Ping?
On Mon, Oct 20, 2014 at 7:18 PM, NightStrike <nightstrike@xxxxxxxxx> wrote:
> I have been studying the asm generated by a typical clamping function,
> and I am confused about the results. This is done on an Opteron 6k
> series compiled with -fverbose-asm, -O3 and -march=native.
>
> float clamp(float const x, float const min, float const max) {
> #if defined (BRANCH)
> if ( x > max )
> return max;
> else if ( x < min )
> return min;
> else
> return x;
> #elif defined (BRANCH2)
> return x > max ? max : ( x < min ? min : x );
> #elif defined (CALL)
> return __builtin_fminf(__builtin_fmaxf(x, min), max);
> #else
> float const t = x < min ? min : x;
> return t> max ? max : t;
> #endif
> }
>
> -DBRANCH / -DBRANCH2:
> The first two approaches are obviously identical, and produce:
>
> clamp:
> .LFB0:
> .cfi_startproc
> vucomiss %xmm2, %xmm0 # max, x
> ja .L3 #,
> vmaxss %xmm0, %xmm1, %xmm0 # x, min, D.2214
> ret
> .p2align 4,,7
> .p2align 3
> .L3:
> vmovaps %xmm2, %xmm0 # max, D.2214
> ret
> .cfi_endproc
>
>
> -DCALL:
> This one I figured would be great, given the use of builtins:
>
> clamp:
> .LFB0:
> .cfi_startproc
> subq $24, %rsp #,
> .cfi_def_cfa_offset 32
> vmovss %xmm2, 12(%rsp) # max, %sfp
> call fmaxf #
> vmovss 12(%rsp), %xmm2 # %sfp, max
> addq $24, %rsp #,
> .cfi_def_cfa_offset 8
> vmovaps %xmm2, %xmm1 # max,
> jmp fminf #
> .cfi_endproc
>
>
> But then we have what appears to be the best of them all.... just a
> couple instructions, no branches, no calls, nothing:
>
> .LFB0:
> .cfi_startproc
> vmaxss %xmm0, %xmm1, %xmm0 # x, min, D.2219
> vminss %xmm0, %xmm2, %xmm0 # D.2219, max, D.2219
> ret
> .cfi_endproc
>
>
>
> So I'm curious.... why is the last approach optimized better than the
> naive approach of some nested if statements?
