack |
|
pointers or stack items, and that accesses to the instruction pointer |
|
only occur through special macros. In general you should heed these |
|
restrictions. However, if you need to break these restrictions, read |
|
the following. |
|
|
|
Accessing a stack or stack pointer directly can be a problem for several |
|
reasons: |
|
@cindex stack caching, restriction on C code |
|
@cindex superinstructions, restrictions on components |
|
|
|
@itemize @bullet |
|
|
|
@item |
|
Vmgen optionally supports caching the top-of-stack item in a local |
|
variable (that is allocated to a register). This is the most frequent |
|
source of trouble. You can deal with it either by not using |
|
top-of-stack caching (slowdown factor 1-1.4, depending on machine), or |
|
by inserting flushing code (e.g., @samp{IF_spTOS(sp[...] = spTOS);}) at |
|
the start and reloading code (e.g., @samp{IF_spTOS(spTOS = sp[0])}) at |
|
the end of problematic C code. Vmgen inserts a stack pointer update |
|
before the start of the user-supplied C code, so the flushing code has |
|
to use an index that corrects for that. In the future, this flushing |
|
may be done automatically by mentioning a special string in the C code. |
|
@c sometimes flushing and/or reloading unnecessary |
|
|
|
@item |
|
The Vmgen-erated code loads the stack items from stack-pointer-indexed |
|
memory into variables before the user-supplied C code, and stores them |
|
from variables to stack-pointer-indexed memory afterwards. If you do |
|
any writes to the stack through its stack pointer in your C code, it |
|
will not affect the variables, and your write may be overwritten by the |
|
stores after the C code. Similarly, a read from a stack using a stack |
|
pointer will not reflect computations of stack items in the same VM |
|
instruction. |
|
|
|
@item |
|
Superinstructions keep stack items in variables across the whole |
|
superinstruction. So you should not include VM instructions, that |
|
access a stack or stack pointer, as components of superinstructions |
|
(@pxref{VM profiler}). |
|
|
|
@end itemize |
|
|
|
You should access the instruction pointer only through its special |
|
macros (@samp{IP}, @samp{SET_IP}, @samp{IPTOS}); this ensure that these |
|
macros can be implemented in several ways for best performance. |
|
@samp{IP} points to the next instruction, and @samp{IPTOS} is its |
|
contents. |
|
|
|
@c -------------------------------------------------------------------- |
|
@node Stack growth direction, , C Code restrictions, Simple instructions |
|
@subsection Stack growth direction |
|
@cindex stack growth direction |
|
|
|
@cindex @code{stack-access-transform} |
|
By default, the stacks grow towards lower addresses. You can change |
|
this for a stack by setting the @code{stack-access-transform} field of |
|
the stack to an xt @code{( itemnum -- index )} that performs the |
|
appropriate index transformation. |
|
|
|
E.g., if you want to let @code{data-stack} grow towards higher |
|
addresses, with the stack pointer always pointing just beyond the |
|
top-of-stack, use this right after defining @code{data-stack}: |
|
|
|
@example |
|
\E : sp-access-transform ( itemnum -- index ) negate 1- ; |
|
\E ' sp-access-transform ' data-stack >body stack-access-transform ! |
|
@end example |
|
|
|
This means that @code{sp-access-transform} will be used to generate |
|
indexes for accessing @code{data-stack}. The definition of |
|
@code{sp-access-transform} above transforms n into -n-1, e.g, 1 into -2. |
|
This will access the 0th data-stack element (top-of-stack) at sp[-1], |
|
the 1st at sp[-2], etc., which is the typical way upward-growing |
|
stacks are used. If you need a different transform and do not know |
|
enough Forth to program it, let me know. |
|
|
|
@c -------------------------------------------------------------------- |
|
@node Superinstructions, Store Optimization, Simple instructions, Input File Format |
@section Superinstructions |
@section Superinstructions |
|
@cindex superinstructions, defining |
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@cindex defining superinstructions |
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|
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Note: don't invest too much work in (static) superinstructions; a future |
|
version of Vmgen will support dynamic superinstructions (see Ian |
|
Piumarta and Fabio Riccardi, @cite{Optimizing Direct Threaded Code by |
|
Selective Inlining}, PLDI'98), and static superinstructions have much |
|
less benefit in that context (preliminary results indicate only a factor |
|
1.1 speedup). |
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|
|
Here is an example of a superinstruction definition: |
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|
|
@example |
|
lit_sub = lit sub |
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@end example |
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|
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@code{lit_sub} is the name of the superinstruction, and @code{lit} and |
|
@code{sub} are its components. This superinstruction performs the same |
|
action as the sequence @code{lit} and @code{sub}. It is generated |
|
automatically by the VM code generation functions whenever that sequence |
|
occurs, so if you want to use this superinstruction, you just need to |
|
add this definition (and even that can be partially automatized, |
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@pxref{VM profiler}). |
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|
|
@cindex prefixes of superinstructions |
|
Vmgen requires that the component instructions are simple instructions |
|
defined before superinstructions using the components. Currently, Vmgen |
|
also requires that all the subsequences at the start of a |
|
superinstruction (prefixes) must be defined as superinstruction before |
|
the superinstruction. I.e., if you want to define a superinstruction |
|
|
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@example |
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foo4 = load add sub mul |
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@end example |
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|
|
you first have to define @code{load}, @code{add}, @code{sub} and |
|
@code{mul}, plus |
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|
|
@example |
|
foo2 = load add |
|
foo3 = load add sub |
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@end example |
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|
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Here, @code{sumof4} is the longest prefix of @code{sumof5}, and @code{sumof3} |
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is the longest prefix of @code{sumof4}. |
|
|
|
Note that Vmgen assumes that only the code it generates accesses stack |
|
pointers, the instruction pointer, and various stack items, and it |
|
performs optimizations based on this assumption. Therefore, VM |
|
instructions where your C code changes the instruction pointer should |
|
only be used as last component; a VM instruction where your C code |
|
accesses a stack pointer should not be used as component at all. Vmgen |
|
does not check these restrictions, they just result in bugs in your |
|
interpreter. |
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|
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@cindex include-skipped-insts |
|
The Vmgen flag @code{include-skipped-insts} influences superinstruction |
|
code generation. Currently there is no support in the peephole |
|
optimizer for both variations, so leave this flag alone for now. |
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|
@c ------------------------------------------------------------------- |
|
@node Store Optimization, Register Machines, Superinstructions, Input File Format |
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@section Store Optimization |
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@cindex store optimization |
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@cindex optimization, stack stores |
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@cindex stack stores, optimization |
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@cindex eliminating stack stores |
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|
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This minor optimization (0.6\%--0.8\% reduction in executed instructions |
|
for Gforth) puts additional requirements on the instruction descriptions |
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and is therefore disabled by default. |
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|
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What does it do? Consider an instruction like |
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|
|
@example |
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dup ( n -- n n ) |
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@end example |
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|
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For simplicity, also assume that we are not caching the top-of-stack in |
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a register. Now, the C code for dup first loads @code{n} from the |
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stack, and then stores it twice to the stack, one time to the address |
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where it came from; that time is unnecessary, but gcc does not optimize |
|
it away, so vmgen can do it instead (if you turn on the store |
|
optimization). |
|
|
|
Vmgen uses the stack item's name to determine if the stack item contains |
|
the same value as it did at the start. Therefore, if you use the store |
|
optimization, you have to ensure that stack items that have the same |
|
name on input and output also have the same value, and are not changed |
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in the C code you supply. I.e., the following code could fail if you |
|
turn on the store optimization: |
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|
|
@example |
|
add1 ( n -- n ) |
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n++; |
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@end example |
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|
|
Instead, you have to use different names, i.e.: |
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|
|
@example |
|
add1 ( n1 -- n2 ) |
|
n2=n1+1; |
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@end example |
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|
|
Similarly, the store optimization assumes that the stack pointer is only |
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changed by Vmgen-erated code. If your C code changes the stack pointer, |
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use different names in input and output stack items to avoid a (probably |
|
wrong) store optimization, or turn the store optimization off for this |
|
VM instruction. |
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|
|
To turn on the store optimization, write |
|
|
|
@example |
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\E store-optimization on |
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@end example |
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|
|
at the start of the file. You can turn this optimization on or off |
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between any two VM instruction descriptions. For turning it off again, |
|
you can use |
|
|
|
@example |
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\E store-optimization off |
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@end example |
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|
|
@c ------------------------------------------------------------------- |
|
@node Register Machines, , Store Optimization, Input File Format |
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@section Register Machines |
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@cindex Register VM |
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@cindex Superinstructions for register VMs |
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@cindex tracing of register VMs |
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|
|
If you want to implement a register VM rather than a stack VM with |
|
Vmgen, there are two ways to do it: Directly and through |
|
superinstructions. |
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|
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If you use the direct way, you define instructions that take the |
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register numbers as immediate arguments, like this: |
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|
|
@example |
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add3 ( #src1 #src2 #dest -- ) |
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reg[dest] = reg[src1]+reg[src2]; |
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@end example |
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|
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A disadvantage of this method is that during tracing you only see the |
|
register numbers, but not the register contents. Actually, with an |
|
appropriate definition of @code{printarg_src} (@pxref{VM engine}), you |
|
can print the values of the source registers on entry, but you cannot |
|
print the value of the destination register on exit. |
|
|
|
If you use superinstructions to define a register VM, you define simple |
|
instructions that use a stack, and then define superinstructions that |
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have no overall stack effect, like this: |
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|
|
@example |
|
loadreg ( #src -- n ) |
|
n = reg[src]; |
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|
|
storereg ( n #dest -- ) |
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reg[dest] = n; |
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|
|
adds ( n1 n2 -- n ) |
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n = n1+n2; |
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|
|
add3 = loadreg loadreg adds storereg |
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@end example |
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|
|
An advantage of this method is that you see the values and not just the |
|
register numbers in tracing. A disadvantage of this method is that |
|
currently you cannot generate superinstructions directly, but only |
|
through generating a sequence of simple instructions (we might change |
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this in the future if there is demand). |
|
|
|
Could the register VM support be improved, apart from the issues |
|
mentioned above? It is hard to see how to do it in a general way, |
|
because there are a number of different designs that different people |
|
mean when they use the term @emph{register machine} in connection with |
|
VM interpreters. However, if you have ideas or requests in that |
|
direction, please let me know (@pxref{Contact}). |
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|
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@c ******************************************************************** |
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@node Error messages, Using the generated code, Input File Format, Top |
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@chapter Error messages |
|
@cindex error messages |
|
|
|
These error messages are created by Vmgen: |
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|
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@table @code |
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|
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@cindex @code{# can only be on the input side} error |
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@item # can only be on the input side |
|
You have used an instruction-stream prefix (usually @samp{#}) after the |
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@samp{--} (the output side); you can only use it before (the input |
|
side). |
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|
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@cindex @code{prefix for this combination must be defined earlier} error |
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@item the prefix for this superinstruction must be defined earlier |
|
You have defined a superinstruction (e.g. @code{abc = a b c}) without |
|
defining its direct prefix (e.g., @code{ab = a b}), |
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@xref{Superinstructions}. |
|
|
|
@cindex @code{sync line syntax} error |
|
@item sync line syntax |
|
If you are using a preprocessor (e.g., @command{m4}) to generate Vmgen |
|
input code, you may want to create @code{#line} directives (aka sync |
|
lines). This error indicates that such a line is not in th syntax |
|
expected by Vmgen (this should not happen; please report the offending |
|
line in a bug report). |
|
|
|
@cindex @code{syntax error, wrong char} error |
|
@item syntax error, wrong char |
|
A syntax error. If you do not see right away where the error is, it may |
|
be helpful to check the following: Did you put an empty line in a VM |
|
instruction where the C code is not delimited by braces (then the empty |
|
line ends the VM instruction)? If you used brace-delimited C code, did |
|
you put the delimiting braces (and only those) at the start of the line, |
|
without preceding white space? Did you forget a delimiting brace? |
|
|
|
@cindex @code{too many stacks} error |
|
@item too many stacks |
|
Vmgen currently supports 3 stacks (plus the instruction stream); if you |
|
need more, let us know. |
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|
|
@cindex @code{unknown prefix} error |
|
@item unknown prefix |
|
The stack item does not match any defined type prefix (after stripping |
|
away any stack prefix). You should either declare the type prefix you |
|
want for that stack item, or use a different type prefix |
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|
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@cindex @code{unknown primitive} error |
|
@item unknown primitive |
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You have used the name of a simple VM instruction in a superinstruction |
|
definition without defining the simple VM instruction first. |
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|
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@end table |
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|
|
In addition, the C compiler can produce errors due to code produced by |
|
Vmgen; e.g., you need to define type cast functions. |
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|
|
@c ******************************************************************** |
|
@node Using the generated code, Hints, Error messages, Top |
|
@chapter Using the generated code |
|
@cindex generated code, usage |
|
@cindex Using vmgen-erated code |
|
|
|
The easiest way to create a working VM interpreter with Vmgen is |
|
probably to start with @file{vmgen-ex}, and modify it for your purposes. |
|
This chapter explains what the various wrapper and generated files do. |
|
It also contains reference-manual style descriptions of the macros, |
|
variables etc. used by the generated code, and you can skip that on |
|
first reading. |
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|
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@menu |
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* VM engine:: Executing VM code |
|
* VM instruction table:: |
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* VM code generation:: Creating VM code (in the front-end) |
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* Peephole optimization:: Creating VM superinstructions |
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* VM disassembler:: for debugging the front end |
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* VM profiler:: for finding worthwhile superinstructions |
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@end menu |
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|
|
@c -------------------------------------------------------------------- |
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@node VM engine, VM instruction table, Using the generated code, Using the generated code |
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@section VM engine |
|
@cindex VM instruction execution |
|
@cindex engine |
|
@cindex executing VM code |
|
@cindex @file{engine.c} |
|
@cindex @file{-vm.i} output file |
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|
|
The VM engine is the VM interpreter that executes the VM code. It is |
|
essential for an interpretive system. |
|
|
|
Vmgen supports two methods of VM instruction dispatch: @emph{threaded |
|
code} (fast, but gcc-specific), and @emph{switch dispatch} (slow, but |
|
portable across C compilers); you can use conditional compilation |
|
(@samp{defined(__GNUC__)}) to choose between these methods, and our |
|
example does so. |
|
|
|
For both methods, the VM engine is contained in a C-level function. |
|
Vmgen generates most of the contents of the function for you |
|
(@file{@var{name}-vm.i}), but you have to define this function, and |
|
macros and variables used in the engine, and initialize the variables. |
|
In our example the engine function also includes |
|
@file{@var{name}-labels.i} (@pxref{VM instruction table}). |
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|
|
@cindex tracing VM code |
|
@cindex superinstructions and tracing |
|
In addition to executing the code, the VM engine can optionally also |
|
print out a trace of the executed instructions, their arguments and |
|
results. For superinstructions it prints the trace as if only component |
|
instructions were executed; this allows to introduce new |
|
superinstructions while keeping the traces comparable to old ones |
|
(important for regression tests). |
|
|
|
It costs significant performance to check in each instruction whether to |
|
print tracing code, so we recommend producing two copies of the engine: |
|
one for fast execution, and one for tracing. See the rules for |
|
@file{engine.o} and @file{engine-debug.o} in @file{vmgen-ex/Makefile} |
|
for an example. |
|
|
|
The following macros and variables are used in @file{@var{name}-vm.i}: |
|
|
|
@table @code |
|
|
|
@findex LABEL |
|
@item LABEL(@var{inst_name}) |
|
This is used just before each VM instruction to provide a jump or |
|
@code{switch} label (the @samp{:} is provided by Vmgen). For switch |
|
dispatch this should expand to @samp{case @var{label}:}; for |
|
threaded-code dispatch this should just expand to @samp{@var{label}:}. |
|
In either case @var{label} is usually the @var{inst_name} with some |
|
prefix or suffix to avoid naming conflicts. |
|
|
|
@findex LABEL2 |
|
@item LABEL2(@var{inst_name}) |
|
This will be used for dynamic superinstructions; at the moment, this |
|
should expand to nothing. |
|
|
|
@findex NAME |
|
@item NAME(@var{inst_name_string}) |
|
Called on entering a VM instruction with a string containing the name of |
|
the VM instruction as parameter. In normal execution this should be |
|
expand to nothing, but for tracing this usually prints the name, and |
|
possibly other information (several VM registers in our example). |
|
|
|
@findex DEF_CA |
|
@item DEF_CA |
|
Usually empty. Called just inside a new scope at the start of a VM |
|
instruction. Can be used to define variables that should be visible |
|
during every VM instruction. If you define this macro as non-empty, you |
|
have to provide the finishing @samp{;} in the macro. |
|
|
|
@findex NEXT_P0 |
|
@findex NEXT_P1 |
|
@findex NEXT_P2 |
|
@item NEXT_P0 NEXT_P1 NEXT_P2 |
|
The three parts of instruction dispatch. They can be defined in |
|
different ways for best performance on various processors (see |
|
@file{engine.c} in the example or @file{engine/threaded.h} in Gforth). |
|
@samp{NEXT_P0} is invoked right at the start of the VM instruction (but |
|
after @samp{DEF_CA}), @samp{NEXT_P1} right after the user-supplied C |
|
code, and @samp{NEXT_P2} at the end. The actual jump has to be |
|
performed by @samp{NEXT_P2} (if you would do it earlier, important parts |
|
of the VM instruction would not be executed). |
|
|
|
The simplest variant is if @samp{NEXT_P2} does everything and the other |
|
macros do nothing. Then also related macros like @samp{IP}, |
|
@samp{SET_IP}, @samp{IP}, @samp{INC_IP} and @samp{IPTOS} are very |
|
straightforward to define. For switch dispatch this code consists just |
|
of a jump to the dispatch code (@samp{goto next_inst;} in our example); |
|
for direct threaded code it consists of something like |
|
@samp{(@{cfa=*ip++; goto *cfa;@})}. |
|
|
|
Pulling code (usually the @samp{cfa=*ip++;}) up into @samp{NEXT_P1} |
|
usually does not cause problems, but pulling things up into |
|
@samp{NEXT_P0} usually requires changing the other macros (and, at least |
|
for Gforth on Alpha, it does not buy much, because the compiler often |
|
manages to schedule the relevant stuff up by itself). An even more |
|
extreme variant is to pull code up even further, into, e.g., NEXT_P1 of |
|
the previous VM instruction (prefetching, useful on PowerPCs). |
|
|
|
@findex INC_IP |
|
@item INC_IP(@var{n}) |
|
This increments @code{IP} by @var{n}. |
|
|
|
@findex SET_IP |
|
@item SET_IP(@var{target}) |
|
This sets @code{IP} to @var{target}. |
|
|
|
@cindex type cast macro |
|
@findex vm_@var{A}2@var{B} |
|
@item vm_@var{A}2@var{B}(a,b) |
|
Type casting macro that assigns @samp{a} (of type @var{A}) to @samp{b} |
|
(of type @var{B}). This is mainly used for getting stack items into |
|
variables and back. So you need to define macros for every combination |
|
of stack basic type (@code{Cell} in our example) and type-prefix types |
|
used with that stack (in both directions). For the type-prefix type, |
|
you use the type-prefix (not the C type string) as type name (e.g., |
|
@samp{vm_Cell2i}, not @samp{vm_Cell2Cell}). In addition, you have to |
|
define a vm_@var{X}2@var{X} macro for the stack's basic type @var{X} |
|
(used in superinstructions). |
|
|
|
@cindex instruction stream, basic type |
|
The stack basic type for the predefined @samp{inst-stream} is |
|
@samp{Cell}. If you want a stack with the same item size, making its |
|
basic type @samp{Cell} usually reduces the number of macros you have to |
|
define. |
|
|
|
@cindex unions in type cast macros |
|
@cindex casts in type cast macros |
|
@cindex type casting between floats and integers |
|
Here our examples differ a lot: @file{vmgen-ex} uses casts in these |
|
macros, whereas @file{vmgen-ex2} uses union-field selection (or |
|
assignment to union fields). Note that casting floats into integers and |
|
vice versa changes the bit pattern (and you do not want that). In this |
|
case your options are to use a (temporary) union, or to take the address |
|
of the value, cast the pointer, and dereference that (not always |
|
possible, and sometimes expensive). |
|
|
|
@findex vm_two@var{A}2@var{B} |
|
@findex vm_@var{B}2two@var{A} |
|
@item vm_two@var{A}2@var{B}(a1,a2,b) |
|
@item vm_@var{B}2two@var{A}(b,a1,a2) |
|
Type casting between two stack items (@code{a1}, @code{a2}) and a |
|
variable @code{b} of a type that takes two stack items. This does not |
|
occur in our small examples, but you can look at Gforth for examples |
|
(see @code{vm_twoCell2d} in @file{engine/forth.h}). |
|
|
|
@cindex stack pointer definition |
|
@cindex instruction pointer definition |
|
@item @var{stackpointer} |
|
For each stack used, the stackpointer name given in the stack |
|
declaration is used. For a regular stack this must be an l-expression; |
|
typically it is a variable declared as a pointer to the stack's basic |
|
type. For @samp{inst-stream}, the name is @samp{IP}, and it can be a |
|
plain r-value; typically it is a macro that abstracts away the |
|
differences between the various implementations of @code{NEXT_P*}. |
|
|
|
@cindex IMM_ARG |
|
@findex IMM_ARG |
|
@item IMM_ARG(access,value) |
|
Define this to expland to ``(access)''. This is just a placeholder for |
|
future extensions. |
|
|
|
@cindex top of stack caching |
|
@cindex stack caching |
|
@cindex TOS |
|
@findex IPTOS |
|
@item @var{stackpointer}TOS |
|
The top-of-stack for the stack pointed to by @var{stackpointer}. If you |
|
are using top-of-stack caching for that stack, this should be defined as |
|
variable; if you are not using top-of-stack caching for that stack, this |
|
should be a macro expanding to @samp{@var{stackpointer}[0]}. The stack |
|
pointer for the predefined @samp{inst-stream} is called @samp{IP}, so |
|
the top-of-stack is called @samp{IPTOS}. |
|
|
|
@findex IF_@var{stackpointer}TOS |
|
@item IF_@var{stackpointer}TOS(@var{expr}) |
|
Macro for executing @var{expr}, if top-of-stack caching is used for the |
|
@var{stackpointer} stack. I.e., this should do @var{expr} if there is |
|
top-of-stack caching for @var{stackpointer}; otherwise it should do |
|
nothing. |
|
|
|
@findex SUPER_END |
|
@item SUPER_END |
|
This is used by the VM profiler (@pxref{VM profiler}); it should not do |
|
anything in normal operation, and call @code{vm_count_block(IP)} for |
|
profiling. |
|
|
|
@findex SUPER_CONTINUE |
|
@item SUPER_CONTINUE |
|
This is just a hint to Vmgen and does nothing at the C level. |
|
|
|
@findex MAYBE_UNUSED |
|
@item MAYBE_UNUSED |
|
This should be defined as @code{__attribute__((unused))} for gcc-2.7 and |
|
higher. It suppresses the warnings about unused variables in the code |
|
for superinstructions. You need to define this only if you are using |
|
superinstructions. |
|
|
|
@findex VM_DEBUG |
|
@item VM_DEBUG |
|
If this is defined, the tracing code will be compiled in (slower |
|
interpretation, but better debugging). Our example compiles two |
|
versions of the engine, a fast-running one that cannot trace, and one |
|
with potential tracing and profiling. |
|
|
|
@findex vm_debug |
|
@item vm_debug |
|
Needed only if @samp{VM_DEBUG} is defined. If this variable contains |
|
true, the VM instructions produce trace output. It can be turned on or |
|
off at any time. |
|
|
|
@findex vm_out |
|
@item vm_out |
|
Needed only if @samp{VM_DEBUG} is defined. Specifies the file on which |
|
to print the trace output (type @samp{FILE *}). |
|
|
|
@findex printarg_@var{type} |
|
@item printarg_@var{type}(@var{value}) |
|
Needed only if @samp{VM_DEBUG} is defined. Macro or function for |
|
printing @var{value} in a way appropriate for the @var{type}. This is |
|
used for printing the values of stack items during tracing. @var{Type} |
|
is normally the type prefix specified in a @code{type-prefix} definition |
|
(e.g., @samp{printarg_i}); in superinstructions it is currently the |
|
basic type of the stack. |
|
|
|
@end table |
|
|
|
|
|
@c -------------------------------------------------------------------- |
|
@node VM instruction table, VM code generation, VM engine, Using the generated code |
|
@section VM instruction table |
|
@cindex instruction table |
|
@cindex opcode definition |
|
@cindex labels for threaded code |
|
@cindex @code{vm_prim}, definition |
|
@cindex @file{-labels.i} output file |
|
|
|
For threaded code we also need to produce a table containing the labels |
|
of all VM instructions. This is needed for VM code generation |
|
(@pxref{VM code generation}), and it has to be done in the engine |
|
function, because the labels are not visible outside. It then has to be |
|
passed outside the function (and assigned to @samp{vm_prim}), to be used |
|
by the VM code generation functions. |
|
|
|
This means that the engine function has to be called first to produce |
|
the VM instruction table, and later, after generating VM code, it has to |
|
be called again to execute the generated VM code (yes, this is ugly). |
|
In our example program, these two modes of calling the engine function |
|
are differentiated by the value of the parameter ip0 (if it equals 0, |
|
then the table is passed out, otherwise the VM code is executed); in our |
|
example, we pass the table out by assigning it to @samp{vm_prim} and |
|
returning from @samp{engine}. |
|
|
|
In our example (@file{vmgen-ex/engine.c}), we also build such a table for |
|
switch dispatch; this is mainly done for uniformity. |
|
|
|
For switch dispatch, we also need to define the VM instruction opcodes |
|
used as case labels in an @code{enum}. |
|
|
|
For both purposes (VM instruction table, and enum), the file |
|
@file{@var{name}-labels.i} is generated by Vmgen. You have to define |
|
the following macro used in this file: |
|
|
|
@table @code |
|
|
|
@findex INST_ADDR |
|
@item INST_ADDR(@var{inst_name}) |
|
For switch dispatch, this is just the name of the switch label (the same |
|
name as used in @samp{LABEL(@var{inst_name})}), for both uses of |
|
@file{@var{name}-labels.i}. For threaded-code dispatch, this is the |
|
address of the label defined in @samp{LABEL(@var{inst_name})}); the |
|
address is taken with @samp{&&} (@pxref{Labels as Values, , Labels as |
|
Values, gcc.info, GNU C Manual}). |
|
|
|
@end table |
|
|
|
|
|
@c -------------------------------------------------------------------- |
|
@node VM code generation, Peephole optimization, VM instruction table, Using the generated code |
|
@section VM code generation |
|
@cindex VM code generation |
|
@cindex code generation, VM |
|
@cindex @file{-gen.i} output file |
|
|
|
Vmgen generates VM code generation functions in @file{@var{name}-gen.i} |
|
that the front end can call to generate VM code. This is essential for |
|
an interpretive system. |
|
|
|
@findex gen_@var{inst} |
|
For a VM instruction @samp{x ( #a b #c -- d )}, Vmgen generates a |
|
function with the prototype |
|
|
|
@example |
|
void gen_x(Inst **ctp, a_type a, c_type c) |
|
@end example |
|
|
|
The @code{ctp} argument points to a pointer to the next instruction. |
|
@code{*ctp} is increased by the generation functions; i.e., you should |
|
allocate memory for the code to be generated beforehand, and start with |
|
*ctp set at the start of this memory area. Before running out of |
|
memory, allocate a new area, and generate a VM-level jump to the new |
|
area (this overflow handling is not implemented in our examples). |
|
|
|
@cindex immediate arguments, VM code generation |
|
The other arguments correspond to the immediate arguments of the VM |
|
instruction (with their appropriate types as defined in the |
|
@code{type_prefix} declaration. |
|
|
|
The following types, variables, and functions are used in |
|
@file{@var{name}-gen.i}: |
|
|
|
@table @code |
|
|
|
@findex Inst |
|
@item Inst |
|
The type of the VM instruction; if you use threaded code, this is |
|
@code{void *}; for switch dispatch this is an integer type. |
|
|
|
@cindex @code{vm_prim}, use |
|
@item vm_prim |
|
The VM instruction table (type: @code{Inst *}, @pxref{VM instruction table}). |
|
|
|
@findex gen_inst |
|
@item gen_inst(Inst **ctp, Inst i) |
|
This function compiles the instruction @code{i}. Take a look at it in |
|
@file{vmgen-ex/peephole.c}. It is trivial when you don't want to use |
|
superinstructions (just the last two lines of the example function), and |
|
slightly more complicated in the example due to its ability to use |
|
superinstructions (@pxref{Peephole optimization}). |
|
|
|
@findex genarg_@var{type_prefix} |
|
@item genarg_@var{type_prefix}(Inst **ctp, @var{type} @var{type_prefix}) |
|
This compiles an immediate argument of @var{type} (as defined in a |
|
@code{type-prefix} definition). These functions are trivial to define |
|
(see @file{vmgen-ex/support.c}). You need one of these functions for |
|
every type that you use as immediate argument. |
|
|
|
@end table |
|
|
|
@findex BB_BOUNDARY |
|
In addition to using these functions to generate code, you should call |
|
@code{BB_BOUNDARY} at every basic block entry point if you ever want to |
|
use superinstructions (or if you want to use the profiling supported by |
|
Vmgen; but this support is also useful mainly for selecting |
|
superinstructions). If you use @code{BB_BOUNDARY}, you should also |
|
define it (take a look at its definition in @file{vmgen-ex/mini.y}). |
|
|
|
You do not need to call @code{BB_BOUNDARY} after branches, because you |
|
will not define superinstructions that contain branches in the middle |
|
(and if you did, and it would work, there would be no reason to end the |
|
superinstruction at the branch), and because the branches announce |
|
themselves to the profiler. |
|
|
|
|
|
@c -------------------------------------------------------------------- |
|
@node Peephole optimization, VM disassembler, VM code generation, Using the generated code |
|
@section Peephole optimization |
|
@cindex peephole optimization |
|
@cindex superinstructions, generating |
|
@cindex @file{peephole.c} |
|
@cindex @file{-peephole.i} output file |
|
|
|
You need peephole optimization only if you want to use |
|
superinstructions. But having the code for it does not hurt much if you |
|
do not use superinstructions. |
|
|
|
A simple greedy peephole optimization algorithm is used for |
|
superinstruction selection: every time @code{gen_inst} compiles a VM |
|
instruction, it checks if it can combine it with the last VM instruction |
|
(which may also be a superinstruction resulting from a previous peephole |
|
optimization); if so, it changes the last instruction to the combined |
|
instruction instead of laying down @code{i} at the current @samp{*ctp}. |
|
|
|
The code for peephole optimization is in @file{vmgen-ex/peephole.c}. |
|
You can use this file almost verbatim. Vmgen generates |
|
@file{@var{file}-peephole.i} which contains data for the peephole |
|
optimizer. |
|
|
|
@findex init_peeptable |
|
You have to call @samp{init_peeptable()} after initializing |
|
@samp{vm_prim}, and before compiling any VM code to initialize data |
|
structures for peephole optimization. After that, compiling with the VM |
|
code generation functions will automatically combine VM instructions |
|
into superinstructions. Since you do not want to combine instructions |
|
across VM branch targets (otherwise there will not be a proper VM |
|
instruction to branch to), you have to call @code{BB_BOUNDARY} |
|
(@pxref{VM code generation}) at branch targets. |
|
|
|
|
|
@c -------------------------------------------------------------------- |
|
@node VM disassembler, VM profiler, Peephole optimization, Using the generated code |
|
@section VM disassembler |
|
@cindex VM disassembler |
|
@cindex disassembler, VM code |
|
@cindex @file{disasm.c} |
|
@cindex @file{-disasm.i} output file |
|
|
|
A VM code disassembler is optional for an interpretive system, but |
|
highly recommended during its development and maintenance, because it is |
|
very useful for detecting bugs in the front end (and for distinguishing |
|
them from VM interpreter bugs). |
|
|
|
Vmgen supports VM code disassembling by generating |
|
@file{@var{file}-disasm.i}. This code has to be wrapped into a |
|
function, as is done in @file{vmgen-ex/disasm.c}. You can use this file |
|
almost verbatim. In addition to @samp{vm_@var{A}2@var{B}(a,b)}, |
|
@samp{vm_out}, @samp{printarg_@var{type}(@var{value})}, which are |
|
explained above, the following macros and variables are used in |
|
@file{@var{file}-disasm.i} (and you have to define them): |
|
|
|
@table @code |
|
|
|
@item ip |
|
This variable points to the opcode of the current VM instruction. |
|
|
|
@cindex @code{IP}, @code{IPTOS} in disassmbler |
|
@item IP IPTOS |
|
@samp{IPTOS} is the first argument of the current VM instruction, and |
|
@samp{IP} points to it; this is just as in the engine, but here |
|
@samp{ip} points to the opcode of the VM instruction (in contrast to the |
|
engine, where @samp{ip} points to the next cell, or even one further). |
|
|
|
@findex VM_IS_INST |
|
@item VM_IS_INST(Inst i, int n) |
|
Tests if the opcode @samp{i} is the same as the @samp{n}th entry in the |
|
VM instruction table. |
|
|
|
@end table |
|
|
|
|
|
@c -------------------------------------------------------------------- |
|
@node VM profiler, , VM disassembler, Using the generated code |
|
@section VM profiler |
|
@cindex VM profiler |
|
@cindex profiling for selecting superinstructions |
|
@cindex superinstructions and profiling |
|
@cindex @file{profile.c} |
|
@cindex @file{-profile.i} output file |
|
|
|
The VM profiler is designed for getting execution and occurence counts |
|
for VM instruction sequences, and these counts can then be used for |
|
selecting sequences as superinstructions. The VM profiler is probably |
|
not useful as profiling tool for the interpretive system. I.e., the VM |
|
profiler is useful for the developers, but not the users of the |
|
interpretive system. |
|
|
|
The output of the profiler is: for each basic block (executed at least |
|
once), it produces the dynamic execution count of that basic block and |
|
all its subsequences; e.g., |
|
|
|
@example |
|
9227465 lit storelocal |
|
9227465 storelocal branch |
|
9227465 lit storelocal branch |
|
@end example |
|
|
|
I.e., a basic block consisting of @samp{lit storelocal branch} is |
|
executed 9227465 times. |
|
|
|
@cindex @file{stat.awk} |
|
@cindex @file{seq2rule.awk} |
|
This output can be combined in various ways. E.g., |
|
@file{vmgen-ex/stat.awk} adds up the occurences of a given sequence wrt |
|
dynamic execution, static occurence, and per-program occurence. E.g., |
|
|
|
@example |
|
2 16 36910041 loadlocal lit |
|
@end example |
|
|
|
@noindent |
|
indicates that the sequence @samp{loadlocal lit} occurs in 2 programs, |
|
in 16 places, and has been executed 36910041 times. Now you can select |
|
superinstructions in any way you like (note that compile time and space |
|
typically limit the number of superinstructions to 100--1000). After |
|
you have done that, @file{vmgen/seq2rule.awk} turns lines of the form |
|
above into rules for inclusion in a Vmgen input file. Note that this |
|
script does not ensure that all prefixes are defined, so you have to do |
|
that in other ways. So, an overall script for turning profiles into |
|
superinstructions can look like this: |
|
|
|
@example |
|
awk -f stat.awk fib.prof test.prof| |
|
awk '$3>=10000'| #select sequences |
|
fgrep -v -f peephole-blacklist| #eliminate wrong instructions |
|
awk -f seq2rule.awk| #turn into superinstructions |
|
sort -k 3 >mini-super.vmg #sort sequences |
|
@end example |
|
|
|
Here the dynamic count is used for selecting sequences (preliminary |
|
results indicate that the static count gives better results, though); |
|
the third line eliminates sequences containing instructions that must not |
|
occur in a superinstruction, because they access a stack directly. The |
|
dynamic count selection ensures that all subsequences (including |
|
prefixes) of longer sequences occur (because subsequences have at least |
|
the same count as the longer sequences); the sort in the last line |
|
ensures that longer superinstructions occur after their prefixes. |
|
|
|
But before using this, you have to have the profiler. Vmgen supports its |
|
creation by generating @file{@var{file}-profile.i}; you also need the |
|
wrapper file @file{vmgen-ex/profile.c} that you can use almost verbatim. |
|
|
|
@cindex @code{SUPER_END} in profiling |
|
@cindex @code{BB_BOUNDARY} in profiling |
|
The profiler works by recording the targets of all VM control flow |
|
changes (through @code{SUPER_END} during execution, and through |
|
@code{BB_BOUNDARY} in the front end), and counting (through |
|
@code{SUPER_END}) how often they were targeted. After the program run, |
|
the numbers are corrected such that each VM basic block has the correct |
|
count (entering a block without executing a branch does not increase the |
|
count, and the correction fixes that), then the subsequences of all |
|
basic blocks are printed. To get all this, you just have to define |
|
@code{SUPER_END} (and @code{BB_BOUNDARY}) appropriately, and call |
|
@code{vm_print_profile(FILE *file)} when you want to output the profile |
|
on @code{file}. |
|
|
|
@cindex @code{VM_IS_INST} in profiling |
|
The @file{@var{file}-profile.i} is similar to the disassembler file, and |
|
it uses variables and functions defined in @file{vmgen-ex/profile.c}, |
|
plus @code{VM_IS_INST} already defined for the VM disassembler |
|
(@pxref{VM disassembler}). |
|
|
|
@c ********************************************************** |
|
@node Hints, The future, Using the generated code, Top |
|
@chapter Hints |
|
@cindex hints |
|
|
|
@menu |
|
* Floating point:: and stacks |
|
@end menu |
|
|
|
@c -------------------------------------------------------------------- |
|
@node Floating point, , Hints, Hints |
|
@section Floating point |
|
|
|
How should you deal with floating point values? Should you use the same |
|
stack as for integers/pointers, or a different one? This section |
|
discusses this issue with a view on execution speed. |
|
|
|
The simpler approach is to use a separate floating-point stack. This |
|
allows you to choose FP value size without considering the size of the |
|
integers/pointers, and you avoid a number of performance problems. The |
|
main downside is that this needs an FP stack pointer (and that may not |
|
fit in the register file on the 386 arhitecture, costing some |
|
performance, but comparatively little if you take the other option into |
|
account). If you use a separate FP stack (with stack pointer @code{fp}), |
|
using an fpTOS is helpful on most machines, but some spill the fpTOS |
|
register into memory, and fpTOS should not be used there. |
|
|
|
The other approach is to share one stack (pointed to by, say, @code{sp}) |
|
between integer/pointer and floating-point values. This is ok if you do |
|
not use @code{spTOS}. If you do use @code{spTOS}, the compiler has to |
|
decide whether to put that variable into an integer or a floating point |
|
register, and the other type of operation becomes quite expensive on |
|
most machines (because moving values between integer and FP registers is |
|
quite expensive). If a value of one type has to be synthesized out of |
|
two values of the other type (@code{double} types), things are even more |
|
interesting. |
|
|
|
One way around this problem would be to not use the @code{spTOS} |
|
supported by Vmgen, but to use explicit top-of-stack variables (one for |
|
integers, one for FP values), and having a kind of accumulator+stack |
|
architecture (e.g., Ocaml bytecode uses this approach); however, this is |
|
a major change, and it's ramifications are not completely clear. |
|
|
|
@c ********************************************************** |
|
@node The future, Changes, Hints, Top |
|
@chapter The future |
|
@cindex future ideas |
|
|
|
We have a number of ideas for future versions of Vmgen. However, there |
|
are so many possible things to do that we would like some feedback from |
|
you. What are you doing with Vmgen, what features are you missing, and |
|
why? |
|
|
|
One idea we are thinking about is to generate just one @file{.c} file |
|
instead of letting you copy and adapt all the wrapper files (you would |
|
still have to define stuff like the type-specific macros, and stack |
|
pointers etc. somewhere). The advantage would be that, if we change the |
|
wrapper files between versions, you would not need to integrate your |
|
changes and our changes to them; Vmgen would also be easier to use for |
|
beginners. The main disadvantage of that is that it would reduce the |
|
flexibility of Vmgen a little (well, those who like flexibility could |
|
still patch the resulting @file{.c} file, like they are now doing for |
|
the wrapper files). In any case, if you are doing things to the wrapper |
|
files that would cause problems in a generated-@file{.c}-file approach, |
|
please let us know. |
|
|
|
@c ********************************************************** |
|
@node Changes, Contact, The future, Top |
|
@chapter Changes |
|
@cindex Changes from old versions |
|
|
|
User-visible changes between 0.5.9-20020822 and 0.5.9-20020901: |
|
|
|
The store optimization is now disabled by default, but can be enabled by |
|
the user (@pxref{Store Optimization}). Documentation for this |
|
optimization is also new. |
|
|
|
User-visible changes between 0.5.9-20010501 and 0.5.9-20020822: |
|
|
|
There is now a manual (in info, HTML, Postscript, or plain text format). |
|
|
|
There is the vmgen-ex2 variant of the vmgen-ex example; the new |
|
variant uses a union type instead of lots of casting. |
|
|
|
Both variants of the example can now be compiled with an ANSI C compiler |
|
(using switch dispatch and losing quite a bit of performance); tested |
|
with @command{lcc}. |
|
|
|
Users of the gforth-0.5.9-20010501 version of Vmgen need to change |
|
several things in their source code to use the current version. I |
|
recommend keeping the gforth-0.5.9-20010501 version until you have |
|
completed the change (note that you can have several versions of Gforth |
|
installed at the same time). I hope to avoid such incompatible changes |
|
in the future. |
|
|
|
The required changes are: |
|
|
|
@table @code |
|
|
|
@cindex @code{TAIL;}, changes |
|
@item TAIL; |
|
has been renamed into @code{INST_TAIL;} (less chance of an accidental |
|
match). |
|
|
|
@cindex @code{vm_@var{A}2@var{B}}, changes |
|
@item vm_@var{A}2@var{B} |
|
now takes two arguments. |
|
|
|
@cindex @code{vm_two@var{A}2@var{B}}, changes |
|
@item vm_two@var{A}2@var{B}(b,a1,a2); |
|
changed to vm_two@var{A}2@var{B}(a1,a2,b) (note the absence of the @samp{;}). |
|
|
|
@end table |
|
|
|
Also some new macros have to be defined, e.g., @code{INST_ADDR}, and |
|
@code{LABEL}; some macros have to be defined in new contexts, e.g., |
|
@code{VM_IS_INST} is now also needed in the disassembler. |
|
|
|
@c ********************************************************* |
|
@node Contact, Copying This Manual, Changes, Top |
|
@chapter Contact |
|
|
|
To report a bug, use |
|
@url{https://savannah.gnu.org/bugs/?func=addbug&group_id=2672}. |
|
|
|
For discussion on Vmgen (e.g., how to use it), use the mailing list |
|
@email{bug-vmgen@@mail.freesoftware.fsf.org} (use |
|
@url{http://mail.gnu.org/mailman/listinfo/help-vmgen} to subscribe). |
|
|
|
You can find vmgen information at |
|
@url{http://www.complang.tuwien.ac.at/anton/vmgen/}. |
|
|
@section Stacks, types, and prefixes |
@c *********************************************************** |
|
@node Copying This Manual, Index, Contact, Top |
|
@appendix Copying This Manual |
|
|
|
@menu |
|
* GNU Free Documentation License:: License for copying this manual. |
|
@end menu |
|
|
|
@include fdl.texi |
|
|
Invocation |
|
|
|
Input Syntax |
@node Index, , Copying This Manual, Top |
|
@unnumbered Index |
|
|
Concepts: Front end, VM, Stacks, Types, input stream |
@printindex cp |
|
|
Contact |
@bye |