Non-deterministic builds with parallel-frontend-threads option

[sundeep-kokkonda]

While enabling parallel-frontend-threads option in our build env (Yocto), the build outputs are not reproducible.
The difference is in the librustc_driver***.so file .gnu_debuglink section. Further analysis on debug data shows the .strtab section is differed and there I found the debug symbols are generated with changed *.llvm.{hash} between the builds. For e.g.,

BuildA has: _RINvNtCskNcDw7Prru3_4core3ptr13drop_in_placeINtNtNtNtCsc9RSozNcmFu_3std4sync6poison5mutex10MutexGuarduEECsi7rQHRWG1Yr_12rustc_middle.llvm.12271321240562639378

BuildB has: _RINvNtCskNcDw7Prru3_4core3ptr13drop_in_placeINtNtNtNtCsc9RSozNcmFu_3std4sync6poison5mutex10MutexGuarduEECsi7rQHRWG1Yr_12rustc_middle.llvm.16648798016096118002

In above lines the *.llvm.{hash} is changed. Is there a way or an option to make this hash generation is unique? I tried by setting CGU to 1 but still the same issue is seen.

The same observation is made on rust upstream sources w/o Yocto build env.

[bjorn3]

Probably a duplicate of #150451 tl;dr: We are leaking the raw source map location of spans into the LLVM IR emitted for inline asm. This seems to be non-deterministic for parallel rustc. We need to come up with some alternative for this to make the LLVM IR deterministic without regressing error messages.