[gforth] / gforth / doc / vmgen.texi  

gforth: gforth/doc/vmgen.texi

Diff for /gforth/doc/vmgen.texi between version 1.17 and 1.18

version 1.17, Thu Aug 22 20:07:33 2002 UTC version 1.18, Wed Aug 28 21:46:59 2002 UTC
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 * Input File Grammar::  * Input File Grammar::
 * Simple instructions::  * Simple instructions::
 * Superinstructions::  * Superinstructions::
   * Store Optimization::
 * Register Machines::           How to define register VM instructions  * Register Machines::           How to define register VM instructions
   
 Input File Grammar  Input File Grammar
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 * Input File Grammar::  * Input File Grammar::
 * Simple instructions::  * Simple instructions::
 * Superinstructions::  * Superinstructions::
   * Store Optimization::
 * Register Machines::           How to define register VM instructions  * Register Machines::           How to define register VM instructions
 @end menu  @end menu
   
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 Forth you need for using Vmgen:  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' ('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
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 @findex single  @findex single
 @findex double  @findex double
 @findex stack-prefix  @findex stack-prefix
   @findex store-optimization
 @example  @example
 stack        ( "name" "pointer" "type" -- )  stack        ( "name" "pointer" "type" -- )
              ( name execution: -- stack )               ( name execution: -- stack )
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 single       ( -- item-size )  single       ( -- item-size )
 double       ( -- item-size )  double       ( -- item-size )
 stack-prefix ( stack "prefix" -- )  stack-prefix ( stack "prefix" -- )
   store-optimization ( -- addr )
 @end example  @end example
   
 An @var{item-size} takes three cells on the stack.  An @var{item-size} takes three cells on the stack.
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 @c --------------------------------------------------------------------  @c --------------------------------------------------------------------
 @node Superinstructions, Register Machines, Simple instructions, Input File Format  @node Superinstructions, Store Optimization, Simple instructions, Input File Format
 @section Superinstructions  @section Superinstructions
 @cindex superinstructions, defining  @cindex superinstructions, defining
 @cindex defining superinstructions  @cindex defining superinstructions
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 interpreter.  interpreter.
   
 @c -------------------------------------------------------------------  @c -------------------------------------------------------------------
 @node Register Machines,  , Superinstructions, Input File Format  @node  Store Optimization, Register Machines, Superinstructions, Input File Format
   @section Store Optimization
   @cindex store optimization
   @cindex optimization, stack stores
   @cindex stack stores, optimization
   @cindex eliminating stack stores
   
   This minor optimization (0.6\%--0.8\% reduction in executed instructions
   for Gforth) puts additional requirements on the instruction descriptions
   and is therefore disabled by default.
   
   What does it do?  Consider an instruction like
   
   @example
   dup ( n -- n n )
   @end example
   
   For simplicity, also assume that we are not caching the top-of-stack in
   a register.  Now, the C code for dup first loads @code{n} from the
   stack, and then stores it twice to the stack, one time to the address
   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
   in the C code you supply.  I.e., the following code could fail if you
   turn on the store optimization:
   
   @example
   add1 ( n -- n )
   n++;
   @end example
   
   Instead, you have to use different names, i.e.:
   
   @example
   add1 ( n1 -- n1 )
   n2=n1+1;
   @end example
   
   To turn on the store optimization, write
   
   @example
   \E store-optimization on
   @end example
   
   at the start of the file.  You can turn this optimization on or off
   between any two VM instruction descriptions.  For turning it off again,
   you can use
   
   @example
   \E store-optimization off
   @end example
   
   @c -------------------------------------------------------------------
   @node Register Machines,  , Store Optimization, Input File Format
 @section Register Machines  @section Register Machines
 @cindex Register VM  @cindex Register VM
 @cindex Superinstructions for register VMs  @cindex Superinstructions for register VMs

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