Re: [PATCH] [x86] Avoid builtins for SSE/AVX2 immidiate logical shifts

[Jakub Jelinek <jakub@redhat.com>]

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