[llvm-bugs] [Bug 46261] New: [SLP] SLP vectorizer change causes performance loss

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=46261

            Bug ID: 46261
           Summary: [SLP] SLP vectorizer change causes performance loss
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: normal
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: Wolfgang_Pieb at playstation.sony.com
                CC: llvm-bugs at lists.llvm.org

Created attachment 23593
  --> https://bugs.llvm.org/attachment.cgi?id=23593&action=edit
Test case

After the commit
https://github.com/llvm/llvm-project/commit/6a18a9548761b266b28a49f705a568677c24b59b
the attached test experiences a roughly 50% slowdown.

Compile with clang -O2 -march=btver2.

The IR dump after SLP shows that, before the commit, a substantial number of
shuffle instructions are present, e.g.

%shuffle = shufflevector <2 x float> %14, <2 x float> undef, <4 x i32> <i32 0,
i32 0, i32 0, i32 1> 

whereas after the commit, the IR dump does not show any.

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