[clang] [llvm] Introduce -funique-source-file-names flag. (PR #135728)

[Teresa Johnson via llvm-commits]

teresajohnson wrote:

> > I assume this will give a duplicate symbol linker error if used inappropriately? Any chance of more subtle bugs?
> 
> As far as LTO is concerned I think it can only result in a duplicate symbol error as the resulting symbols will have external strong linkage. There are two non-LTO uses of `getModuleUniqueId` in the tree:
> 
> * The PowerPC AsmPrinter. It looks like this one is used to produce symbols with external linkage, so the usage error would be detected at link time.
> * The ASan pass. It's unclear what would happen if used incorrectly (a discarded section error I think?) but it doesn't look like we can do much about it anyway because LTO isn't necessarily enabled with ASan.

Could that be addressed by only allowing this flag under LTO?

> 
> > Should there be some error detection at LTO link time? I.e. if the module flag is set on the LTO linked modules can and should we detect and error on duplicate source file names?
> 
> I don't think it is needed as the LTO-specific cases are already detectable.

Probably at least add a note to the new documentation for the option that it will result in linker errors if used inappropriately.

I was mostly thinking that detecting and erroring at LTO time would result in a clearer error (like at the start of LTO::addModule, where it is looking for partially split LTO units, which we do by adding the module flag to the index flags). E.g. I could see a case where someone adds this option to their makefiles, time passes and they forget about it, a new file is added that violates the constraints, and boom they get a linker error that they don't understand (and then the compiler gets an bug report).

https://github.com/llvm/llvm-project/pull/135728