[llvm-bugs] [Bug 52178] New: [VectorCombine] transform shuffle of binops with common operand

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=52178

            Bug ID: 52178
           Summary: [VectorCombine] transform shuffle of binops with
                    common operand
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: spatel+llvm at rotateright.com
                CC: llvm-bugs at lists.llvm.org

Adapted/reduced from the regression test in https://reviews.llvm.org/D111800
(that example could have further shuffle combining which would eliminate a
shuffle and almost definitely be a win):

define <3 x i3> @src(<3 x i3> %x, <3 x i3> %y, <3 x i3> %z) {
  %a1 = sub <3 x i3> %x, %y
  %a2 = sub <3 x i3> %x, %z
  %r = shufflevector <3 x i3> %a1, <3 x i3> %a2, <3 x i32> <i32 0, i32 5, i32
3>
  ret <3 x i3> %r
}

define <3 x i3> @tgt(<3 x i3> %x, <3 x i3> %y, <3 x i3> %z) {
  %s1 = shufflevector <3 x i3> %x, <3 x i3> %x, <3 x i32> <i32 0, i32 5, i32 3>
  %s2 = shufflevector <3 x i3> %y, <3 x i3> %z, <3 x i32> <i32 0, i32 5, i32 3>
  %r2 = sub <3 x i3> %s1, %s2
  ret <3 x i3> %r2
}

https://alive2.llvm.org/ce/z/d8zB4D

Note: the shuffle of '%x' can be reduced to a unary shuffle; I left the mask
as-is to show that we're re-using the incoming shuffle mask.

As long as the shuffle is no more expensive than the binop, this should be a
win and would potentially allow subsequent combines.

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