[llvm-bugs] [Bug 30371] New: [x86, AVX] poor codegen for shufflevector with constants

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https://llvm.org/bugs/show_bug.cgi?id=30371

            Bug ID: 30371
           Summary: [x86, AVX] poor codegen for shufflevector with
                    constants
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: spatel+llvm at rotateright.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

Testing with llc built from r281342:

define <4 x float> @does_it_blend(float %f) {
  %a0 = insertelement <4 x float> undef, float %f, i32 0
  %ret = shufflevector <4 x float> %a0, <4 x float> <float undef, float 4.0,
float 5.0, float 3.0>, <4 x i32> <i32 4, i32 5, i32 6, i32 7>
  ret <4 x float> %ret
}

This should be a blendps or insertps with a vector constant, but we scalarized
the constant loads, so what could have been 2 ops (load+blend) became 6 ops:

$ ./llc blender.ll -o - -mattr=avx
    vinsertps    $16, LCPI0_0(%rip), %xmm0, %xmm0 ## xmm0 =
xmm0[0],mem[0],xmm0[2,3]
    vinsertps    $32, LCPI0_1(%rip), %xmm0, %xmm0 ## xmm0 =
xmm0[0,1],mem[0],xmm0[3]
    vinsertps    $48, LCPI0_2(%rip), %xmm0, %xmm0 ## xmm0 = xmm0[0,1,2],mem[0]
    retq

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