[llvm-bugs] [Bug 36417] New: LLVM 6.0 compile time regression with large amounts of debuginfo

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=36417

            Bug ID: 36417
           Summary: LLVM 6.0 compile time regression with large amounts of
                    debuginfo
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: DebugInfo
          Assignee: unassignedbugs at nondot.org
          Reporter: alex at crichton.co
                CC: aprantl at apple.com, karl-johan.karlsson at ericsson.com,
                    llvm-bugs at lists.llvm.org, rnk at google.com

Created attachment 19883
  --> https://bugs.llvm.org/attachment.cgi?id=19883&action=edit
slow bytecode

We've recently upgraded to LLVM 6.0 (or at least what's close to the release_60
branch of LLVM) in Rust, but unfortunately we're seeing a regression in compile
time now!

First reported to us [1] it looks like the compile time to emit debug
information has increased dramatically between LLVM 4.0 and LLVM 6.0. Doing
some testing locally I've managed to extract a test case for LLVM I think. The
attached IR completes codegen in ~2s on LLVM 4.0 but takes minutes on LLVM 6.0

I ran a bisection based on the results of the command:

    llc test.bc -filetype=obj -O2 -relocation-model=pic

and this bisection pointed at https://reviews.llvm.org/D38229. I've cc'd those
involved with that patch here, would y'all mind helping take a look into this
regression? Also this may also just be rustc not producing debug information
that LLVM is expecting, so if something's off about that please just let us
know!

[1]: https://github.com/rust-lang/rust/issues/48226

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