[llvm-bugs] [Bug 40318] New: [x86] try harder to reduce shuffle with zero operand

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=40318

            Bug ID: 40318
           Summary: [x86] try harder to reduce shuffle with zero operand
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: spatel+llvm at rotateright.com
                CC: craig.topper at gmail.com, llvm-bugs at lists.llvm.org,
                    llvm-dev at redking.me.uk, spatel+llvm at rotateright.com

As noted here:
https://reviews.llvm.org/D56281
...we should be able to do better than this:

define <8 x i16> @shift_zext(<8 x i16> %x) {
  %r = shufflevector <8 x i16> %x, <8 x i16> zeroinitializer, <8 x i32> <i32 1,
i32 9, i32 10, i32 11, i32 undef, i32 undef, i32 undef, i32 undef>
  ret <8 x i16> %r
}

$ llc -o - shufsse.ll 
        pxor    %xmm1, %xmm1
        punpcklwd       %xmm0, %xmm1    ## xmm1 =
xmm1[0],xmm0[0],xmm1[1],xmm0[1],xmm1[2],xmm0[2],xmm1[3],xmm0[3]
        pshufhw $232, %xmm1, %xmm0      ## xmm0 = xmm1[0,1,2,3,4,6,6,7]
        pshufd  $230, %xmm0, %xmm0      ## xmm0 = xmm0[2,1,2,3]
        pshuflw $75, %xmm0, %xmm0       ## xmm0 = xmm0[3,2,0,1,4,5,6,7]
        retq


Or an even worse example:

define <8 x i16> @shift_zext_worse
(<8 x i16> %x) {
  %r = shufflevector <8 x i16> %x, <8 x i16> zeroinitializer, <8 x i32> <i32 2,
i32 9, i32 10, i32 11, i32 undef, i32 undef, i32 undef, i32 undef>
  ret <8 x i16> %r
}

$ llc -o - shufsse.ll
        pxor    %xmm1, %xmm1
        punpcklwd       %xmm0, %xmm1    ## xmm1 =
xmm1[0],xmm0[0],xmm1[1],xmm0[1],xmm1[2],xmm0[2],xmm1[3],xmm0[3]
        pshufd  $39, %xmm1, %xmm0       ## xmm0 = xmm1[3,1,2,0]
        pshuflw $232, %xmm0, %xmm0      ## xmm0 = xmm0[0,2,2,3,4,5,6,7]
        pshufd  $232, %xmm0, %xmm0      ## xmm0 = xmm0[0,2,2,3]
        pshuflw $39, %xmm0, %xmm0       ## xmm0 = xmm0[3,1,2,0,4,5,6,7]
        retq

----------------------------------------------------------------------------

When shuffling with a zero operand, we should try harder to use logical shifts?
In all cases, 2 opposing shifts can put zero elements into all lanes.

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