Messages in this thread | | | Date | Fri, 25 Apr 2008 11:47:42 -0700 | From | "H. Peter Anvin" <> | Subject | Re: [PATCH 1/1] x86: fix text_poke |
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Mathieu Desnoyers wrote: > > Thinking about it, there could be a way to insure limited ZF and %al > liveliness: adding an epilogue to the expected instruction sequence > formed by an asm statement which clobbers the flags (flags are clobbered > in any asm statement on x86) and clobbers %al. > > From that point, we just have to find a specific signature that gcc > could not imitate to put in this asm statement, so we can detect if > other instructions have been placed in the middle of our sequence by > gcc. Actually, I think the best thing to do with this asm statement is > to put the instruction pointer in a special section, so we know that > this code location marks the end of ZF and %al liveliness. There would > be therefore no added code, just asm constraints. > > This epilogue should then be used on both branches of the condition, > like this : > > if (unlikely(imv_cond(var))) { > imv_cond_end(); > ... > } else { > imv_cond_end(); > ... > } >
[...]
> > Does it make sense ? >
I don't think so. You're making way too many assumptions about the code generated by gcc.
This kind of stuff absolutely can be done, *BUT* it requires the cooperation of the compiler. The right way to do this is to negotiate a set of appropriate builtins with the gcc people, and use them. This means this optimization will only work when compiled with the new gcc, so there is a substantial lag, but it's the only sane way to do this kind of stuff.
-hpa
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