[DWARF] Suppress split line tables more carefully.

If a given split type unit does not have source locations, don't have
it refer to the split line table.
If no split type unit refers to the split line table, don't emit the
line table at all.

This will save a little space on rare occasions, but also refactors
things a bit to improve which class is responsible for what.

Responding to review comments on r326395.

Differential Revision: https://reviews.llvm.org/D44220

llvm-svn: 328670

llvm/include/llvm/MC/MCDwarf.h

@@ -250,7 +250,8 @@ class MCDwarfDwoLineTable {
     return cantFail(Header.tryGetFile(Directory, FileName, Checksum, Source));
   }
 
-  void Emit(MCStreamer &MCOS, MCDwarfLineTableParams Params) const;
+  void Emit(MCStreamer &MCOS, MCDwarfLineTableParams Params,
+            MCSection *Section) const;
 };
 
 class MCDwarfLineTable {

llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp

@@ -2122,11 +2122,9 @@ void DwarfDebug::emitDebugAbbrevDWO() {
 
 void DwarfDebug::emitDebugLineDWO() {
   assert(useSplitDwarf() && "No split dwarf?");
-  if (!HasSplitTypeUnits)
-    return;
-  Asm->OutStreamer->SwitchSection(
+  SplitTypeUnitFileTable.Emit(
+      *Asm->OutStreamer, MCDwarfLineTableParams(),
       Asm->getObjFileLowering().getDwarfLineDWOSection());
-  SplitTypeUnitFileTable.Emit(*Asm->OutStreamer, MCDwarfLineTableParams());
 }
 
 void DwarfDebug::emitStringOffsetsTableHeaderDWO() {
@@ -2200,8 +2198,9 @@ void DwarfDebug::addDwarfTypeUnitType(DwarfCompileUnit &CU,
   if (useSplitDwarf())
     NewTU.setSection(Asm->getObjFileLowering().getDwarfTypesDWOSection());
   else {
-    CU.applyStmtList(UnitDie);
     NewTU.setSection(Asm->getObjFileLowering().getDwarfTypesSection(Signature));
+    // Non-split type units reuse the compile unit's line table.
+    CU.applyStmtList(UnitDie);
   }
 
   // Add DW_AT_str_offsets_base to the type unit DIE, but not for split type
@@ -2239,7 +2238,6 @@ void DwarfDebug::addDwarfTypeUnitType(DwarfCompileUnit &CU,
       InfoHolder.computeSizeAndOffsetsForUnit(TU.first.get());
       InfoHolder.emitUnit(TU.first.get(), useSplitDwarf());
     }
-    HasSplitTypeUnits = useSplitDwarf();
   }
   CU.addDIETypeSignature(RefDie, Signature);
 }

llvm/lib/CodeGen/AsmPrinter/DwarfDebug.h

@@ -280,10 +280,6 @@ class DwarfDebug : public DebugHandlerBase {
   /// a monolithic sequence of string offsets.
   bool UseSegmentedStringOffsetsTable;
 
-  /// Whether we have emitted any type units with split DWARF (and therefore
-  /// need to emit a line table to the .dwo file).
-  bool HasSplitTypeUnits = false;
-
   /// Separated Dwarf Variables
   /// In general these will all be for bits that are left in the
   /// original object file, rather than things that are meant

llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp

@@ -85,8 +85,6 @@ DwarfTypeUnit::DwarfTypeUnit(DwarfCompileUnit &CU, AsmPrinter *A,
                              MCDwarfDwoLineTable *SplitLineTable)
     : DwarfUnit(dwarf::DW_TAG_type_unit, CU.getCUNode(), A, DW, DWU), CU(CU),
       SplitLineTable(SplitLineTable) {
-  if (SplitLineTable)
-    addSectionOffset(getUnitDie(), dwarf::DW_AT_stmt_list, 0);
 }
 
 DwarfUnit::~DwarfUnit() {
@@ -300,12 +298,15 @@ MD5::MD5Result *DwarfUnit::getMD5AsBytes(const DIFile *File) {
 }
 
 unsigned DwarfTypeUnit::getOrCreateSourceID(const DIFile *File) {
-  return SplitLineTable
-             ? SplitLineTable->getFile(File->getDirectory(),
-                                       File->getFilename(),
-                                       getMD5AsBytes(File),
-                                       File->getSource())
-             : getCU().getOrCreateSourceID(File);
+  if (!SplitLineTable)
+    return getCU().getOrCreateSourceID(File);
+  if (!UsedLineTable) {
+    UsedLineTable = true;
+    // This is a split type unit that needs a line table.
+    addSectionOffset(getUnitDie(), dwarf::DW_AT_stmt_list, 0);
+  }
+  return SplitLineTable->getFile(File->getDirectory(), File->getFilename(),
+                                 getMD5AsBytes(File), File->getSource());
 }
 
 void DwarfUnit::addOpAddress(DIELoc &Die, const MCSymbol *Sym) {

llvm/lib/CodeGen/AsmPrinter/DwarfUnit.h

@@ -367,6 +367,7 @@ class DwarfTypeUnit final : public DwarfUnit {
   const DIE *Ty;
   DwarfCompileUnit &CU;
   MCDwarfDwoLineTable *SplitLineTable;
+  bool UsedLineTable = false;
 
   unsigned getOrCreateSourceID(const DIFile *File) override;
   bool isDwoUnit() const override;

llvm/lib/MC/MCDwarf.cpp

@@ -257,9 +257,12 @@ void MCDwarfLineTable::Emit(MCObjectStreamer *MCOS,
     LineStr->emitSection(MCOS);
 }
 
-void MCDwarfDwoLineTable::Emit(MCStreamer &MCOS,
-                               MCDwarfLineTableParams Params) const {
+void MCDwarfDwoLineTable::Emit(MCStreamer &MCOS, MCDwarfLineTableParams Params,
+                               MCSection *Section) const {
+  if (Header.MCDwarfFiles.empty())
+    return;
   Optional<MCDwarfLineStr> NoLineStr(None);
+  MCOS.SwitchSection(Section);
   MCOS.EmitLabel(Header.Emit(&MCOS, Params, None, NoLineStr).second);
 }
 

llvm/test/CodeGen/X86/dwarf-split-line-1.ll

@@ -0,0 +1,40 @@
+; Verify that split type units with no source locations don't have a
+; DW_AT_stmt_list attribute, and the .debug_line.dwo section is suppressed.
+
+; RUN: llc -split-dwarf-file=foo.dwo -dwarf-version=5 -generate-type-units \
+; RUN:     -filetype=obj -O0 -mtriple=x86_64-unknown-linux-gnu < %s \
+; RUN:     | llvm-dwarfdump -v - | FileCheck %s
+
+; FIXME: V5 wants type units in .debug_info.dwo not .debug_types.dwo.
+; CHECK-NOT: .debug_line.dwo
+; CHECK: .debug_types.dwo contents:
+; CHECK: 0x00000000: Type Unit: {{.*}} version = 0x0005 unit_type = DW_UT_split_type abbr_offset
+; CHECK: 0x00000018: DW_TAG_type_unit
+; CHECK-NOT: DW_AT_stmt_list
+; CHECK-NOT: DW_AT_decl_file
+; CHECK-NOT: .debug_line.dwo
+
+; ModuleID = 't.cpp'
+source_filename = "t.cpp"
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+%struct.S = type { i32 }
+
+@s = global %struct.S zeroinitializer, align 4, !dbg !0
+
+!llvm.dbg.cu = !{!2}
+!llvm.module.flags = !{!8, !9}
+!llvm.ident = !{!10}
+
+!0 = !DIGlobalVariableExpression(var: !1, expr: !DIExpression())
+!1 = distinct !DIGlobalVariable(name: "s", scope: !2, file: !3, line: 5, type: !6, isLocal: false, isDefinition: true)
+!2 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: !3, producer: "clang version 5.0.0 (trunk 295942)", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !4, globals: !5)
+!3 = !DIFile(filename: "t.cpp", directory: "/home/probinson/projects/scratch")
+!4 = !{}
+!5 = !{!0}
+!6 = distinct !DICompositeType(tag: DW_TAG_structure_type, name: "S", size: 32, elements: !7, identifier: "_ZTS1S")
+!7 = !{}
+!8 = !{i32 2, !"Dwarf Version", i32 4}
+!9 = !{i32 2, !"Debug Info Version", i32 3}
+!10 = !{!"clang version 5.0.0 (trunk 295942)"}

llvm/test/CodeGen/X86/dwarf-split-line-2.ll

@@ -0,0 +1,57 @@
+; Verify that if we have two split type units, one with source locations and
+; one without, the one without locations doesn't have a DW_AT_stmt_list
+; attribute, but the other one does and the .debug_line.dwo section is present.
+
+; RUN: llc -split-dwarf-file=foo.dwo -dwarf-version=5 -generate-type-units \
+; RUN:     -filetype=obj -O0 -mtriple=x86_64-unknown-linux-gnu < %s \
+; RUN:     | llvm-dwarfdump -v - | FileCheck %s
+
+; Currently the no-source-location type comes out first.
+; FIXME: V5 wants type units in .debug_info.dwo not .debug_types.dwo.
+; CHECK: .debug_types.dwo contents:
+; CHECK: 0x00000000: Type Unit: {{.*}} name = 'S'
+; CHECK-SAME: (next unit at [[TU2:0x[0-9a-f]+]])
+; CHECK: DW_TAG_type_unit
+; CHECK-NOT: DW_AT_stmt_list
+; CHECK-NOT: DW_AT_decl_file
+; CHECK: [[TU2]]: Type Unit: {{.*}} name = 'T'
+; CHECK: DW_TAG_type_unit
+; CHECK: DW_AT_stmt_list
+; CHECK: DW_AT_decl_file
+
+; CHECK: .debug_line.dwo
+; CHECK-NOT: standard_opcode_lengths
+; CHECK: file_names[ 0]:
+; CHECK-NEXT: name: "t.cpp"
+; CHECK-NEXT: dir_index: 0
+
+; ModuleID = 't.cpp'
+source_filename = "t.cpp"
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+%struct.S = type { i32 }
+%struct.T = type { i32 }
+
+@s = global %struct.S zeroinitializer, align 4, !dbg !0
+@t = global %struct.T zeroinitializer, align 4, !dbg !14
+
+!llvm.dbg.cu = !{!2}
+!llvm.module.flags = !{!8, !9}
+!llvm.ident = !{!10}
+
+!0 = !DIGlobalVariableExpression(var: !1, expr: !DIExpression())
+!1 = distinct !DIGlobalVariable(name: "s", scope: !2, file: !3, line: 5, type: !6, isLocal: false, isDefinition: true)
+!2 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: !3, producer: "clang version 5.0.0 (trunk 295942)", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !4, globals: !5)
+!3 = !DIFile(filename: "t.cpp", directory: "/home/probinson/projects/scratch")
+!4 = !{}
+!5 = !{!0,!14}
+!6 = distinct !DICompositeType(tag: DW_TAG_structure_type, name: "S", size: 32, elements: !7, identifier: "_ZTS1S")
+!7 = !{}
+!8 = !{i32 2, !"Dwarf Version", i32 4}
+!9 = !{i32 2, !"Debug Info Version", i32 3}
+!10 = !{!"clang version 5.0.0 (trunk 295942)"}
+!11 = distinct !DIGlobalVariable(name: "t", scope: !2, file: !3, line: 10, type: !12, isLocal: false, isDefinition: true)
+!12 = distinct !DICompositeType(tag: DW_TAG_structure_type, name: "T", file: !3, line: 8, size: 32, elements: !13, identifier: "_ZTS1S")
+!13 = !{}
+!14 = !DIGlobalVariableExpression(var: !11, expr: !DIExpression())