[Driver] -gsplit-dwarf: Produce .dwo regardless of -gN for -fthinlto-index=

-g is an IR generation option while -gsplit-dwarf is an object file generation option.
For -gsplit-dwarf in the backend phase of a distributed ThinLTO (-fthinlto-index=) which does object file generation and no IR generation, -g should not be needed.

This patch makes `-fthinlto-index= -gsplit-dwarf` emit .dwo even in the absence of -g.
This should fix https://crbug.com/1158215 after D80391.

```
// Distributed ThinLTO usage
clang -g -O2 -c -flto=thin -fthin-link-bitcode=a.indexing.o a.c
clang -g -O2 -c -flto=thin -fthin-link-bitcode=b.indexing.o b.c
clang -fuse-ld=lld -Wl,--thinlto-index-only=a.rsp -Wl,--thinlto-prefix-replace=';lto/' -Wl,--thinlto-object-suffix-replace='.indexing.o;.o' a.indexing.o b.indexing.o
clang -gsplit-dwarf -O2 -c -fthinlto-index=lto/a.o.thinlto.bc a.o -o lto/a.o
clang -gsplit-dwarf -O2 -c -fthinlto-index=lto/b.o.thinlto.bc b.o -o lto/b.o
clang -fuse-ld=lld @a.rsp -o exe
```

Note: for implicit regular/Thin LTO, .dwo emission works without this patch:
`clang -flto=thin -gsplit-dwarf a.o b.o` passes `-plugin-opt=dwo_dir=` to the linker.
The linker forwards the option to LTO. LTOBackend.cpp emits `$dwo_dir/[01234].dwo`.

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D94647

2 files changed, 16 insertions, 2 deletions

diff --git a/clang/lib/Driver/ToolChains/Clang.cpp b/clang/lib/Driver/ToolChains/Clang.cpp
index d83835a0f7b8..0c821b85c229 100644
--- a/clang/lib/Driver/ToolChains/Clang.cpp
+++ b/clang/lib/Driver/ToolChains/Clang.cpp
@@ -3754,7 +3754,12 @@ static void RenderDebugOptions(const ToolChain &TC, const Driver &D,
       Args.hasFlag(options::OPT_fsplit_dwarf_inlining,
                    options::OPT_fno_split_dwarf_inlining, false);
 
-  if (const Arg *A = Args.getLastArg(options::OPT_g_Group)) {
+  // Normally -gsplit-dwarf is only useful with -gN. For -gsplit-dwarf in the
+  // backend phase of a distributed ThinLTO which does object file generation
+  // and no IR generation, -gN should not be needed. So allow -gsplit-dwarf with
+  // either -gN or -fthinlto-index=.
+  if (Args.hasArg(options::OPT_g_Group) ||
+      Args.hasArg(options::OPT_fthinlto_index_EQ)) {
     Arg *SplitDWARFArg;
     DwarfFission = getDebugFissionKind(D, Args, SplitDWARFArg);
     if (DwarfFission != DwarfFissionKind::None &&
@@ -3762,7 +3767,8 @@ static void RenderDebugOptions(const ToolChain &TC, const Driver &D,
       DwarfFission = DwarfFissionKind::None;
       SplitDWARFInlining = false;
     }
-
+  }
+  if (const Arg *A = Args.getLastArg(options::OPT_g_Group)) {
     DebugInfoKind = codegenoptions::LimitedDebugInfo;
 
     // If the last option explicitly specified a debug-info level, use it.
diff --git a/clang/test/Driver/split-debug.c b/clang/test/Driver/split-debug.c
index de1258dcb13c..2ce4ca980c3f 100644
--- a/clang/test/Driver/split-debug.c
+++ b/clang/test/Driver/split-debug.c
@@ -24,6 +24,14 @@
 /// -gsplit-dwarf is a no-op if no -g is specified.
 // RUN: %clang -### -c -target x86_64 -gsplit-dwarf %s 2>&1 | FileCheck %s --check-prefix=G0
 
+/// ... unless -fthinlto-index= is specified.
+// RUN: echo > %t.bc
+// RUN: %clang -### -c -target x86_64 -fthinlto-index=dummy -gsplit-dwarf %t.bc 2>&1 | FileCheck %s --check-prefix=THINLTO
+
+// THINLTO-NOT:  "-debug-info-kind=
+// THINLTO:      "-ggnu-pubnames"
+// THINLTO-SAME: "-split-dwarf-file" "{{.*}}.dwo" "-split-dwarf-output" "{{.*}}.dwo"
+
 /// -gno-split-dwarf disables debug fission.
 // RUN: %clang -### -c -target x86_64 -gsplit-dwarf -g -gno-split-dwarf %s 2>&1 | FileCheck %s --check-prefix=NOSPLIT
 // RUN: %clang -### -c -target x86_64 -gsplit-dwarf=single -g -gno-split-dwarf %s 2>&1 | FileCheck %s --check-prefix=NOSPLIT