[llvm-bugs] [Bug 31551] New: Lower shuffles with equivalent truncates as good as

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https://llvm.org/bugs/show_bug.cgi?id=31551

            Bug ID: 31551
           Summary: Lower shuffles with equivalent truncates as good as
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: normal
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: zvi.rackover at intel.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

Spinning off from bug 31443:

We should ensure that each 'shufflevector' that has an equivalent 'trunc' form
will be lowered optimally as the trunc.

A random example of two equivalent functions:

define void @shuffle_v16i16_to_v4i16(<16 x i16>* %L, <4 x i16>* %S) nounwind {
  %vec = load <16 x i16>, <16 x i16>* %L
  %strided.vec = shufflevector <16 x i16> %vec, <16 x i16> undef, <4 x i32>
<i32 0, i32 4, i32 8, i32 12>
  store <4 x i16> %strided.vec, <4 x i16>* %S
  ret void
}

define void @trunc_v4i64_to_v4i16(<16 x i16>* %L, <4 x i16>* %S) nounwind {
  %vec = load <16 x i16>, <16 x i16>* %L
  %bc = bitcast <16 x i16> %vec to <4 x i64>
  %strided.vec = trunc <4 x i64> %bc to <4 x i16>
  store <4 x i16> %strided.vec, <4 x i16>* %S
  ret void
}

shuffle_v16i16_to_v4i16 is lowered to:
  vmovdqa (%rdi), %ymm0
  vextracti128 $1, %ymm0, %xmm1
  vpshufd {{.*#+}} xmm1 = xmm1[0,2,2,3]
  vpshuflw {{.*#+}} xmm1 = xmm1[0,2,2,3,4,5,6,7]
  vpshufd {{.*#+}} xmm0 = xmm0[0,2,2,3]
  vpshuflw {{.*#+}} xmm0 = xmm0[0,2,2,3,4,5,6,7]
  vpunpckldq {{.*#+}} xmm0 = xmm0[0],xmm1[0],xmm0[1],xmm1[1]
  vmovq %xmm0, (%rsi)

trunc_v4i64_to_v4i16 is lowered to (and can also be improved):
  vmovdqa (%rdi), %ymm0
  vpmovqd %zmm0, %ymm0
  vpshufb {{.*#+}} xmm0 = xmm0[0,1,4,5,8,9,12,13,8,9,12,13,12,13,14,15]
  vmovq %xmm0, (%rsi)

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