[Driver] Add -dumpdir and change -gsplit-dwarf .dwo names for linking

When the final phase is linking, Clang currently places `.dwo` files in the
current directory (like the `-c` behavior for multiple inputs).
Strangely, -fdebug-compilation-dir=/-ffile-compilation-dir= is considered, which
is untested.

GCC has a more useful behavior that derives auxiliary filenames from the final
output (-o).
```
gcc -c -g -gsplit-dwarf d/a.c d/b.c      # a.dwo b.dwo
gcc -g -gsplit-dwarf d/a.c d/b.c -o e/x  # e/x-a.dwo e/x-b.dwo
gcc -g -gsplit-dwarf d/a.c d/b.c         # a-a.dwo a-b.dwo
```

Port a useful subset of GCC behaviors that are easy to describe to Clang.

* Add a driver and cc1 option -dumpdir
* When the final phase is link, add a default -dumpdir if not specified by the user
* Forward -dumpdir to -cc1 command lines
* tools::SplitDebugName prefers -dumpdir when constructing the .dwo filename

GCC provides -dumpbase. If we use just one of -dumpdir and -dumpbase,
-dumpbase isn't very useful as it appends a dash.

```
gcc -g -gsplit-dwarf -dumpdir e d/a.c   # ea.dwo
gcc -g -gsplit-dwarf -dumpdir e/ d/a.c  # e/a.dwo
gcc -g -gsplit-dwarf -dumpbase e d/a.c  # e-a.dwo
gcc -g -gsplit-dwarf -dumpbase e/ d/a.c # e/-a.dwo
```

If we specify both `-dumpdir` and `-dumpbase`, we can avoid the influence of the
source filename when there is one input file.
```
gcc -g -gsplit-dwarf -dumpdir f/ -dumpbase x d/a.c        # f/x.dwo
gcc -g -gsplit-dwarf -dumpdir f/ -dumpbase x d/a.c d/b.c  # f/x-a.dwo f/x-b.dwo
```

Given the above examples, I think -dumpbase is not useful.

GCC -save-temps has interesting interaction with -dumpdir as -save-temps
generated files are considered auxiliary files like .dwo files.

For Clang, with this patch, -save-temps and -dumpdir are orthogonal, which is
easier to explain.

```
gcc -g -gsplit-dwarf d/a.c -o e/x -dumpdir f/ -save-temps=obj # e/a.{i,s,o,dwo}
gcc -g -gsplit-dwarf d/a.c -o e/x -save-temps=obj -dumpdir f/ # f/a.{i,s,o,dwo}

clang -g -gsplit-dwarf d/a.c -o e/x -save-temps=obj -dumpdir f/ # e/a.{i,s,o} f/a.dwo
```

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D149193

1 files changed, 15 insertions, 0 deletions

diff --git a/clang/lib/Driver/Driver.cpp b/clang/lib/Driver/Driver.cpp
index a2deb9b..879f3bf 100644
--- a/clang/lib/Driver/Driver.cpp
+++ b/clang/lib/Driver/Driver.cpp
@@ -3878,6 +3878,21 @@ void Driver::handleArguments(Compilation &C, DerivedArgList &Args,
         !Args.getLastArgValue(options::OPT_fuse_ld_EQ)
              .equals_insensitive("lld"))
       Diag(clang::diag::err_drv_lto_without_lld);
+
+    // If -dumpdir is not specified, give a default prefix derived from the link
+    // output filename. For example, `clang -g -gsplit-dwarf a.c -o x` passes
+    // `-dumpdir x-` to cc1. If -o is unspecified, use
+    // stem(getDefaultImageName()) (usually stem("a.out") = "a").
+    if (!Args.hasArg(options::OPT_dumpdir)) {
+      Arg *Arg = Args.MakeSeparateArg(
+          nullptr, getOpts().getOption(options::OPT_dumpdir),
+          Args.MakeArgString(Args.getLastArgValue(
+                                 options::OPT_o,
+                                 llvm::sys::path::stem(getDefaultImageName())) +
+                             "-"));
+      Arg->claim();
+      Args.append(Arg);
+    }
   }
 
   if (FinalPhase == phases::Preprocess || Args.hasArg(options::OPT__SLASH_Y_)) {