[llvm-bugs] [Bug 33276] New: [X86][SSE] Perform unsigned compare as signed when signbits are zero

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=33276

            Bug ID: 33276
           Summary: [X86][SSE] Perform unsigned compare as signed when
                    signbits are zero
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: llvm-dev at redking.me.uk
                CC: llvm-bugs at lists.llvm.org, spatel+llvm at rotateright.com

SSE doesn't have an unsigned integer compare, so we have to flip the sign bits
and use the signed compare instructions. But if we know that the sign bits of
both values are zero then we should be able to avoid the flip:

llc -mtriple=x86_64-unknown -mcpu=btver2

define <4 x i32> @cmp_ugt_4i32(<4 x i32> %a0, <4 x i32> %a1) {
  %1 = lshr <4 x i32> %a0, <i32 1, i32 1, i32 1, i32 1>
  %2 = lshr <4 x i32> %a1, <i32 1, i32 1, i32 1, i32 1>
  %3 = icmp ugt <4 x i32> %1, %2
  %4 = sext <4 x i1> %3 to <4 x i32>
  ret <4 x i32> %4
}

cmp_ugt:
        vmovdqa .LCPI0_0(%rip), %xmm2   # xmm2 =
[2147483648,2147483648,2147483648,2147483648]
        vpsrld  $1, %xmm0, %xmm0
        vpsrld  $1, %xmm1, %xmm1
        vpxor   %xmm2, %xmm1, %xmm1
        vpxor   %xmm2, %xmm0, %xmm0
        vpcmpgtd        %xmm1, %xmm0, %xmm0
        retq

Ideally:

cmp_ugt:
        vpsrld  $1, %xmm0, %xmm0
        vpsrld  $1, %xmm1, %xmm1
        vpcmpgtd        %xmm1, %xmm0, %xmm0
        retq

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