[PATCH] D37954: Try to shorten system header paths when using -MD depfiles

[Peter Wu via Phabricator via cfe-commits]

Lekensteyn added a comment.

(FWIW, the suggestion to use `FileManager:getCanonicalName` did not work because I have no DirectoryEntry. And `-no-canonical-prefixes` it used very early and stripped from normal options, it seems used for a different purpose. Given the opposition, I think that this patch is a no-go.)

In https://reviews.llvm.org/D37954#902979, @jyknight wrote:

> It seems to me that it was caused by the prefix being set as "/bin" instead of "/usr/bin", because clang _doesn't_ actually canonicalize its prefix, even when -no-canonical-prefixes isn't specified! All it does, now, is to make the prefix absolute -- without fully canonicalizing. I think that's simply a bug.
>
> If the prefix had been, properly, /usr/bin, then the include path would've been:
>
>   /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/7.2.0/../../../../include/c++/7.2.0/iostream
>
> And in that case, it would've worked properly with ninja, without adding the feature in this patch.
>
> Reference clang/tools/driver/driver.cpp:297:
>
>   // FIXME: We don't actually canonicalize this, we just make it absolute.
>   if (CanonicalPrefixes)
>     llvm::sys::fs::make_absolute(InstalledPath);


If this fixes the issue, then it would definitely be a much better approach. If this has changed between Clang 4 -> 5, then it is possible the root cause.

On my system (Arch Linux) `/bin` is a symlink to `usr/bin`. Output of `clang -v`:

  clang version 5.0.0 (tags/RELEASE_500/final)
  Target: x86_64-unknown-linux-gnu
  Thread model: posix
  InstalledDir: /bin
  Found candidate GCC installation: /bin/../lib/gcc/x86_64-pc-linux-gnu/7.2.0
  Found candidate GCC installation: /bin/../lib64/gcc/x86_64-pc-linux-gnu/7.2.0
  Found candidate GCC installation: /usr/lib/gcc/x86_64-pc-linux-gnu/7.2.0
  Found candidate GCC installation: /usr/lib64/gcc/x86_64-pc-linux-gnu/7.2.0
  Selected GCC installation: /bin/../lib64/gcc/x86_64-pc-linux-gnu/7.2.0
  Candidate multilib: .;@m64
  Candidate multilib: 32;@m32
  Selected multilib: .;@m64


Repository:
  rL LLVM

https://reviews.llvm.org/D37954