[llvm-bugs] [Bug 51857] New: Interpreter project is up to 15% slower with clang 13

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=51857

            Bug ID: 51857
           Summary: Interpreter project is up to 15% slower with clang 13
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: chfast at gmail.com
                CC: llvm-bugs at lists.llvm.org

I maintain a relatively simple interpreter project (single main function with
big switch statement).
I tested clang 13 RC and it gives 0-15% slowdown depending on the workload. In
none of the cases the clang 13 is faster than clang 12.

I tried to look at generated assembly and LLVM IR but I cannot stop anything
interesting there. Maybe just that clang 13 produces smaller code probably due
to better register allocation / spilling.

Also perf tool does not show anything specific.

I'm happy to provide additional data if you can tell we where to start and what
would be useful.

I also started bisecting LLVM from 12.x to 13.x. So far I have found single
commit that decreases performance by ~6%:


[LVI] Don't bail on overdefined value in select
https://github.com/llvm/llvm-project/commit/72e0846ef87d0d3b5960238bc47fc0cc6f04d848
https://reviews.llvm.org/rG72e0846ef87d0d3b5960238bc47fc0cc6f04d848

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