[llvm-bugs] [Bug 40135] New: [x86] keep math/logic on vector unit to avoid insert/extract

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=40135

            Bug ID: 40135
           Summary: [x86] keep math/logic on vector unit to avoid
                    insert/extract
           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

We have a potential regression with shift lowering via rotates while trying to
do scalarization in:
https://reviews.llvm.org/D55722

I don't think we're allowed to do anything with this in IR even if the pattern
was appearing there:

define <4 x i32> @shiftamount_math(<4 x i32> %x, <4 x i32> %y) {
  %e = extractelement <4 x i32> %x, i32 0
  %s = sub i32 32, %e
  %i = insertelement <4 x i32> undef, i32 %s, i32 0
  %splat = shufflevector <4 x i32> %i, <4 x i32> undef, <4 x i32>
zeroinitializer
  %shift = shl <4 x i32> %y, %splat
  ret <4 x i32> %shift
}

$ llc -o - vecshift.ll
...
        movd    %xmm0, %eax
        movl    $32, %ecx
        subl    %eax, %ecx
        movd    %ecx, %xmm0
        pslld   %xmm0, %xmm1
        movdqa  %xmm1, %xmm0
        retq


We would probably be better off loading the '32' as a splat constant and doing
the 'sub' as a vector op:
        movdqa  LCPI1_0(%rip), %xmm2    ## xmm2 = [32,32,32,32]
        psubd   %xmm0, %xmm2
        pxor    %xmm0, %xmm0
        movss   %xmm2, %xmm0            ## xmm0 = xmm2[0],xmm0[1,2,3]
        pslld   %xmm0, %xmm1
        movdqa  %xmm1, %xmm0

It's ugly because of the x86
vector-shift-by-variable-but-not-really-a-vector-shift instruction that takes a
64-bit shift amount, but that beats transferring to scalar and back?

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