[PATCH] D137058: [Driver] [Modules] Support -fmodule-output (1/2)

[Tom Honermann via Phabricator via cfe-commits]

tahonermann added inline comments.


================
Comment at: clang/lib/Driver/Driver.cpp:5541-5542
+  //
+  // FIXME: Do we need to handle the case that the calculated output is
+  // conflicting with the specified output file or the input file?
+  if (!AtTopLevel && isa<PrecompileJobAction>(JA) &&
----------------
tahonermann wrote:
> ChuanqiXu wrote:
> > dblaikie wrote:
> > > tahonermann wrote:
> > > > ChuanqiXu wrote:
> > > > > dblaikie wrote:
> > > > > > Do we do that for `-o` today? (eg: if you try to `-o` and specify the input file name, such that the output would overwrite the input, what happens? I'm not sure - but I guess it's OK to do whatever that is for this case too)
> > > > > > Do we do that for -o today? (eg: if you try to -o and specify the input file name, such that the output would overwrite the input, what happens? I'm not sure - but I guess it's OK to do whatever that is for this case too)
> > > > > 
> > > > > In this case, the input file will be overwrite and no warning shows. So it looks like we didn't take special treatment here. So I remove the FIXME.
> > > > Basing the location of the module output on the presence of `-o` sounds confusing to me. Likewise, defaulting to writing next to the input file as opposed to the current directory (where a `.o` file is written by default) sounds wrong. I think this option should be handled similarly to `-o`; it should accept a path and:
> > > >   - If an absolute path is provided, write to that location.
> > > >   - If a relative path is provided, write to that location relative to the current working directory.
> > > > Leave it up to the user or build system to ensure that the `.o` and `.pcm` file locations coincide if that is what they want. In general, I don't expect colocation of `.o` and `.pcm` files to be what is desired.
> > > > 
> > > > 
> > > @tahonermann there's precedent for this with Split DWARF (.dwo files go next to the .o file) & I'd argued for possibly only providing this behavior, letting consumers move files elsewhere if they needed to, but got voted down there.
> > > 
> > > I do think this is a reasonable default, though. Users and build systems have the option to pass a path to place the .pcm somewhere else (in the follow-up patch to this one).
> > @tahonermann here is another patch which implements the behavior you described: https://reviews.llvm.org/D137059
> I'm ok with a default that consistently writes the PCM relative to the location of the `.o` file (if not overridden with an absolute path). What I'm not ok with is having the default be next to the `.o` file if `-o` is specified and next to the input file if `-o` is not specified. I don't think writing the PCM relative to the input file is a good default in any case. If `-o` is not specified, then I think it should be written relative to the current working directory; which matches where the `.o` file will be written.
Summary of the consensus of the Clang Modules WG:
  # If `-fmodule-output=` is provided, the PCM is written to the indicated file (relative to the current working directory for a relative path).
  # If `-fmodule-output` is provided, the PCM is written relative to the location of the `.o` file (the current working directory by default and the location indicated by `-o` otherwise).


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D137058/new/

https://reviews.llvm.org/D137058