Messages in this thread | | | Date | Fri, 1 Sep 2023 10:24:42 +0700 | From | Ammar Faizi <> | Subject | Re: [RFC PATCH v1 3/5] tools/nolibc: x86-64: Use `rep cmpsb` for `memcmp()` |
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On Wed, Aug 30, 2023 at 11:26:57PM +0200, Willy Tarreau wrote: > Out of curiosity, given that you implemented the 3 other ones directly > in an asm statement, is there a particular reason this one mixes a bit > of C and asm ?
Because this one maybe unused. The other are explicitly exported.
> It would probably be something around this, in the same vein: > > memcmp: > xchg %esi,%eax // source1 > mov %rdx,%rcx // count > rep cmpsb // source2 in rdi; sets ZF on equal, CF if src1<src2 > seta %al // 0 if src2 <= src1, 1 if src2 > src1 > sbb $0, %al // 0 if src2 == src1, -1 if src2 < src1, 1 if src2 > src1 > movsx %al, %eax // sign extend to %eax > ret > > Note that the output logic could have to be revisited, I'm not certain but > at first glance it looks valid.
After thinking about this more, I think I'll drop the memcmp() patch because it will prevent optimization when comparing a small value.
For example, without __asm__:
memcmp(var, "abcd", 4);
may compile to:
cmpl $0x64636261, %reg ...something...
But with __asm__, the compiler can't do that. Thus, it's not worth optimizing the memcmp() in this case.
-- Ammar Faizi
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