[llvm-bugs] [Bug 38088] New: Compile time regression introduced in r322401

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=38088

            Bug ID: 38088
           Summary: Compile time regression introduced in r322401
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: alex at crichton.co
                CC: brzycki at gmail.com, llvm-bugs at lists.llvm.org

We're currently attempting to upgrade LLVM in the Rust project [1] but we're
unfortunately losing performance from this upgrade on a number of our
benchmarks [2]. I've been doing some bisection and it's somewhat difficult to
narrow down on any one regression because the regression to track down is
smaller and smaller.

In any case though I've tracked down one regression to r322401 or
https://reviews.llvm.org/D40146. The IR file [3] takes awhile to pass through
the `opt` tool, but before this commit it took 16.8s locally and afterwards it
takes 17.9. 

I haven't dove too much further into the patch, but let me know if I can help
out!

[1]: https://github.com/rust-lang/rust/pull/51966
[2]:
https://perf.rust-lang.org/compare.html?start=478226cc2ab768d45cc8284652c5997559b7b05f&end=cde23ec54231f5454f8c128383fe9962b9e598b0
[3]: https://gist.github.com/alexcrichton/fda0ea23be40b34f9927840179a82e93

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