[clang] No relative path support for precompiled headers

[matts1]

To reproduce:

touch a.h
clang++ -x c++-header a.h -Xclang -fmodule-file-home-is-cwd -o a.pcm
clang++ -module-file-info a.pcm | grep "Input file"    

Got: /absolute/path/to/a.h
Want: a.h

The same applies to input pcm files, though I don't have a simple reproducer for that.
When I add -fmodule-file=obj/buildtools/third_party/libc++/std/module.pcm, I find that the module info output contains Imports module 'std': /absolute/path/to/obj/buildtools/third_party/libc++/std/module.pcm

(And yes, I checked - the file itself contains absolute paths, it's not just output by the tool)

This is a problem for us, because we want to use remote execution, which requires that our paths be relative.

[llvmbot]

@llvm/issue-subscribers-clang-driver

Author: Matt (matts1)

To reproduce: ``` touch a.h clang++ -x c++-header a.h -Xclang -fmodule-file-home-is-cwd -o a.pcm clang++ -module-file-info a.pcm | grep "Input file" ```

Got: /absolute/path/to/a.h
Want: a.h

The same applies to input pcm files, though I don't have a simple reproducer for that.
When I add -fmodule-file=obj/buildtools/third_party/libc++/std/module.pcm, I find that the module info output contains Imports module 'std': /absolute/path/to/obj/buildtools/third_party/libc++/std/module.pcm

(And yes, I checked - the file itself contains absolute paths, it's not just output by the tool)

This is a problem for us, because we want to use remote execution, which requires that our paths be relative.

[llvmbot]

@llvm/issue-subscribers-clang-modules

Author: Matt (matts1)

To reproduce: ``` touch a.h clang++ -x c++-header a.h -Xclang -fmodule-file-home-is-cwd -o a.pcm clang++ -module-file-info a.pcm | grep "Input file" ```

Got: /absolute/path/to/a.h
Want: a.h

The same applies to input pcm files, though I don't have a simple reproducer for that.
When I add -fmodule-file=obj/buildtools/third_party/libc++/std/module.pcm, I find that the module info output contains Imports module 'std': /absolute/path/to/obj/buildtools/third_party/libc++/std/module.pcm

(And yes, I checked - the file itself contains absolute paths, it's not just output by the tool)

This is a problem for us, because we want to use remote execution, which requires that our paths be relative.

[matts1]

After hooking up a debugger, root cause appears to be that:

• getEmittingModule returns null when -x c++-header is set because PP.getLangOpts().isCompilingModule() returns false.
• This is then propogated to ASTWriter.WritingModule, which checks if WritingModule is null before configuring path normalization.

Also, I considered using relocatable precompiled headers, but looking at the documentation, they appear to serve a completely different purpose.

[matts1]

I think that we should allow the use of -fmodule-file-home-is-cwd when using precompiled headers as well as modules.