version 1.28, 2005/01/26 21:24:15
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version 1.29, 2005/10/02 11:30:34
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Line 94 Input File Grammar
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Line 94 Input File Grammar
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Simple instructions |
Simple instructions |
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* Explicit stack access:: If the C code accesses a stack pointer |
* C Code Macros:: Macros recognized by Vmgen |
* C Code Macros:: Macros recognized by Vmgen |
* C Code restrictions:: Vmgen makes assumptions about C code |
* C Code restrictions:: Vmgen makes assumptions about C code |
* Stack growth direction:: is configurable per stack |
* Stack growth direction:: is configurable per stack |
Line 829 name. Stack prefixes are defined like t
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Line 830 name. Stack prefixes are defined like t
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@example |
@example |
\E inst-stream stack-prefix # |
\E inst-stream stack-prefix # |
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\E data-stack stack-prefix S: |
@end example |
@end example |
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This definition defines that the stack prefix @code{#} specifies the |
This definition defines that the stack prefix @code{#} specifies the |
Line 845 If there are multiple instruction stream
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Line 847 If there are multiple instruction stream
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first one (just as the intuition suggests). |
first one (just as the intuition suggests). |
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@menu |
@menu |
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* Explicit stack access:: If the C code accesses a stack pointer |
* C Code Macros:: Macros recognized by Vmgen |
* C Code Macros:: Macros recognized by Vmgen |
* C Code restrictions:: Vmgen makes assumptions about C code |
* C Code restrictions:: Vmgen makes assumptions about C code |
* Stack growth direction:: is configurable per stack |
* Stack growth direction:: is configurable per stack |
@end menu |
@end menu |
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@c -------------------------------------------------------------------- |
@c -------------------------------------------------------------------- |
@node C Code Macros, C Code restrictions, Simple instructions, Simple instructions |
@node Explicit stack access, C Code Macros, Simple instructions, Simple instructions |
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@subsection Explicit stack access |
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@cindex stack access, explicit |
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@cindex Stack pointer access |
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@cindex explicit stack access |
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Not all stack effects can be specified using the stack effect |
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specifications above. For VM instructions that have other stack |
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effects, you can specify them explicitly by accessing the stack |
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pointer in the C code; however, you have to notify Vmgen of such |
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explicit stack accesses, otherwise Vmgens optimizations could conflict |
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with your explicit stack accesses. |
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You notify Vmgen by putting @code{...} with the appropriate stack |
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prefix into the stack comment. Then the VM instruction will first |
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take the other stack items specified in the stack effect into C |
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variables, then make sure that all other stack items for that stack |
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are in memory, and that the stack pointer for the stack points to the |
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top-of-stack (by default, unless you change the stack access |
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transformation: @pxref{Stack growth direction}). |
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The general rule is: If you mention a stack pointer in the C code of a |
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VM instruction, you should put a @code{...} for that stack in the stack |
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effect. |
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Consider this example: |
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@example |
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return ( #iadjust S:... target afp i1 -- i2 ) |
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SET_IP(target); |
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sp = (Cell *)(((char *)sp)+iadjust); |
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fp = afp; |
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i2=i1; |
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@end example |
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First the variables @code{target afp i1} are popped off the stack, |
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then the stack pointer @code{sp} is set correctly for the new stack |
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depth, then the C code changes the stack depth and does other things, |
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and finally @code{i2} is pushed on the stack with the new depth. |
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The position of the @code{...} within the stack effect does not |
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matter. You can use several @code{...}s, for different stacks, and |
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also several for the same stack (that has no additional effect). If |
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you use @code{...} without a stack prefix, this specifies all the |
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stacks except the instruction stream. |
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You cannot use @code{...} for the instruction stream, but that is not |
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necessary: At the start of the C code, @code{IP} points to the start |
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of the next VM instruction (i.e., right beyond the end of the current |
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VM instruction), and you can change the instruction pointer with |
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@code{SET_IP} (@pxref{VM engine}). |
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@c -------------------------------------------------------------------- |
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@node C Code Macros, C Code restrictions, Explicit stack access, Simple instructions |
@subsection C Code Macros |
@subsection C Code Macros |
@cindex macros recognized by Vmgen |
@cindex macros recognized by Vmgen |
@cindex basic block, VM level |
@cindex basic block, VM level |