[llvm-bugs] [Bug 38792] New: [InstCombine] Fails to combine three shufflevectors produced by LoopVectorizer

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=38792

            Bug ID: 38792
           Summary: [InstCombine]  Fails to combine three shufflevectors
                    produced by LoopVectorizer
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: paulsson at linux.vnet.ibm.com
                CC: llvm-bugs at lists.llvm.org

Created attachment 20815
  --> https://bugs.llvm.org/attachment.cgi?id=20815&action=edit
reduced testcase

The LoopVectorizer has interleaved loads and stores. Basically, the loaded
elements should pairwise be reversed like

[0 1 2 3]  -> [1 0 3 2]

The Vectorizer does not understand this but generates from two interleave
groups first a result for the load group, and then makes another shuffle for
the store group

 %tmp6 = load <4 x i64>, <4 x i64>* %tmp5, align 8
  %tmp7 = shufflevector <4 x i64> %tmp6, <4 x i64> undef, <2 x i32> <i32 0, i32
2>
  %tmp8 = shufflevector <4 x i64> %tmp6, <4 x i64> undef, <2 x i32> <i32 1, i32
3>
  %tmp9 = shufflevector <2 x i64> %tmp8, <2 x i64> %tmp7, <4 x i32> <i32 0, i32
1, i32 2, i32 3>
  %tmp10 = shufflevector <4 x i64> %tmp9, <4 x i64> undef, <4 x i32> <i32 0,
i32 2, i32 1, i32 3>
  store <4 x i64> %tmp10, <4 x i64>* undef, align 8

This results in [1 0 3 2], and I would have hoped that this would become a
single shufflevector after instcombine, but this does not happen.

There are comments in InstCombine that this is purposely done very
conservatively. It is however clear that this does not give good code on
SystemZ.

I wonder if anyone has any idea if InstCombiner should handle this case, or if
not, where should this be done. A custom DAGCombine by the target? 

bin/opt  ./tc_instcombine.ll -mtriple=systemz-unknown -mcpu=z13 -S -o
out.opt.ll -instcombine

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