[DEBUGINFO] Add flag for DWARF2 to use sections as references.

Summary:
Some targets does not support labels inside debug sections, but support
references in form `section+offset`. Patch adds initial support
for this.

Reviewers: echristo, probinson, jlebar

Subscribers: llvm-commits, JDevlieghere

Differential Revision: https://reviews.llvm.org/D43943

llvm-svn: 328314

llvm/lib/CodeGen/AsmPrinter/DwarfCompileUnit.cpp

@@ -270,15 +270,20 @@ void DwarfCompileUnit::addRange(RangeSpan Range) {
 
 void DwarfCompileUnit::initStmtList() {
   // Define start line table label for each Compile Unit.
-  MCSymbol *LineTableStartSym =
-      Asm->OutStreamer->getDwarfLineTableSymbol(getUniqueID());
+  MCSymbol *LineTableStartSym;
+  const TargetLoweringObjectFile &TLOF = Asm->getObjFileLowering();
+  if (DD->useSectionsAsReferences()) {
+    LineTableStartSym = TLOF.getDwarfLineSection()->getBeginSymbol();
+  } else {
+    LineTableStartSym =
+        Asm->OutStreamer->getDwarfLineTableSymbol(getUniqueID());
+  }
 
   // DW_AT_stmt_list is a offset of line number information for this
   // compile unit in debug_line section. For split dwarf this is
   // left in the skeleton CU and so not included.
   // The line table entries are not always emitted in assembly, so it
   // is not okay to use line_table_start here.
-  const TargetLoweringObjectFile &TLOF = Asm->getObjFileLowering();
   StmtListValue =
       addSectionLabel(getUnitDie(), dwarf::DW_AT_stmt_list, LineTableStartSym,
                       TLOF.getDwarfLineSection()->getBeginSymbol());
@@ -835,7 +840,7 @@ void DwarfCompileUnit::createAbstractVariable(const DILocalVariable *Var,
 
 void DwarfCompileUnit::emitHeader(bool UseOffsets) {
   // Don't bother labeling the .dwo unit, as its offset isn't used.
-  if (!Skeleton) {
+  if (!Skeleton && !DD->useSectionsAsReferences()) {
     LabelBegin = Asm->createTempSymbol("cu_begin");
     Asm->OutStreamer->EmitLabel(LabelBegin);
   }

llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp

@@ -133,6 +133,13 @@ static cl::opt<bool>
                          cl::desc("Disable emission .debug_ranges section."),
                          cl::init(false));
 
+static cl::opt<DefaultOnOff> DwarfSectionsAsReferences(
+    "dwarf-sections-as-references", cl::Hidden,
+    cl::desc("Use sections+offset as references rather than labels."),
+    cl::values(clEnumVal(Default, "Default for platform"),
+               clEnumVal(Enable, "Enabled"), clEnumVal(Disable, "Disabled")),
+    cl::init(Default));
+
 enum LinkageNameOption {
   DefaultLinkageNames,
   AllLinkageNames,
@@ -323,6 +330,9 @@ DwarfDebug::DwarfDebug(AsmPrinter *A, Module *M)
   UsePubSections = !NoDwarfPubSections;
   UseRangesSection = !NoDwarfRangesSection;
 
+  // Use sections as references.
+  UseSectionsAsReferences = DwarfSectionsAsReferences == Enable;
+
   // Work around a GDB bug. GDB doesn't support the standard opcode;
   // SCE doesn't support GNU's; LLDB prefers the standard opcode, which
   // is defined as of DWARF 3.
@@ -1559,6 +1569,14 @@ void DwarfDebug::emitDebugPubSections() {
   }
 }
 
+void DwarfDebug::emitSectionReference(const DwarfCompileUnit &CU) {
+  if (useSectionsAsReferences())
+    Asm->EmitDwarfOffset(CU.getSection()->getBeginSymbol(),
+                         CU.getDebugSectionOffset());
+  else
+    Asm->emitDwarfSymbolReference(CU.getLabelBegin());
+}
+
 void DwarfDebug::emitDebugPubSection(bool GnuStyle, StringRef Name,
                                      DwarfCompileUnit *TheU,
                                      const StringMap<const DIE *> &Globals) {
@@ -1577,7 +1595,7 @@ void DwarfDebug::emitDebugPubSection(bool GnuStyle, StringRef Name,
   Asm->EmitInt16(dwarf::DW_PUBNAMES_VERSION);
 
   Asm->OutStreamer->AddComment("Offset of Compilation Unit Info");
-  Asm->emitDwarfSymbolReference(TheU->getLabelBegin());
+  emitSectionReference(*TheU);
 
   Asm->OutStreamer->AddComment("Compilation Unit Length");
   Asm->EmitInt32(TheU->getLength());
@@ -1876,7 +1894,7 @@ void DwarfDebug::emitDebugARanges() {
     Asm->OutStreamer->AddComment("DWARF Arange version number");
     Asm->EmitInt16(dwarf::DW_ARANGES_VERSION);
     Asm->OutStreamer->AddComment("Offset Into Debug Info Section");
-    Asm->emitDwarfSymbolReference(CU->getLabelBegin());
+    emitSectionReference(*CU);
     Asm->OutStreamer->AddComment("Address Size (in bytes)");
     Asm->EmitInt8(PtrSize);
     Asm->OutStreamer->AddComment("Segment Size (in bytes)");

llvm/lib/CodeGen/AsmPrinter/DwarfDebug.h

@@ -264,6 +264,10 @@ class DwarfDebug : public DebugHandlerBase {
   /// Allow emission of .debug_ranges section.
   bool UseRangesSection = true;
 
+  /// True if the sections itself must be used as references and don't create
+  /// temp symbols inside DWARF sections.
+  bool UseSectionsAsReferences = false;
+
   /// DWARF5 Experimental Options
   /// @{
   bool HasDwarfAccelTables;
@@ -448,6 +452,9 @@ class DwarfDebug : public DebugHandlerBase {
   void collectVariableInfoFromMFTable(DwarfCompileUnit &TheCU,
                                       DenseSet<InlinedVariable> &P);
 
+  /// Emit the reference to the section.
+  void emitSectionReference(const DwarfCompileUnit &CU);
+
 protected:
   /// Gather pre-function debug information.
   void beginFunctionImpl(const MachineFunction *MF) override;
@@ -513,6 +520,11 @@ class DwarfDebug : public DebugHandlerBase {
   /// Returns whether ranges section should be emitted.
   bool useRangesSection() const { return UseRangesSection; }
 
+  /// Returns whether to use sections as labels rather than temp symbols.
+  bool useSectionsAsReferences() const {
+    return UseSectionsAsReferences;
+  }
+
   // Experimental DWARF5 features.
 
   /// Returns whether or not to emit tables that dwarf consumers can

llvm/test/DebugInfo/X86/sections_as_references.ll

@@ -0,0 +1,54 @@
+; RUN: llc -filetype=asm -O0 -mtriple=x86_64-linux-gnu < %s -dwarf-sections-as-references=Enable -dwarf-inlined-strings=Enable -no-dwarf-pub-sections -no-dwarf-ranges-section -dwarf-version 2 -debugger-tune=gdb | FileCheck %s
+
+; CHECK:      .file
+
+; CHECK-NOT:  .L
+
+; CHECK:      .section .debug_abbrev
+; CHECK-NOT:  DW_FORM_str{{p|x}}
+; CHECK-NOT: .L
+
+; CHECK:      .section .debug_info
+; CHECK-NOT:  .L
+; CHECK:      .short 2             # DWARF version number
+; CHECK-NOT:  .L
+; CHECK:      .long .debug_abbrev  # Offset Into Abbrev. Section
+; CHECK-NOT:  .L
+; CHECK:      .long .debug_line    # DW_AT_stmt_list
+; CHECK-NOT:  .L
+; CHECK:      .long .debug_abbrev  # Offset Into Abbrev. Section
+; CHECK-NOT:  .L
+; CHECK:      .long .debug_line    # DW_AT_stmt_list
+; CHECK-NOT:  .L
+; CHECK:      .quad .debug_info+{{[0-9]+}} # DW_AT_type
+; CHECK-NOT:  .L
+; CHECK:      .byte 0              # End Of Children Mark
+; CHECK-NOT:  .L
+
+source_filename = "test/DebugInfo/X86/sections_as_references.ll"
+
+%struct.foo = type { i8 }
+
+@f = global %struct.foo zeroinitializer, align 1, !dbg !0
+@g = global %struct.foo zeroinitializer, align 1, !dbg !6
+
+!llvm.dbg.cu = !{!9, !12}
+!llvm.module.flags = !{!14, !15}
+
+!0 = !DIGlobalVariableExpression(var: !1, expr: !DIExpression())
+!1 = !DIGlobalVariable(name: "f", scope: null, file: !2, line: 2, type: !3, isLocal: false, isDefinition: true)
+!2 = !DIFile(filename: "tu1.cpp", directory: "/dir")
+!3 = !DICompositeType(tag: DW_TAG_structure_type, name: "foo", file: !4, line: 1, size: 8, align: 8, elements: !5, identifier: "_ZTS3foo")
+!4 = !DIFile(filename: "./hdr.h", directory: "/dir")
+!5 = !{}
+!6 = !DIGlobalVariableExpression(var: !7, expr: !DIExpression())
+!7 = !DIGlobalVariable(name: "g", scope: null, file: !8, line: 2, type: !3, isLocal: false, isDefinition: true)
+!8 = !DIFile(filename: "tu2.cpp", directory: "/dir")
+!9 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: !2, producer: "clang", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !5, retainedTypes: !10, globals: !11, imports: !5)
+!10 = !{!3}
+!11 = !{!0}
+!12 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: !8, producer: "clang", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !5, retainedTypes: !10, globals: !13, imports: !5)
+!13 = !{!6}
+!14 = !{i32 2, !"Dwarf Version", i32 2}
+!15 = !{i32 1, !"Debug Info Version", i32 3}
+