clang never finishes on one particular C++ file

[yurivict]

unity_1935_cxx.cxx.E.cxx.bz2

FreeBSD bug report.

[VoxSciurorum]

To reproduce feed the attached file to llc. It hangs in DAGCombiner::Run. If you run the LLVM 14

clang++ -fno-strict-aliasing -fno-omit-frame-pointer -march=native -O3 -fPIC -emit-llvm

on the original file you get an IR file that causes llc to hang. From there I narrowed down the crash by dumping after each pass. The file is from the last pass that didn't hang.

You need the clang++ from LLVM 14 because the builtins changed in LLVM 15. Both 14 and 15 llc hang.

Target is amd64.

bad-llc-ir.txt

[xgupta]

Not reproduce with the current trunk - https://godbolt.org/z/PzdPdvrcz..

[yurivict]

@xgupta
Are you trying on FreeBSD?

[xgupta]

No it is Linux system.

[yurivict]

C++ bugs like this likely depend on the C++ library that is used, which is different on FreeBSD.

[VoxSciurorum]

Not reproduce with the current trunk - https://godbolt.org/z/PzdPdvrcz..

Run llc on the IR file I attached. llc hangs at -O1, -O2, or with default optimization. llc completes in a second or two at -O0. Verified on FreeBSD/amd64, FreeBSD/arm64, and Linux/amd64. Apparently at all versions from 14 up to latest trunk.

I forgot to include a target triple in the IR file. It would have been generated on FreeBSD/amd64.

This is not a FreeBSD-specific bug. It is a codegen bug that happens to be triggered by libraries and headers on a FreeBSD system.

[VoxSciurorum]

It is worth mentioning that the IR file contains a basic block with about 43,000 instructions and 40,000 values. The "hang" may in fact be a superlinear run time. Whatever optimization it is attempting should be disabled when the block size is so large.