LLVM does not support PGO on Windows with -Cpanic=unwind

[michaelwoerister]

LLVM's "IR-level" instrumentation, which is used by rustc to generate PGO instrumented binaries, does not yet work with exception handling on Windows MSVC. The problem has been reported to LLVM for C++ here: https://bugs.llvm.org/show_bug.cgi?id=41279

This also affects Rust programs built with -Cpanic=unwind for Windows MSVC. As long as LLVM does not support exception handling there, it is a known limitation that PGO can only be used with -Cpanic=abort on this platform.

[michaelwoerister]

PGO does not yet work on windows-gnu for other reasons (see #61002). Once it does, we should check if this problem is specific to MSVC.

[mati865]

I have working x86_64 windows-gnu (64 bit builds use SEH) build with PGO I'll open PR once #60981 is merged.
I can do testing in meantime.

FWIW simple hello world example works fine after going through PGO.

[michaelwoerister]

@mati865 if you test, be sure to compile your LLVM with assertions: https://github.com/rust-lang/rust/pull/61005/files#diff-1c226b200b9385d77428f0b4418366e4R1277

Thanks for looking into it!

[mati865]

I'll recompile it with assertions today later. Should the test case be using any specific code like panic!()?

[michaelwoerister]

The following test case runs into the problem with MSVC:
https://github.com/rust-lang/rust/blob/master/src/test/run-make-fulldeps/pgo-use/main.rs

See https://github.com/rust-lang/rust/blob/master/src/test/run-make-fulldeps/pgo-use/Makefile for how the file is compiled.

[mati865]

It does not reproduce with gnu toolchain with Rust and LLVM assertions enabled, I'll try to remember about fixing #61005 along with gnu profiler.

[michaelwoerister]

Cool!

[michaelwoerister]

I just confirmed that (for me locally) PGO seems to work on Windows GNU. That is, I built ripgrep with instrumentation, ran it, and rebuilt it with the profiling data collected. It worked just fine, no crashes. So I updated the original issue text to just mention MSVC instead of Windows in general.