[llvm-bugs] [Bug 43230] New: [X86] Incorrect shuffle of shift optimization on haswell

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=43230

            Bug ID: 43230
           Summary: [X86] Incorrect shuffle of shift optimization on
                    haswell
           Product: libraries
           Version: 9.0
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: nikita.ppv at gmail.com
                CC: craig.topper at gmail.com, llvm-bugs at lists.llvm.org,
                    llvm-dev at redking.me.uk, spatel+llvm at rotateright.com

define <16 x i16> @test(<16 x i16> %a, <16 x i16> %b) {
  %shr = lshr <16 x i16> %a, %b
  %shuf = shufflevector <16 x i16> zeroinitializer, <16 x i16> %shr, <16 x i32>
<i32 0, i32 1, i32 2, i32 3, i32 4, i32 5, i32 6, i32 7, i32 8, i32 9, i32 10,
i32 11, i32 12, i32 13, i32 30, i32 15>
  ret <16 x i16> %shuf
} 

With -mcpu=haswell results in:

        vpxor   %xmm2, %xmm2, %xmm2
        vpunpckhwd      %ymm2, %ymm1, %ymm1 # ymm1 =
ymm1[4],ymm2[4],ymm1[5],ymm2[5],ymm1[6],ymm2[6],ymm1[7],ymm2[7],ymm1[12],ymm2[12],ymm1[13],ymm2[13],ymm1[14],ymm2[14],ymm1[15],ymm2[15]
        vpunpckhwd      %ymm0, %ymm2, %ymm0 # ymm0 =
ymm2[4],ymm0[4],ymm2[5],ymm0[5],ymm2[6],ymm0[6],ymm2[7],ymm0[7],ymm2[12],ymm0[12],ymm2[13],ymm0[13],ymm2[14],ymm0[14],ymm2[15],ymm0[15]
        vpsrlvd %ymm1, %ymm0, %ymm0
        vpand   .LCPI0_0(%rip), %ymm0, %ymm0

While in LLVM 7 it was:

        vpxor   xmm2, xmm2, xmm2
        vpunpckhwd      ymm1, ymm1, ymm2 # ymm1 =
ymm1[4],ymm2[4],ymm1[5],ymm2[5],ymm1[6],ymm2[6],ymm1[7],ymm2[7],ymm1[12],ymm2[12],ymm1[13],ymm2[13],ymm1[14],ymm2[14],ymm1[15],ymm2[15]
        vpunpckhwd      ymm0, ymm2, ymm0 # ymm0 =
ymm2[4],ymm0[4],ymm2[5],ymm0[5],ymm2[6],ymm0[6],ymm2[7],ymm0[7],ymm2[12],ymm0[12],ymm2[13],ymm0[13],ymm2[14],ymm0[14],ymm2[15],ymm0[15]
        vpsrlvd ymm0, ymm0, ymm1
        vpand   ymm0, ymm0, ymmword ptr [rip + .LCPI0_0]

Godbolt: https://godbolt.org/z/4SyEhQ

I *think* this transformation is not correct, though maybe my vector foo is too
weak.

The debug log has:

With: t53: v32i8 = BUILD_VECTOR undef:i8, undef:i8, undef:i8, undef:i8,
undef:i8, undef:i8, undef:i8, undef:i8, undef:i8, undef:i8, undef:i8, undef:i8,
undef:i8, undef:i8, undef:i8, undef:i8, undef:i8, undef:i8, undef:i8, undef:i8,
undef:i8, undef:i8, undef:i8, undef:i8, undef:i8, undef:i8, undef:i8, undef:i8,
Constant:i8<26>, Constant:i8<27>, undef:i8, undef:i8


Combining: t50: v32i8 = X86ISD::PSHUFB t48, t53
Creating new node: t54: v8i32 = undef
Creating new node: t55: v16i16 = bitcast t23
Creating constant: t56: i8 = Constant<-28>
Creating new node: t57: v16i16 = X86ISD::PSHUFLW t55, Constant:i8<-28>
Creating new node: t58: v32i8 = bitcast t57
 ... into: t58: v32i8 = bitcast t57

Which looks like a non-identity pshufb is replaced with an identity pshuflw.

This happens via matchUnaryPermuteShuffle(), though I haven't looked further.

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