[Bug target/98934] New: Very poor code generation for SSE 8-bit vector right shift

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From: "gabravier at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/98934] New: Very poor code generation for SSE 8-bit vector right shift
Date: Tue, 02 Feb 2021 15:39:37 +0000	[thread overview]
Message-ID: <bug-98934-4@http.gcc.gnu.org/bugzilla/> (raw)

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98934

            Bug ID: 98934
           Summary: Very poor code generation for SSE 8-bit vector right
                    shift
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gabravier at gmail dot com
  Target Milestone: ---

typedef char __attribute__((vector_size(16))) v16i8;

v16i8 f(v16i8 x, v16i8 y)
{
    return x >> y;
}

With -O3, LLVM outputs this:

f(char __vector(16), char __vector(16)):
  punpckhbw xmm2, xmm0 # xmm2 =
xmm2[8],xmm0[8],xmm2[9],xmm0[9],xmm2[10],xmm0[10],xmm2[11],xmm0[11],xmm2[12],xmm0[12],xmm2[13],xmm0[13],xmm2[14],xmm0[14],xmm2[15],xmm0[15]
  psllw xmm1, 5
  punpckhbw xmm4, xmm1 # xmm4 =
xmm4[8],xmm1[8],xmm4[9],xmm1[9],xmm4[10],xmm1[10],xmm4[11],xmm1[11],xmm4[12],xmm1[12],xmm4[13],xmm1[13],xmm4[14],xmm1[14],xmm4[15],xmm1[15]
  pxor xmm3, xmm3
  pxor xmm5, xmm5
  pcmpgtw xmm5, xmm4
  movdqa xmm6, xmm5
  pandn xmm6, xmm2
  psraw xmm2, 4
  pand xmm2, xmm5
  por xmm2, xmm6
  paddw xmm4, xmm4
  pxor xmm5, xmm5
  pcmpgtw xmm5, xmm4
  movdqa xmm6, xmm5
  pandn xmm6, xmm2
  psraw xmm2, 2
  pand xmm2, xmm5
  por xmm2, xmm6
  paddw xmm4, xmm4
  pxor xmm5, xmm5
  pcmpgtw xmm5, xmm4
  movdqa xmm4, xmm5
  pandn xmm4, xmm2
  psraw xmm2, 1
  pand xmm2, xmm5
  por xmm2, xmm4
  psrlw xmm2, 8
  punpcklbw xmm0, xmm0 # xmm0 = xmm0[0,0,1,1,2,2,3,3,4,4,5,5,6,6,7,7]
  punpcklbw xmm1, xmm1 # xmm1 = xmm1[0,0,1,1,2,2,3,3,4,4,5,5,6,6,7,7]
  pxor xmm4, xmm4
  pcmpgtw xmm4, xmm1
  movdqa xmm5, xmm4
  pandn xmm5, xmm0
  psraw xmm0, 4
  pand xmm0, xmm4
  por xmm0, xmm5
  paddw xmm1, xmm1
  pxor xmm4, xmm4
  pcmpgtw xmm4, xmm1
  movdqa xmm5, xmm4
  pandn xmm5, xmm0
  psraw xmm0, 2
  pand xmm0, xmm4
  por xmm0, xmm5
  paddw xmm1, xmm1
  pcmpgtw xmm3, xmm1
  movdqa xmm1, xmm3
  pandn xmm1, xmm0
  psraw xmm0, 1
  pand xmm0, xmm3
  por xmm0, xmm1
  psrlw xmm0, 8
  packuswb xmm0, xmm2
  ret

GCC outputs this:

f(char __vector(16), char __vector(16)):
  push r15
  movd edx, xmm0
  movd ecx, xmm1
  push r14
  sar dl, cl
  push r13
  movzx edx, dl
  push r12
  push rbp
  push rbx
  sub rsp, 400
  movaps XMMWORD PTR [rsp+376], xmm0
  movzx ebx, BYTE PTR [rsp+377]
  movaps XMMWORD PTR [rsp+360], xmm1
  movzx ecx, BYTE PTR [rsp+361]
  movaps XMMWORD PTR [rsp+344], xmm0
  movzx ebp, BYTE PTR [rsp+346]
  sar bl, cl
  movaps XMMWORD PTR [rsp+328], xmm1
  movzx ecx, BYTE PTR [rsp+330]
  movaps XMMWORD PTR [rsp+312], xmm0
  movzx ebx, bl
  movzx r12d, BYTE PTR [rsp+315]
  sar bpl, cl
  movaps XMMWORD PTR [rsp+296], xmm1
  movzx ecx, BYTE PTR [rsp+299]
  movaps XMMWORD PTR [rsp+280], xmm0
  movzx ebp, bpl
  movzx r13d, BYTE PTR [rsp+284]
  sar r12b, cl
  movaps XMMWORD PTR [rsp+264], xmm1
  movzx ecx, BYTE PTR [rsp+268]
  movaps XMMWORD PTR [rsp+248], xmm0
  movzx r12d, r12b
  movzx r14d, BYTE PTR [rsp+253]
  sar r13b, cl
  movaps XMMWORD PTR [rsp+232], xmm1
  movzx ecx, BYTE PTR [rsp+237]
  movaps XMMWORD PTR [rsp+216], xmm0
  movzx r13d, r13b
  movzx r15d, BYTE PTR [rsp+222]
  sar r14b, cl
  movaps XMMWORD PTR [rsp+200], xmm1
  movzx ecx, BYTE PTR [rsp+206]
  movaps XMMWORD PTR [rsp+184], xmm0
  movzx r14d, r14b
  movaps XMMWORD PTR [rsp+168], xmm1
  sar r15b, cl
  movzx eax, BYTE PTR [rsp+191]
  movzx ecx, BYTE PTR [rsp+175]
  movaps XMMWORD PTR [rsp+152], xmm0
  movzx esi, BYTE PTR [rsp+160]
  movzx r15d, r15b
  sar al, cl
  movaps XMMWORD PTR [rsp+136], xmm1
  movzx ecx, BYTE PTR [rsp+144]
  movaps XMMWORD PTR [rsp+120], xmm0
  movzx edi, BYTE PTR [rsp+129]
  sar sil, cl
  movaps XMMWORD PTR [rsp+104], xmm1
  movzx ecx, BYTE PTR [rsp+113]
  movaps XMMWORD PTR [rsp+88], xmm0
  sar dil, cl
  mov BYTE PTR [rsp-89], sil
  movaps XMMWORD PTR [rsp+72], xmm1
  movzx esi, dil
  movzx ecx, BYTE PTR [rsp+82]
  movzx edi, BYTE PTR [rsp+98]
  movaps XMMWORD PTR [rsp+56], xmm0
  movzx r8d, BYTE PTR [rsp+67]
  sar dil, cl
  movaps XMMWORD PTR [rsp+40], xmm1
  movzx ecx, BYTE PTR [rsp+51]
  movaps XMMWORD PTR [rsp+24], xmm0
  movzx r9d, BYTE PTR [rsp+36]
  movzx edi, dil
  sar r8b, cl
  movaps XMMWORD PTR [rsp+8], xmm1
  movzx ecx, BYTE PTR [rsp+20]
  movaps XMMWORD PTR [rsp-8], xmm0
  movzx r10d, BYTE PTR [rsp+5]
  movzx r8d, r8b
  sar r9b, cl
  movaps XMMWORD PTR [rsp-24], xmm1
  movzx ecx, BYTE PTR [rsp-11]
  movaps XMMWORD PTR [rsp-40], xmm0
  movzx r11d, BYTE PTR [rsp-26]
  movzx r9d, r9b
  sar r10b, cl
  movaps XMMWORD PTR [rsp-56], xmm1
  movzx ecx, BYTE PTR [rsp-42]
  mov BYTE PTR [rsp-120], al
  movzx r10d, r10b
  movaps XMMWORD PTR [rsp-72], xmm0
  sar r11b, cl
  movzx eax, BYTE PTR [rsp-57]
  movaps XMMWORD PTR [rsp-88], xmm1
  movzx ecx, BYTE PTR [rsp-73]
  movzx r11d, r11b
  sar al, cl
  movzx ecx, al
  movzx eax, BYTE PTR [rsp-120]
  sal rcx, 8
  sal rax, 8
  or rcx, r11
  or rax, r15
  sal rax, 8
  or rax, r14
  sal rax, 8
  or rax, r13
  sal rax, 8
  or rax, r12
  sal rax, 8
  or rax, rbp
  sal rax, 8
  or rax, rbx
  movzx ebx, BYTE PTR [rsp-89]
  sal rax, 8
  sal rcx, 8
  or rcx, r10
  or rax, rdx
  sal rcx, 8
  mov QWORD PTR [rsp-120], rax
  or rcx, r9
  sal rcx, 8
  or rcx, r8
  sal rcx, 8
  or rcx, rdi
  sal rcx, 8
  or rcx, rsi
  sal rcx, 8
  or rcx, rbx
  mov QWORD PTR [rsp-112], rcx
  movdqa xmm0, XMMWORD PTR [rsp-120]
  add rsp, 400
  pop rbx
  pop rbp
  pop r12
  pop r13
  pop r14
  pop r15
  ret

I have not studied the generated code much, but I can tell at a glance that
GCC's code spills to memory and does the entire operation with normal
registers, whereas LLVM manages to keep it all in xmm registers, no spills to
memory and 3 times less assembly code.

next             reply	other threads:[~2021-02-02 15:39 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-02-02 15:39 gabravier at gmail dot com [this message]
2021-02-02 16:05 ` [Bug target/98934] " rguenth at gcc dot gnu.org
2021-08-25  5:26 ` pinskia at gcc dot gnu.org
2021-08-25  5:29 ` pinskia at gcc dot gnu.org
2021-08-25  7:35 ` crazylht at gmail dot com

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