[clang] Add -funique-source-file-output-paths option

There's build setups where source file names aren't necessarily unique
(e.g. the same source files can be used for multiple targets) but output
paths are. It's theoretically possible to get the build system to pass
`-funique-source-file-identifier` with the output path, but it can also
be quite complicated to do so. Introduce a driver option that specifies
the output path as the unique source identifier to support this.

Author: smeenai

clang/docs/UsersManual.rst

@@ -2341,10 +2341,11 @@ are listed below.
 
    When enabled, allows the compiler to assume that each object file
    passed to the linker has a unique identifier. The identifier for
-   an object file is either the source file path or the value of the
-   argument `-funique-source-file-identifier` if specified. This is
-   useful for reducing link times when doing ThinLTO in combination with
-   whole-program devirtualization or CFI.
+   an object file is either the source file path, the output file path
+   if the ``-funique-source-file-output-paths`` argument is passed, or
+   the value of the argument ``-funique-source-file-identifier`` if
+   specified. This is useful for reducing link times when doing ThinLTO
+   in combination with whole-program devirtualization or CFI.
 
    The full source path or identifier passed to the compiler must be
    unique. This means that, for example, the following is a usage error:
@@ -2371,9 +2372,14 @@ are listed below.
 
 .. option:: -funique-source-file-identifier=IDENTIFIER
 
-   Used with `-funique-source-file-names` to specify a source file
+   Used with ``-funique-source-file-names`` to specify a source file
    identifier.
 
+.. option:: -funique-source-file-output-paths
+
+   Used with ``-funique-source-file-names`` to specify the output path
+   as the source file identifier.
+
 .. option:: -fforce-emit-vtables
 
    In order to improve devirtualization, forces emitting of vtables even in

clang/include/clang/Basic/DiagnosticDriverKinds.td

@@ -890,4 +890,7 @@ def warn_drv_gcc_install_dir_libstdcxx : Warning<
     "future releases of the clang compiler will prefer GCC installations "
     "containing libstdc++ include directories; '%0' would be chosen over '%1'">,
     InGroup<DiagGroup<"gcc-install-dir-libstdcxx">>;
+
+def err_drv_no_output_filename : Error<
+  "option '%0' requires an output file name">;
 }

clang/include/clang/Driver/Options.td

@@ -4354,6 +4354,8 @@ def unique_source_file_identifier_EQ: Joined<["-"], "funique-source-file-identif
   HelpText<"Specify the source file identifier for -funique-source-file-names; "
            "uses the source file path if not specified">,
   MarshallingInfoString<CodeGenOpts<"UniqueSourceFileIdentifier">>;
+def funique_source_file_output_paths: Joined<["-"], "funique-source-file-output-paths">, Group<f_Group>,
+  HelpText<"Use the output path as the source file identifier for -funique-source-file-names">;
 def funsigned_bitfields : Flag<["-"], "funsigned-bitfields">, Group<f_Group>;
 def funsigned_char : Flag<["-"], "funsigned-char">, Group<f_Group>;
 def fno_unsigned_char : Flag<["-"], "fno-unsigned-char">;

clang/lib/Driver/ToolChains/Clang.cpp

@@ -7578,11 +7578,21 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
 
   if (Args.hasFlag(options::OPT_funique_source_file_names,
                    options::OPT_fno_unique_source_file_names, false)) {
-    if (Arg *A = Args.getLastArg(options::OPT_unique_source_file_identifier_EQ))
-      A->render(Args, CmdArgs);
-    else
+    Arg *A = Args.getLastArg(options::OPT_unique_source_file_identifier_EQ,
+                             options::OPT_funique_source_file_output_paths);
+    if (!A) {
       CmdArgs.push_back(Args.MakeArgString(
           Twine("-funique-source-file-identifier=") + Input.getBaseInput()));
+    } else if (A->getOption().matches(
+                   options::OPT_funique_source_file_output_paths)) {
+      if (Output.isFilename())
+        CmdArgs.push_back(Args.MakeArgString(
+            Twine("-funique-source-file-identifier=") + Output.getFilename()));
+      else
+        D.Diag(diag::err_drv_no_output_filename) << A->getSpelling();
+    } else {
+      A->render(Args, CmdArgs);
+    }
   }
 
   // Setup statistics file output.

clang/test/Driver/unique-source-file-names.c

@@ -9,3 +9,9 @@
 
 // ID: "-cc1"
 // ID: "-funique-source-file-identifier=foo"
+
+// RUN: %clang -funique-source-file-names -funique-source-file-output-paths -o out.o -c -### %s 2> %t
+// RUN: FileCheck --check-prefix=OUTPUT < %t %s
+
+// OUTPUT: "-cc1"
+// OUTPUT: "-funique-source-file-identifier=out.o"