[llvm-bugs] [Bug 26376] New: SROA may create redundant PHI nodes

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=26376

            Bug ID: 26376
           Summary: SROA may create redundant PHI nodes
           Product: new-bugs
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: andrew.kaylor at intel.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

Created attachment 15755
  --> https://llvm.org/bugs/attachment.cgi?id=15755&action=edit
test case

I've come across a situation where SROA will create redundant PHI nodes while
promoting an alloca.  The old SSAUpdater (optional prior to r246028) was able
to detect the redundancy in this case and avoid it.  The PromoteRegToMem class
does not detect the redundancy.

I have seen a case (on 32-bit x86) where this led to an extra select and
resulted in a measurable performance degradation.

The attached test case demonstrates the problem.

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