[llvm-bugs] [Bug 29124] New: [InstCombine] reduce insertelements of constants to shufflevector

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=29124

            Bug ID: 29124
           Summary: [InstCombine] reduce insertelements of constants to
                    shufflevector
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: spatel+llvm at rotateright.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

If we're inserting more than one constant into a vector:

define <4 x float> @foo(<4 x float> %x) {
  %ins1 = insertelement <4 x float> %x, float 1.0, i32 1
  %ins2 = insertelement <4 x float> %ins1, float 2.0, i32 2
  ret <4 x float> %ins2
}

InstCombine could reduce that to a shufflevector:

define <4 x float> @goo(<4 x float> %x) {
  %shuf = shufflevector <4 x float> %x, <4 x float> <float undef, float 1.0,
float 2.0, float undef>, <4 x i32><i32 0, i32 5, i32 6, i32 3>
  ret <4 x float> %shuf
}

This can improve codegen (possibly a separate bug that the backend doesn't do
this too?):

$ ./llc -o - min.ll -mattr=avx
LCPI0_0:
    .long    1065353216              ## float 1
LCPI0_1:
    .long    1073741824              ## float 2
## BB#0:
    vinsertps    $16, LCPI0_0(%rip), %xmm0, %xmm0 
    vinsertps    $32, LCPI0_1(%rip), %xmm0, %xmm0 
    retq

LCPI1_0:
    .space    4
    .long    1065353216              ## float 1
    .long    1073741824              ## float 2
    .space    4
## BB#0:
    vblendps    $6, LCPI1_0(%rip), %xmm0, %xmm0 
    retq

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