thakis added inline comments.

================
Comment at: include/clang/Driver/CLCompatOptions.td:68
@@ +67,3 @@
+def _SLASH_FC : CLFlag<"FC">,
+  HelpText<"Use absolute paths in diagnostics and __FILE__">,
+  Alias<fabsolute_paths>;
----------------
hans wrote:
> thakis wrote:
> > Hm, I'm not sure this should affect __FILE__. It sounds people want this 
> > for diagnostics, but making it affect __FILE__ makes reproducible builds 
> > harder. (There's -ffile-macro-prefix-to-remove from 
> > http://reviews.llvm.org/D17741 for that too, but that's not in.)
> The MSVC docs say it applies to __FILE__ though 
> https://msdn.microsoft.com/en-us/library/027c4t2s.aspx and I worry that 
> implementing only half of it might surprise users.
> 
> Chromium currently builds with MSVC and /FC, so we should already have full 
> paths in __FILE__ there.
Sure, but the MSVC build is not deterministic in several ways that folks aren't 
happy about, and we're supposed to improve that :-)

As far as I know __FILE__ defaults to just the file's basename in cl.exe (is 
that true?). So without /FC __FILE__ is useless there and people more or less 
have to pass /FC. With clang, we emit the full relative path to the file, so 
it's not useless already.

For diagnostics, I can kind of see how full paths are useful: You likely 
develop on the same machine as you see the diagnostics on, so people want real 
absolute paths so they can click on them in MSVC. For __FILE__, that's less 
convincing, since that gets embedded into the binary that's shipped elsewhere.


https://reviews.llvm.org/D23816



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