Messages in this thread | | | Date | Fri, 1 Sep 2023 05:35:08 +0200 | From | Willy Tarreau <> | Subject | Re: [RFC PATCH v1 3/5] tools/nolibc: x86-64: Use `rep cmpsb` for `memcmp()` |
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On Fri, Sep 01, 2023 at 10:24:42AM +0700, Ammar Faizi wrote: > 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.
Makes sense, indeed.
> > 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.
Ah you're totally right!
Willy
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