DebugInfo: Omit an empty CU when a subprogram was moved into its use

When the only use of a CU is for a subprogram that's only emitted into
the using CU (to avoid cross-CU references in DWO files), avoid creating
that CU at all.

llvm-svn: 304111

Author: dwblaikie

llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp

@@ -374,14 +374,18 @@ void DwarfDebug::constructAbstractSubprogramScopeDIE(DwarfCompileUnit &SrcCU,
 
   // Find the subprogram's DwarfCompileUnit in the SPMap in case the subprogram
   // was inlined from another compile unit.
-  auto &CU = getOrCreateDwarfCompileUnit(SP->getUnit());
-  if (auto *SkelCU = CU.getSkeleton()) {
-    (shareAcrossDWOCUs() ? CU : SrcCU)
-        .constructAbstractSubprogramScopeDIE(Scope);
-    if (CU.getCUNode()->getSplitDebugInlining())
-      SkelCU->constructAbstractSubprogramScopeDIE(Scope);
-  } else {
-    CU.constructAbstractSubprogramScopeDIE(Scope);
+  if (useSplitDwarf() && !shareAcrossDWOCUs() && !SP->getUnit()->getSplitDebugInlining())
+    // Avoid building the original CU if it won't be used
+    SrcCU.constructAbstractSubprogramScopeDIE(Scope);
+  else {
+    auto &CU = getOrCreateDwarfCompileUnit(SP->getUnit());
+    if (auto *SkelCU = CU.getSkeleton()) {
+      (shareAcrossDWOCUs() ? CU : SrcCU)
+          .constructAbstractSubprogramScopeDIE(Scope);
+      if (CU.getCUNode()->getSplitDebugInlining())
+        SkelCU->constructAbstractSubprogramScopeDIE(Scope);
+    } else
+      CU.constructAbstractSubprogramScopeDIE(Scope);
   }
 }
 

llvm/test/DebugInfo/split-dwarf-omit-empty.ll

@@ -0,0 +1,52 @@
+; RUN: %llc_dwarf -split-dwarf-file=foo.dwo  %s -filetype=obj -o - | llvm-dwarfdump -debug-dump=info - | FileCheck %s
+
+; Created from:
+; a.cpp:
+;   void f1();
+;   inline __attribute__((always_inline)) __attribute__((used)) void f2() { f1(); }
+; b.cpp:
+;   void f2();
+;   void f3() {
+;     f2();
+;   }
+; $ clang++ -fno-split-dwarf-inlining {a,b}.cpp -emit-llvm -S -g
+; $ llvm-link {a,b}.ll -S -o ab.ll
+; Then strip out the @llvm.used global, so no out of line definition of 'f2'
+; will be emitted. This emulates something more like the available_externally
+; import performed by ThinLTO.
+
+; CHECK: Compile Unit
+; CHECK-NOT: Compile Unit
+
+declare void @_Z2f1v()
+
+; Function Attrs: noinline norecurse uwtable
+define i32 @main() !dbg !9 {
+entry:
+  call void @_Z2f1v(), !dbg !13
+  ret i32 0, !dbg !18
+}
+
+!llvm.dbg.cu = !{!0, !3}
+!llvm.ident = !{!5, !5}
+!llvm.module.flags = !{!6, !7, !8}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: !1, producer: "clang version 5.0.0 (trunk 304054) (llvm/trunk 304080)", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, splitDebugInlining: false)
+!1 = !DIFile(filename: "a.cpp", directory: "/usr/local/google/home/blaikie/dev/scratch")
+!2 = !{}
+!3 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: !4, producer: "clang version 5.0.0 (trunk 304054) (llvm/trunk 304080)", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, splitDebugInlining: false)
+!4 = !DIFile(filename: "b.cpp", directory: "/usr/local/google/home/blaikie/dev/scratch")
+!5 = !{!"clang version 5.0.0 (trunk 304054) (llvm/trunk 304080)"}
+!6 = !{i32 2, !"Dwarf Version", i32 4}
+!7 = !{i32 2, !"Debug Info Version", i32 3}
+!8 = !{i32 1, !"wchar_size", i32 4}
+!9 = distinct !DISubprogram(name: "main", scope: !4, file: !4, line: 2, type: !10, isLocal: false, isDefinition: true, scopeLine: 2, flags: DIFlagPrototyped, isOptimized: false, unit: !3, variables: !2)
+!10 = !DISubroutineType(types: !11)
+!11 = !{!12}
+!12 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!13 = !DILocation(line: 2, column: 73, scope: !14, inlinedAt: !17)
+!14 = distinct !DISubprogram(name: "f2", linkageName: "_Z2f2v", scope: !1, file: !1, line: 2, type: !15, isLocal: false, isDefinition: true, scopeLine: 2, flags: DIFlagPrototyped, isOptimized: false, unit: !0, variables: !2)
+!15 = !DISubroutineType(types: !16)
+!16 = !{null}
+!17 = distinct !DILocation(line: 3, column: 3, scope: !9)
+!18 = !DILocation(line: 4, column: 1, scope: !9)