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Date: Mon, 07 Nov 2022 06:26:57 +0000
From: i.nixman@autistici.org
To: Hongtao Liu <crazylht@gmail.com>, gcc-help@gcc.gnu.org
Subject: Re: simd, redundant pcmpeqb and pxor
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On 2022-11-07 03:32, Hongtao Liu wrote:
> On Sun, Nov 6, 2022 at 6:54 PM i.nixman--- via Gcc-help
> <gcc-help@gcc.gnu.org> wrote:
>> 
>> 
>> Hello,
>> 
>> look at this example(https://godbolt.org/z/TnGMsfMs6):
>> ```
>> auto foo(const char *p) {
>>      const auto substr = _mm_loadu_si128((const __m128i *)p);
>>      return _mm_cmplt_epi8(substr, _mm_set1_epi8('0'));
>> }
>> ```
>> and to the generated asm:
>> ```
>> 1: foo(char const*):
>> 2:    movdqu  xmm0, XMMWORD PTR [rdi]
>> 3:    pxor    xmm1, xmm1
>> 4:    pcmpgtb xmm0, XMMWORD PTR .LC0[rip]
>> 5:    pcmpeqb xmm0, xmm1
>> 6:    ret
>> ```
>> look at line 5.
>> is there any reason for `pcmpeqb` instruction?

hi,

> Looks like a mis optimization from
> 
> _4 = VIEW_CONVERT_EXPR<__v16qs>(_7);
> _3 = _4 <= { 47, 47, 47, 47, 47, 47, 47, 47, 47, 47, 47, 47, 47, 47, 
> 47, 47 };
> _5 = VIEW_CONVERT_EXPR<vector(16) signed char>(_3);  --- this?
> 
> Could you open a bugzilla for it
> https://gcc.gnu.org/bugzilla/

sure, but for which component?



>> 
>> just for info, clang's output(https://godbolt.org/z/MPnvEMdhr):
>> ```
>> 1: foo(char const*):
>> 2:    movdqu  xmm1, xmmword ptr [rdi]
>> 3:    movdqa  xmm0, xmmword ptr [rip + .LCPI0_0]
>> 4:    pcmpgtb xmm0, xmm1
>> 5:    ret
>> ```
>> 
>> 
>> best!