[llvm-bugs] [Bug 33900] New: After r308322, clang hangs on compiling lib/IR/Function.cpp from llvm 3.5.2

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=33900

            Bug ID: 33900
           Summary: After r308322, clang hangs on compiling
                    lib/IR/Function.cpp from llvm 3.5.2
           Product: new-bugs
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: dimitry at andric.com
                CC: llvm-bugs at lists.llvm.org

Created attachment 18836
  --> https://bugs.llvm.org/attachment.cgi?id=18836&action=edit
Preprocessed Function.cpp and reproduction script

I recently imported llvm/clang trunk r308421 into FreeBSD 12-CURRENT, and
afterwards, I got a report that building the port for llvm 3.5.2 was hanging:
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=219139#c13 (it gets
automatically killed after 7200 seconds, as a "runaway build").

It turned out it was hanging on compiling lib/IR/Function.cpp, and bisecting
shows this started at https://reviews.llvm.org/rL308322.  Since this commit
seems to only change two defaults (MaxLoadsPerMemcmp and
MaxLoadsPerMemcmpOptSize), I suspect the hang exposes another, deeper lying
issue.

I've attached a tarball with the fully preprocessed Function.cpp, and a shell
script for reproduction.  In the mean time, I'm trying to reduce the test case,
but it is tricky to do with a hang. :)  Normally, the .ii file should compile
within ~20 seconds, depending on the speed of the machine.

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