[llvm-bugs] [Bug 33438] New: Performance regression: SROA apepars to add unnecessary code to critical path function

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=33438

            Bug ID: 33438
           Summary: Performance regression: SROA apepars to add
                    unnecessary code to critical path function
           Product: new-bugs
           Version: 4.0
          Hardware: Other
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: eric.schweitz at pgroup.com
                CC: llvm-bugs at lists.llvm.org

Created attachment 18630
  --> https://bugs.llvm.org/attachment.cgi?id=18630&action=edit
SROA adding many phi-nodes

A performance regression in a large code has been tracked down to a difference
between the way LLVM 3.9 and LLVM 4.0 produce code for a particular function.
The reproducer is attached.

In LLVM 4.0, one can see that the function performs saves on all registers

# BB#0:                                 # %L.entry
        std 20, -96(1)                  # 8-byte Folded Spill
        std 21, -88(1)                  # 8-byte Folded Spill
        std 22, -80(1)                  # 8-byte Folded Spill
        std 23, -72(1)                  # 8-byte Folded Spill
        std 24, -64(1)                  # 8-byte Folded Spill
        std 25, -56(1)                  # 8-byte Folded Spill
        std 26, -48(1)                  # 8-byte Folded Spill
        std 27, -40(1)                  # 8-byte Folded Spill

lots of register-to-register moves

        mr 27, 0
        mr 30, 0
        mr 29, 0
        mr 28, 0
        mr 26, 0
        mr 8, 0
        mr 7, 0

etc.

The code produced by LLVM 3.9 does not contain these instructions.

It appears that SROA may play some role in this regression.

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