[llvm-bugs] [Bug 41429] New: vector select optimization failures?

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=41429

            Bug ID: 41429
           Summary: vector select optimization failures?
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: lebedev.ri at gmail.com
                CC: llvm-bugs at lists.llvm.org

>From IRC:
https://godbolt.org/z/NgGuw9

#include <immintrin.h>
#include <inttypes.h>
void test_restrict(__m256i * __restrict__ dest, const __m256i * __restrict__ a)
{
    (*dest)[2] = (*a)[2];
    (*dest)[3] = (*a)[3];
}

Currently results in:
define dso_local void @example2(<4 x i64>* noalias nocapture, <4 x i64>*
noalias nocapture readonly) local_unnamed_addr #0 {
  %3 = load <4 x i64>, <4 x i64>* %1, align 32, !tbaa !2
  %4 = load <4 x i64>, <4 x i64>* %0, align 32
  %5 = shufflevector <4 x i64> %4, <4 x i64> %3, <4 x i32> <i32 0, i32 1, i32
6, i32 undef>
  %6 = shufflevector <4 x i64> %5, <4 x i64> %3, <4 x i32> <i32 0, i32 1, i32
2, i32 7>
  store <4 x i64> %6, <4 x i64>* %0, align 32
  ret void
}

Since these are simple identity shuffles), can we combine this into a single
shuffle?
I'm guessing it is the `undef` that is the problem?

; Function Attrs: norecurse nounwind uwtable
define dso_local void @test_restrict(<4 x i64>* noalias nocapture, <4 x i64>*
noalias nocapture readonly) local_unnamed_addr #0 {
  %3 = load <4 x i64>, <4 x i64>* %1, align 32, !tbaa !2
  %4 = load <4 x i64>, <4 x i64>* %0, align 32
  %5 = shufflevector <4 x i64> %4, <4 x i64> %3, <4 x i32> <i32 0, i32 1, i32
6, i32 7>
  store <4 x i64> %5, <4 x i64>* %0, align 32
  ret void
}

https://godbolt.org/z/NgGuw9

The final x86 asm is identical.

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