[PATCH] D59673: [Clang] Harmonize Split DWARF options with llc

[Aaron Puchert via Phabricator via cfe-commits]

aaronpuchert added inline comments.


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Comment at: lib/CodeGen/BackendUtil.cpp:1345
   Conf.RemarksPasses = CGOpts.OptRecordPasses;
-  Conf.DwoPath = CGOpts.SplitDwarfFile;
+  Conf.DwoPath = CGOpts.SplitDwarfOutput;
   switch (Action) {
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tejohnson wrote:
> aaronpuchert wrote:
> > tejohnson wrote:
> > > aaronpuchert wrote:
> > > > aaronpuchert wrote:
> > > > > @pcc Your documentation for `DwoPath` suggests that this should be the actual output filename. However, the test that you added together with this line in rC333677 doesn't fail whatever garbage I write into that field here. What can I add to that so that it fails when we don't do the right thing here?
> > > > @pcc Could you (or perhaps @tejohnson) comment on what the intended behavior is here, and how I can change the test so that it fails when I do the wrong thing? Is this the name of the file we write the split debug info to, or is it the value we use for the DW_AT_[GNU_]dwo_name attribute in the skeleton CU?
> > > It is the name of the file the split debug info is written to. If you test by changing the file name given to the -split-dwarf-file option in test/CodeGen/thinlto-split-dwarf.c, make sure you clean up the old one in your test output directory. When I tested it just now, it initially still passed because I had an old one sitting in the output directory from a prior run. Once I removed that it failed with the new name (without changing the corresponding llvm-readobj invocation).
> > Thanks, I didn't consider that. I wasn't even aware that test output is persisted.
> > 
> > It seems `DwoPath` is used both as output filename and as value for `DW_AT_[GNU_]dwo_name` in the skeleton CU. `llc` has separate options for both: `-split-dwarf-file` for the attribute and `-split-dwarf-output` for the output filename. I want to do the same for Clang. Then we should probably separate them for LTO, too.
> > 
> > What is the use case for `-split-dwarf-file` with an individual thin backend? Is that used for distributed/remote builds? Also is there any non-cc1 way to use it? There is `--plugin-opt=dwo_dir=...` for the LTO linker plugin, but that seems to go a different route.
> This is the path taken for distributed ThinLTO, the plugin-opt passed to linker are for in-process ThinLTO. While -split-dwarf-file is a cc1 option, it is normally set automatically from the filename under -gsplit-dwarf (see where it is set in Clang::ConstructJob).
Ok, then it makes sense to do the changes for LTO as well.

You're right that `-split-dwarf-file` is produced for `-gsplit-dwarf`, but not with LTO, right? When I set `-gsplit-dwarf` on a vanilla ThinLTO build, then the flag is just ignored and I'm getting the debug info in the final executables/shared objects. (At least with Clang 8.)

I'm asking because this changes the cc1-interface, and while I have adapted `Clang::ConstructJob`, I'm not sure how distributed ThinLTO works, and whether there have to be changes.

If it works like test/CodeGen/thinlto-split-dwarf.c, using cc1-options, then users might have to adapt after this change.


Repository:
  rC Clang

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  https://reviews.llvm.org/D59673/new/

https://reviews.llvm.org/D59673