[llvm-bugs] [Bug 52057] New: Failure to optimize small vectors from using s

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=52057

            Bug ID: 52057
           Summary: Failure to optimize small vectors from using s
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: gabravier at gmail.com
                CC: llvm-bugs at lists.llvm.org

typedef char V __attribute__((vector_size(2)));

V foo(V v)
{
  v[(V){}[0]] <<= 1;
  return v;
}

Code such as this seems to be very badly optimized for all targets that have
SIMD.

With -O3, Clang outputs this:

.LCPI0_0:
        .byte   0                               # 0x0
        .byte   255                             # 0xff
        .byte   255                             # 0xff
        .byte   255                             # 0xff
        .byte   255                             # 0xff
        .byte   255                             # 0xff
        .byte   255                             # 0xff
        .byte   255                             # 0xff
        .byte   255                             # 0xff
        .byte   255                             # 0xff
        .byte   255                             # 0xff
        .byte   255                             # 0xff
        .byte   255                             # 0xff
        .byte   255                             # 0xff
        .byte   255                             # 0xff
        .byte   255                             # 0xff
foo(char __vector(2)):                            # @foo(char __vector(2))
        movd    xmm0, edi
        movdqa  xmmword ptr [rsp - 24], xmm0
        mov     al, byte ptr [rsp - 24]
        add     al, al
        movzx   eax, al
        movd    xmm1, eax
        pxor    xmm2, xmm2
        punpcklbw       xmm1, xmm2              # xmm1 =
xmm1[0],xmm2[0],xmm1[1],xmm2[1],xmm1[2],xmm2[2],xmm1[3],xmm2[3],xmm1[4],xmm2[4],xmm1[5],xmm2[5],xmm1[6],xmm2[6],xmm1[7],xmm2[7]
        pand    xmm0, xmmword ptr [rip + .LCPI0_0]
        por     xmm0, xmm1
        movd    eax, xmm0
        ret

GCC outputs this:

foo(char __vector(2)):
        movsx   edx, dil
        mov     eax, edi
        add     edx, edx
        mov     al, dl
        ret

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