DebugInfo: Split DWARF + gmlt + no-split-dwarf-inlining shouldn't emit anything to the .dwo file

This configuration (due to r349207) was intended not to emit any DWO CU,
but a degenerate CU was still being emitted - containing a header and a
DW_TAG_compile_unit with no attributes.

Under that situation, emit nothing to the .dwo file. (since this is a
dynamic property of the input the .dwo file is still emitted, just with
nothing in it (so a valid, but empty, ELF file) - if some other CU
didn't satisfy this criteria, its DWO CU would still go there, etc)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@353771 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/AsmPrinter/DwarfDebug.cpp

@@ -2124,9 +2124,9 @@ void DwarfDebug::emitDebugLoc() {
 }
 
 void DwarfDebug::emitDebugLocDWO() {
-  Asm->OutStreamer->SwitchSection(
-      Asm->getObjFileLowering().getDwarfLocDWOSection());
   for (const auto &List : DebugLocs.getLists()) {
+    Asm->OutStreamer->SwitchSection(
+        Asm->getObjFileLowering().getDwarfLocDWOSection());
     Asm->OutStreamer->EmitLabel(List.Label);
     for (const auto &Entry : DebugLocs.getEntries(List)) {
       // GDB only supports startx_length in pre-standard split-DWARF.

lib/CodeGen/AsmPrinter/DwarfFile.cpp

@@ -43,6 +43,11 @@ void DwarfFile::emitUnit(DwarfUnit *TheU, bool UseOffsets) {
   if (!S)
     return;
 
+  // Skip CUs that ended up not being needed (split CUs that were abandoned
+  // because they added no information beyond the non-split CU)
+  if (llvm::empty(TheU->getUnitDie().values()))
+    return;
+
   Asm->OutStreamer->SwitchSection(S);
   TheU->emitHeader(UseOffsets);
   Asm->emitDwarfDIE(TheU->getUnitDie());
@@ -62,6 +67,11 @@ void DwarfFile::computeSizeAndOffsets() {
     if (TheU->getCUNode()->isDebugDirectivesOnly())
       continue;
 
+    // Skip CUs that ended up not being needed (split CUs that were abandoned
+    // because they added no information beyond the non-split CU)
+    if (llvm::empty(TheU->getUnitDie().values()))
+      return;
+
     TheU->setDebugSectionOffset(SecOffset);
     SecOffset += computeSizeAndOffsetsForUnit(TheU.get());
   }

test/DebugInfo/X86/gmlt-no-split-dwarf-inlining-empty.ll

@@ -0,0 +1,28 @@
+; RUN: %llc_dwarf -split-dwarf-file=foo.dwo  %s -filetype=obj -o - | llvm-objdump -h - | FileCheck %s
+
+; Created from:
+;   void f1() {
+;   }
+; $ clang-tot gmlt-no-split-dwarf-inlining-empty.c -fno-split-dwarf-inlining -gmlt -gsplit-dwarf -c -emit-llvm -S
+
+; CHECK-NOT: .debug_{{.*}}.dwo
+
+define dso_local void @f1() !dbg !7 {
+entry:
+  ret void, !dbg !9
+}
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4, !5}
+!llvm.ident = !{!6}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 9.0.0 (trunk 353744) (llvm/trunk 353759)", isOptimized: false, runtimeVersion: 0, emissionKind: LineTablesOnly, enums: !2, splitDebugInlining: false, nameTableKind: GNU)
+!1 = !DIFile(filename: "gmlt-no-split-dwarf-inlining-empty.c", directory: "/usr/local/google/home/blaikie/dev/scratch")
+!2 = !{}
+!3 = !{i32 2, !"Dwarf Version", i32 4}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = !{i32 1, !"wchar_size", i32 4}
+!6 = !{!"clang version 9.0.0 (trunk 353744) (llvm/trunk 353759)"}
+!7 = distinct !DISubprogram(name: "f1", scope: !1, file: !1, line: 1, type: !8, scopeLine: 1, spFlags: DISPFlagDefinition, unit: !0, retainedNodes: !2)
+!8 = !DISubroutineType(types: !2)
+!9 = !DILocation(line: 2, column: 1, scope: !7)