On Mon, Apr 24, 2017 at 10:02:40AM +0200, Allan Sandfeld Jensen wrote:
> > That said, both the options I've mentioned above provide the same
> > advantages and don't have the disadvantages of pessimizing normal code.
> >
> What pessimizing? This produce the same or better code for all legal
> arguments. The only difference besides better generated code is that it
> allows
No. Have you really tried that?
> the intrinsics to be used incorrectly with non-literal arguments because we
> lack the C-extension for constexp to prevent that.
Consider e.g. -O2 -mavx2 -mtune=intel:
#include <x86intrin.h>
__m256i
foo (__m256i x, int s)
{
return (__m256i)__builtin_ia32_psllwi256 ((__v16hi)x, s);
}
__m256i
bar (__m256i x, int s)
{
return ((s & 0xff) < 16) ? (__m256i)((__v16hi)x << (s & 0xff)) :
_mm256_setzero_si256 ();
}
The first one generates
movl %edi, %edi
vmovq %rdi, %xmm1
vpsllw %xmm1, %ymm0, %ymm0
ret
(because that is actually what the instruction does), the second one
movzbl %dil, %edi
cmpl $15, %edi
jg .L5
vmovq %rdi, %xmm1
vpsllw %xmm1, %ymm0, %ymm0
ret
.p2align 4,,7
.p2align 3
.L5:
vpxor %xmm0, %xmm0, %xmm0
ret
Jakub
