[llvm-bugs] [Bug 49128] New: new pass manager performance regression: loop unswitch for omnetpp

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=49128

            Bug ID: 49128
           Summary: new pass manager performance regression: loop unswitch
                    for omnetpp
           Product: new-bugs
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: Sanne.Wouda at arm.com
                CC: htmldeveloper at gmail.com, llvm-bugs at lists.llvm.org

We're seeing a regression on omnetpp (both spec 2006 and 2017, -8% and -7%
respectively) as a result of the new pass manager switch. This is AArch64 on a
Neoverse-N1, with -flto -mcpu=native -fomit-frame-pointer.

This seems to boil down to a recent patch to LoopUnswitch
(https://reviews.llvm.org/D93764) which implemented partial loop unswitching.
However, the new pass manager uses SimpleLoopUnswitch and does not benefit from
this improvement.

AFAIU, LoopUnswitch is incompatible with the new pass manager, so the
improvement to LoopUnswitch needs to be ported to SimpleLoopUnswitch.

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