Some were necessary (asm goto), some were not (nested functions, flexible array members not at the end of structs).
> UDB, bugs and all
Luckily, the kernel didn't intentionally rely on GCC specifics this way. Where it did unintentionally, we fixed the kernel sources properly with detailed commit messages explaining why.
> or were there any issues that might be considered as specific to the linux kernel?
and then added to LLVM's existing test suite. Many such tests were also simply manually written.
> Did the gcc project themselves have a regression/test suite that you were able to use as a starting point?
GCC and binutils have their own test suites. Folks in the LLVM community have worked on being able to test clang against GCC's test suite. I personally have never run GCC's test suite or looked at its sources.
Some were necessary (asm goto), some were not (nested functions, flexible array members not at the end of structs).
> UDB, bugs and all
Luckily, the kernel didn't intentionally rely on GCC specifics this way. Where it did unintentionally, we fixed the kernel sources properly with detailed commit messages explaining why.
> or were there any issues that might be considered as specific to the linux kernel?
Yes, https://github.com/ClangBuiltLinux/linux/issues is our issue tracker. We use tags extensively to mark if we triage the issue to be kernel-side vs toolchain-side.
> Did you end up building a gcc compatability test suite as a part of this?
No, but some tricky cases LLVM got wrong were distilled from kernel sources using either:
- creduce - cvise (my favorite) - bugpoint - llvm-reduce
and then added to LLVM's existing test suite. Many such tests were also simply manually written.
> Did the gcc project themselves have a regression/test suite that you were able to use as a starting point?
GCC and binutils have their own test suites. Folks in the LLVM community have worked on being able to test clang against GCC's test suite. I personally have never run GCC's test suite or looked at its sources.