Emit .debug$H section in clang

[zturner]

Summary

This causes emission of .debug$H section unconditionally when -gcodeview is present.

I ran some benchmarks on a self-hosted build and found a ~2% increase in overall build time (within the margin of error, so basically noise), and a ~15% increase in object file size, on average.

Currently the linker doesn't use this, that will work will come in a followup patch. For now, we're just testing that clang emits it correctly with some obj2yaml and llc based tests.

[compnerd]

I really wish that we had some way to indicate that this is a LLVM extension.

llvm/include/llvm/DebugInfo/CodeView/TypeHashing.h

59

Do you mind using Algorithm. I really don't like the abbreviation, especially since the name is already fairly lengthy.

[rnk]

I think we'll need some flag for this. A 15% object file size increase is quite a bit, and it will probably slow down linking with MSVC, which is what most Chromium developers are using today. I suspect you'll want to land this, and then @hans will want to roll clang while you're on vacation, and there is some risk that the larger object files will cause problems and he'll need to revert it.. A hidden cl::opt/-mllvm flag would be fine for this, and we can add a real clang flag later (it would add a module flag like /guard:cf).

llvm/lib/CodeGen/AsmPrinter/CodeViewDebug.cpp

494

I'd check the cl::opt here.

564–566

I bet we can remove this check now that we have the if (TypeTable.empty()) return; line. Want to give it a try here and above in emitTypeInformation?

llvm/test/DebugInfo/COFF/global-type-hashes.ll

1

I'd do an assembly emission test as well, just to get code coverage of the verbose asm comment code. You only really need to look for .debug$H and a few hashes after that.

[hans]

In D40917#954173, @rnk wrote:

I think we'll need some flag for this. A 15% object file size increase is quite a bit, and it will probably slow down linking with MSVC, which is what most Chromium developers are using today. I suspect you'll want to land this, and then @hans will want to roll clang while you're on vacation, and there is some risk that the larger object files will cause problems and he'll need to revert it.

+1 A flag would be great, and we can enable it by default for Chromium's win/lld builds to get coverage without potentially disrupting developers using link.exe.

[zturner]

Added a command line option and an assembly test.

[rnk]

lgtm!

llvm/include/llvm/DebugInfo/CodeView/GlobalTypeTableBuilder.h

+//===- GlobalTypeTableBuilder.h ----------------------------------*- C++-*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_DEBUGINFO_CODEVIEW_GLOBALTYPETABLEBUILDER_H
+#define LLVM_DEBUGINFO_CODEVIEW_GLOBALTYPETABLEBUILDER_H
+
+#include "llvm/ADT/ArrayRef.h"
+#include "llvm/ADT/DenseSet.h"
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/DebugInfo/CodeView/CodeView.h"
+#include "llvm/DebugInfo/CodeView/SimpleTypeSerializer.h"
+#include "llvm/DebugInfo/CodeView/TypeCollection.h"
+#include "llvm/DebugInfo/CodeView/TypeHashing.h"
+#include "llvm/DebugInfo/CodeView/TypeIndex.h"
+#include "llvm/Support/Allocator.h"
+#include <cassert>
+#include <cstdint>
+#include <memory>
+#include <vector>
+
+namespace llvm {
+namespace codeview {
+
+class ContinuationRecordBuilder;
+
+class GlobalTypeTableBuilder : public TypeCollection {
+  /// Storage for records.  These need to outlive the TypeTableBuilder.
+  BumpPtrAllocator &RecordStorage;
+
+  /// A serializer that can write non-continuation leaf types.  Only used as
+  /// a convenience function so that we can provide an interface method to
+  /// write an unserialized record.
+  SimpleTypeSerializer SimpleSerializer;
+
+  /// Hash table.
+  DenseMap<GloballyHashedType, TypeIndex> HashedRecords;
+
+  /// Contains a list of all records indexed by TypeIndex.toArrayIndex().
+  SmallVector<ArrayRef<uint8_t>, 2> SeenRecords;
+
+  /// Contains a list of all hash values inexed by TypeIndex.toArrayIndex().
+  SmallVector<GloballyHashedType, 2> SeenHashes;
+
+public:
+  explicit GlobalTypeTableBuilder(BumpPtrAllocator &Storage);
+  ~GlobalTypeTableBuilder();
+
+  // TypeTableCollection overrides
+  Optional<TypeIndex> getFirst() override;
+  Optional<TypeIndex> getNext(TypeIndex Prev) override;
+  CVType getType(TypeIndex Index) override;
+  StringRef getTypeName(TypeIndex Index) override;
+  bool contains(TypeIndex Index) override;
+  uint32_t size() override;
+  uint32_t capacity() override;
+
+  // public interface
+  void reset();
+  TypeIndex nextTypeIndex() const;
+
+  BumpPtrAllocator &getAllocator() { return RecordStorage; }
+
+  ArrayRef<ArrayRef<uint8_t>> records() const;
+  ArrayRef<GloballyHashedType> hashes() const;
+
+  using CreateRecord = llvm::function_ref<ArrayRef<uint8_t>()>;
+
+  TypeIndex insertRecordAs(GloballyHashedType Hash, CreateRecord Create);
+  TypeIndex insertRecordBytes(ArrayRef<uint8_t> Data);
+  TypeIndex insertRecord(ContinuationRecordBuilder &Builder);
+
+  template <typename T> TypeIndex writeLeafType(T &Record) {
+    ArrayRef<uint8_t> Data = SimpleSerializer.serialize(Record);
+    return insertRecordBytes(Data);
+  }
+};
+
+} // end namespace codeview
+} // end namespace llvm
+
+#endif // LLVM_DEBUGINFO_CODEVIEW_MERGINGTYPETABLEBUILDER_H

llvm/include/llvm/DebugInfo/CodeView/TypeHashing.h

hashTypeCollection(TypeCollection &Types) {
     std::vector<LocallyHashedType> Hashes;
     Types.ForEachRecord([&Hashes](TypeIndex TI, const CVType &Type) {
       Hashes.push_back(hashType(Type.RecordData));
     });
     return Hashes;
   }
 };
 
+enum class GlobalTypeHashAlg : uint16_t { SHA1 = 0 };
+
 /// A globally hashed type represents a hash value that is sufficient to
 /// uniquely identify a record across multiple type streams or type sequences.
 /// This works by, for any given record A which references B, replacing the
 /// TypeIndex that refers to B with a previously-computed global hash for B.  As
 /// this is a recursive algorithm (e.g. the global hash of B also depends on the
 /// global hashes of the types that B refers to), a global hash can uniquely
 /// identify identify that A occurs in another stream that has a completely
 /// different graph structure.  Although the hash itself is slower to compute,

llvm/include/llvm/MC/MCObjectFileInfo.h

protected:
   /// Section for newer gnu pubtypes.
   MCSection *DwarfGnuPubTypesSection;
 
   // Section for Swift AST
   MCSection *DwarfSwiftASTSection;
 
   MCSection *COFFDebugSymbolsSection;
   MCSection *COFFDebugTypesSection;
+  MCSection *COFFGlobalTypeHashesSection;
 
   /// Extra TLS Variable Data section.
   ///
   /// If the target needs to put additional information for a TLS variable,
   /// it'll go here.
   MCSection *TLSExtraDataSection;
 
   /// Section directive for Thread Local data. ELF, MachO, COFF, and Wasm.
public:
   MCSection *getDwarfSwiftASTSection() const { return DwarfSwiftASTSection; }
 
   MCSection *getCOFFDebugSymbolsSection() const {
     return COFFDebugSymbolsSection;
   }
   MCSection *getCOFFDebugTypesSection() const {
     return COFFDebugTypesSection;
   }
-
+  MCSection *getCOFFGlobalTypeHashesSection() const {
+    return COFFGlobalTypeHashesSection;
+  }
 
   MCSection *getTLSExtraDataSection() const { return TLSExtraDataSection; }
   const MCSection *getTLSDataSection() const { return TLSDataSection; }
   MCSection *getTLSBSSSection() const { return TLSBSSSection; }
 
   MCSection *getStackMapSection() const { return StackMapSection; }
   MCSection *getFaultMapSection() const { return FaultMapSection; }
 

llvm/lib/CodeGen/AsmPrinter/CodeViewDebug.h

 #include "DebugHandlerBase.h"
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/DenseSet.h"
 #include "llvm/ADT/MapVector.h"
 #include "llvm/ADT/SetVector.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/DebugInfo/CodeView/CodeView.h"
-#include "llvm/DebugInfo/CodeView/MergingTypeTableBuilder.h"
+#include "llvm/DebugInfo/CodeView/GlobalTypeTableBuilder.h"
 #include "llvm/DebugInfo/CodeView/TypeIndex.h"
 #include "llvm/IR/DebugLoc.h"
 #include "llvm/Support/Allocator.h"
 #include "llvm/Support/Compiler.h"
 #include <cstdint>
 #include <map>
 #include <string>
 #include <tuple>

 class MCStreamer;
 class MCSymbol;
 class MachineFunction;
 
 /// \brief Collects and handles line tables information in a CodeView format.
 class LLVM_LIBRARY_VISIBILITY CodeViewDebug : public DebugHandlerBase {
   MCStreamer &OS;
   BumpPtrAllocator Allocator;
-  codeview::MergingTypeTableBuilder TypeTable;
+  codeview::GlobalTypeTableBuilder TypeTable;
 
   /// Represents the most general definition range.
   struct LocalVarDefRange {
     /// Indicates that variable data is stored in memory relative to the
     /// specified register.
     int InMemory : 1;
 
     /// Offset of variable data in memory.
class LLVM_LIBRARY_VISIBILITY CodeViewDebug : public DebugHandlerBase {
   }
 
   /// Emit the magic version number at the start of a CodeView type or symbol
   /// section. Appears at the front of every .debug$S or .debug$T section.
   void emitCodeViewMagicVersion();
 
   void emitTypeInformation();
 
+  void emitTypeGlobalHashes();
+
   void emitCompilerInformation();
 
   void emitInlineeLinesSubsection();
 
   void emitDebugInfoForFunction(const Function *GV, FunctionInfo &FI);
 
   void emitDebugInfoForGlobals();
 

llvm/lib/CodeGen/AsmPrinter/CodeViewDebug.cpp

 #include "llvm/MC/MCSymbol.h"
 #include "llvm/Support/BinaryByteStream.h"
 #include "llvm/Support/BinaryStreamReader.h"
 #include "llvm/Support/Casting.h"
 #include "llvm/Support/Compiler.h"
 #include "llvm/Support/Endian.h"
 #include "llvm/Support/Error.h"
 #include "llvm/Support/ErrorHandling.h"
+#include "llvm/Support/FormatVariadic.h"
 #include "llvm/Support/SMLoc.h"
 #include "llvm/Support/ScopedPrinter.h"
 #include "llvm/Target/TargetMachine.h"
 #include <algorithm>
 #include <cassert>
 #include <cctype>
 #include <cstddef>
 #include <cstdint>
void CodeViewDebug::endModule() {
   // This subsection holds a file index to offset in string table table.
   OS.AddComment("File index to string table offset subsection");
   OS.EmitCVFileChecksumsDirective();
 
   // This subsection holds the string table.
   OS.AddComment("String table");
   OS.EmitCVStringTableDirective();
 
-  // Emit type information last, so that any types we translate while emitting
-  // function info are included.
+  // Emit type information and hashes last, so that any types we translate while
+  // emitting function info are included.
   emitTypeInformation();
 
+  emitTypeGlobalHashes();
+
   clear();
 }
 
 static void emitNullTerminatedSymbolName(MCStreamer &OS, StringRef S) {
   // The maximum CV record length is 0xFF00. Most of the strings we emit appear
   // after a fixed length portion of the record. The fixed length portion should
   // always be less than 0xF00 (3840) bytes, so truncate the string so that the
   // overall record size is less than the maximum allowed.
if (OS.isVerboseAsm()) {
       OS.emitRawComment(
           CommentOS.str().drop_front(CommentPrefix.size() - 1).rtrim());
     }
     OS.EmitBinaryData(Record.str_data());
     B = Table.getNext(*B);
   }
 }
 
+void CodeViewDebug::emitTypeGlobalHashes() {
+  // Do nothing if we have no debug info or if no non-trivial types were emitted
+  // to TypeTable during codegen.
+  NamedMDNode *CU_Nodes = MMI->getModule()->getNamedMetadata("llvm.dbg.cu");
+  if (!CU_Nodes)
+    return;
+  if (TypeTable.empty())
+    return;
+
+  // Start the .debug$H section with the version and hash algorithm, currently
+  // hardcoded to version 0, SHA1.
+  OS.SwitchSection(Asm->getObjFileLowering().getCOFFGlobalTypeHashesSection());
+
+  OS.EmitValueToAlignment(4);
+  OS.AddComment("Magic");
+  OS.EmitIntValue(COFF::DEBUG_HASHES_SECTION_MAGIC, 4);
+  OS.AddComment("Section Version");
+  OS.EmitIntValue(0, 2);
+  OS.AddComment("Hash Algorithm");
+  OS.EmitIntValue(uint16_t(GlobalTypeHashAlg::SHA1), 2);
+
+  TypeIndex TI(TypeIndex::FirstNonSimpleIndex);
+  for (const auto &GHR : TypeTable.hashes()) {
+    if (OS.isVerboseAsm()) {
+      // Emit an EOL-comment describing which TypeIndex this hash corresponds
+      // to, as well as the stringified SHA1 hash.
+      SmallString<32> Comment;
+      raw_svector_ostream CommentOS(Comment);
+      CommentOS << formatv("{0:X+} [{1}]", TI.getIndex(), GHR);
+      OS.AddComment(Comment);
+      ++TI;
+    }
+    assert(GHR.Hash.size() % 20 == 0);
+    StringRef S(reinterpret_cast<const char *>(GHR.Hash.data()),
+                GHR.Hash.size());
+    OS.EmitBinaryData(S);
+  }
+}
+
 static SourceLanguage MapDWLangToCVLang(unsigned DWLang) {
   switch (DWLang) {
   case dwarf::DW_LANG_C:
   case dwarf::DW_LANG_C89:
   case dwarf::DW_LANG_C99:
   case dwarf::DW_LANG_C11:
   case dwarf::DW_LANG_ObjC:
     return SourceLanguage::C;

llvm/lib/DebugInfo/CodeView/CMakeLists.txt

add_llvm_library(LLVMDebugInfoCodeView
   DebugStringTableSubsection.cpp
   DebugSubsection.cpp
   DebugSubsectionRecord.cpp
   DebugSubsectionVisitor.cpp
   DebugSymbolRVASubsection.cpp
   DebugSymbolsSubsection.cpp
   EnumTables.cpp
   Formatters.cpp
+  GlobalTypeTableBuilder.cpp
   LazyRandomTypeCollection.cpp
   Line.cpp
   MergingTypeTableBuilder.cpp
   RecordName.cpp
   RecordSerialization.cpp
   SimpleTypeSerializer.cpp
   StringsAndChecksums.cpp
   SymbolRecordMapping.cpp

llvm/lib/DebugInfo/CodeView/GlobalTypeTableBuilder.cpp

+//===- GlobalTypeTableBuilder.cpp -----------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/DebugInfo/CodeView/GlobalTypeTableBuilder.h"
+#include "llvm/ADT/ArrayRef.h"
+#include "llvm/ADT/DenseSet.h"
+#include "llvm/ADT/STLExtras.h"
+#include "llvm/DebugInfo/CodeView/CodeView.h"
+#include "llvm/DebugInfo/CodeView/ContinuationRecordBuilder.h"
+#include "llvm/DebugInfo/CodeView/RecordSerialization.h"
+#include "llvm/DebugInfo/CodeView/TypeIndex.h"
+#include "llvm/Support/Allocator.h"
+#include "llvm/Support/BinaryByteStream.h"
+#include "llvm/Support/BinaryStreamWriter.h"
+#include "llvm/Support/Endian.h"
+#include "llvm/Support/Error.h"
+#include <algorithm>
+#include <cassert>
+#include <cstdint>
+#include <cstring>
+
+using namespace llvm;
+using namespace llvm::codeview;
+
+TypeIndex GlobalTypeTableBuilder::nextTypeIndex() const {
+  return TypeIndex::fromArrayIndex(SeenRecords.size());
+}
+
+GlobalTypeTableBuilder::GlobalTypeTableBuilder(BumpPtrAllocator &Storage)
+    : RecordStorage(Storage) {
+  SeenRecords.reserve(4096);
+}
+
+GlobalTypeTableBuilder::~GlobalTypeTableBuilder() = default;
+
+Optional<TypeIndex> GlobalTypeTableBuilder::getFirst() {
+  if (empty())
+    return None;
+
+  return TypeIndex(TypeIndex::FirstNonSimpleIndex);
+}
+
+Optional<TypeIndex> GlobalTypeTableBuilder::getNext(TypeIndex Prev) {
+  if (++Prev == nextTypeIndex())
+    return None;
+  return Prev;
+}
+
+CVType GlobalTypeTableBuilder::getType(TypeIndex Index) {
+  CVType Type;
+  Type.RecordData = SeenRecords[Index.toArrayIndex()];
+  const RecordPrefix *P =
+      reinterpret_cast<const RecordPrefix *>(Type.RecordData.data());
+  Type.Type = static_cast<TypeLeafKind>(uint16_t(P->RecordKind));
+  return Type;
+}
+
+StringRef GlobalTypeTableBuilder::getTypeName(TypeIndex Index) {
+  llvm_unreachable("Method not implemented");
+}
+
+bool GlobalTypeTableBuilder::contains(TypeIndex Index) {
+  if (Index.isSimple() || Index.isNoneType())
+    return false;
+
+  return Index.toArrayIndex() < SeenRecords.size();
+}
+
+uint32_t GlobalTypeTableBuilder::size() { return SeenRecords.size(); }
+
+uint32_t GlobalTypeTableBuilder::capacity() { return SeenRecords.size(); }
+
+ArrayRef<ArrayRef<uint8_t>> GlobalTypeTableBuilder::records() const {
+  return SeenRecords;
+}
+
+ArrayRef<GloballyHashedType> GlobalTypeTableBuilder::hashes() const {
+  return SeenHashes;
+}
+
+void GlobalTypeTableBuilder::reset() {
+  HashedRecords.clear();
+  SeenRecords.clear();
+}
+
+static inline ArrayRef<uint8_t> stabilize(BumpPtrAllocator &Alloc,
+                                          ArrayRef<uint8_t> Data) {
+  uint8_t *Stable = Alloc.Allocate<uint8_t>(Data.size());
+  memcpy(Stable, Data.data(), Data.size());
+  return makeArrayRef(Stable, Data.size());
+}
+
+TypeIndex GlobalTypeTableBuilder::insertRecordAs(GloballyHashedType Hash,
+                                                 CreateRecord Create) {
+  auto Result = HashedRecords.try_emplace(Hash, nextTypeIndex());
+
+  if (Result.second) {
+    ArrayRef<uint8_t> RecordData = stabilize(RecordStorage, Create());
+    SeenRecords.push_back(RecordData);
+    SeenHashes.push_back(Hash);
+  }
+
+  // Update the caller's copy of Record to point a stable copy.
+  return Result.first->second;
+}
+
+TypeIndex GlobalTypeTableBuilder::insertRecordBytes(ArrayRef<uint8_t> Record) {
+  GloballyHashedType GHT =
+      GloballyHashedType::hashType(Record, SeenHashes, SeenHashes);
+  return insertRecordAs(GHT, [Record]() { return Record; });
+}
+
+TypeIndex
+GlobalTypeTableBuilder::insertRecord(ContinuationRecordBuilder &Builder) {
+  TypeIndex TI;
+  auto Fragments = Builder.end(nextTypeIndex());
+  assert(!Fragments.empty());
+  for (auto C : Fragments)
+    TI = insertRecordBytes(C.RecordData);
+  return TI;
+}

llvm/lib/MC/MCObjectFileInfo.cpp

= Ctx->getMachOSection("__DATA", "__thread_ptr",
                            SectionKind::getMetadata());
 
   // Exception Handling.
   LSDASection = Ctx->getMachOSection("__TEXT", "__gcc_except_tab", 0,
                                      SectionKind::getReadOnlyWithRel());
 
   COFFDebugSymbolsSection = nullptr;
   COFFDebugTypesSection = nullptr;
+  COFFGlobalTypeHashesSection = nullptr;
 
   if (useCompactUnwind(T)) {
     CompactUnwindSection =
         Ctx->getMachOSection("__LD", "__compact_unwind", MachO::S_ATTR_DEBUG,
                              SectionKind::getReadOnly());
 
     if (T.getArch() == Triple::x86_64 || T.getArch() == Triple::x86)
       CompactUnwindDwarfEHFrameOnly = 0x04000000;  // UNWIND_X86_64_MODE_DWARF
COFFDebugSymbolsSection =
                                        COFF::IMAGE_SCN_CNT_INITIALIZED_DATA |
                                        COFF::IMAGE_SCN_MEM_READ),
                           SectionKind::getMetadata());
   COFFDebugTypesSection =
       Ctx->getCOFFSection(".debug$T", (COFF::IMAGE_SCN_MEM_DISCARDABLE |
                                        COFF::IMAGE_SCN_CNT_INITIALIZED_DATA |
                                        COFF::IMAGE_SCN_MEM_READ),
                           SectionKind::getMetadata());
+  COFFGlobalTypeHashesSection = Ctx->getCOFFSection(
+      ".debug$H",
+      (COFF::IMAGE_SCN_MEM_DISCARDABLE | COFF::IMAGE_SCN_CNT_INITIALIZED_DATA |
+       COFF::IMAGE_SCN_MEM_READ),
+      SectionKind::getMetadata());
 
   DwarfAbbrevSection = Ctx->getCOFFSection(
       ".debug_abbrev",
       COFF::IMAGE_SCN_MEM_DISCARDABLE | COFF::IMAGE_SCN_CNT_INITIALIZED_DATA |
           COFF::IMAGE_SCN_MEM_READ,
       SectionKind::getMetadata(), "section_abbrev");
   DwarfInfoSection = Ctx->getCOFFSection(
       ".debug_info",

llvm/lib/ObjectYAML/CodeViewYAMLTypeHashing.cpp

 StringRef ScalarTraits<GlobalHash>::input(StringRef Scalar, void *Ctx,
                                           GlobalHash &GH) {
   return ScalarTraits<BinaryRef>::input(Scalar, Ctx, GH.Hash);
 }
 
 } // end namespace yaml
 } // end namespace llvm
 
-DebugHSection llvm::CodeViewYAML::fromDebugH(ArrayRef<uint8_t> DebugT) {
-  assert(DebugT.size() >= 8);
-  assert((DebugT.size() - 8) % 20 == 0);
+DebugHSection llvm::CodeViewYAML::fromDebugH(ArrayRef<uint8_t> DebugH) {
+  assert(DebugH.size() >= 8);
+  assert((DebugH.size() - 8) % 20 == 0);
 
-  BinaryStreamReader Reader(DebugT, llvm::support::little);
+  BinaryStreamReader Reader(DebugH, llvm::support::little);
   DebugHSection DHS;
   cantFail(Reader.readInteger(DHS.Magic));
   cantFail(Reader.readInteger(DHS.Version));
   cantFail(Reader.readInteger(DHS.HashAlgorithm));
   while (Reader.bytesRemaining() != 0) {
     ArrayRef<uint8_t> S;
     cantFail(Reader.readBytes(S, 20));
     DHS.Hashes.emplace_back(S);

llvm/test/DebugInfo/COFF/global-type-hashes.ll

+; RUN: llc -filetype=obj < %s > %t.obj
+; RUN: obj2yaml %t.obj | FileCheck %s --check-prefix=YAML
+
+; C++ source to regenerate:
+; $ cat t.cpp
+; struct Foo {
+;   Foo(int x, int y) : X(x), Y(y) {}
+;   int method() { return X + Y; }
+;   int X;
+;   int Y;
+; };
+; int main(int argc, char **argv) {
+;   Foo F {argc, argc};
+;   return F.method();
+; };
+; $ clang-cc1 -triple i686-pc-windows-msvc19.11.25547 -emit-llvm -gcodeview \
+;   -debug-info-kind=limited -std=c++14 foo.cpp
+;
+
+
+; ModuleID = 'foo.cpp'
+source_filename = "foo.cpp"
+target datalayout = "e-m:x-p:32:32-i64:64-f80:32-n8:16:32-a:0:32-S32"
+target triple = "i686-pc-windows-msvc19.11.25547"
+
+%struct.Foo = type { i32, i32 }
+
+$"\01??0Foo@@QAE@HH@Z" = comdat any
+
+$"\01?method@Foo@@QAEHXZ" = comdat any
+
+; Function Attrs: noinline norecurse nounwind optnone
+define i32 @main(i32 %argc, i8** %argv) #0 !dbg !8 {
+entry:
+  %retval = alloca i32, align 4
+  %argv.addr = alloca i8**, align 4
+  %argc.addr = alloca i32, align 4
+  %F = alloca %struct.Foo, align 4
+  store i32 0, i32* %retval, align 4
+  store i8** %argv, i8*** %argv.addr, align 4
+  call void @llvm.dbg.declare(metadata i8*** %argv.addr, metadata !16, metadata !DIExpression()), !dbg !17
+  store i32 %argc, i32* %argc.addr, align 4
+  call void @llvm.dbg.declare(metadata i32* %argc.addr, metadata !18, metadata !DIExpression()), !dbg !17
+  call void @llvm.dbg.declare(metadata %struct.Foo* %F, metadata !19, metadata !DIExpression()), !dbg !31
+  %0 = load i32, i32* %argc.addr, align 4, !dbg !31
+  %1 = load i32, i32* %argc.addr, align 4, !dbg !31
+  %call = call x86_thiscallcc %struct.Foo* @"\01??0Foo@@QAE@HH@Z"(%struct.Foo* %F, i32 %0, i32 %1), !dbg !31
+  %call1 = call x86_thiscallcc i32 @"\01?method@Foo@@QAEHXZ"(%struct.Foo* %F), !dbg !32
+  ret i32 %call1, !dbg !32
+}
+
+; Function Attrs: nounwind readnone speculatable
+declare void @llvm.dbg.declare(metadata, metadata, metadata) #1
+
+; Function Attrs: noinline nounwind optnone
+define linkonce_odr x86_thiscallcc %struct.Foo* @"\01??0Foo@@QAE@HH@Z"(%struct.Foo* returned %this, i32 %x, i32 %y) unnamed_addr #2 comdat align 2 !dbg !33 {
+entry:
+  %y.addr = alloca i32, align 4
+  %x.addr = alloca i32, align 4
+  %this.addr = alloca %struct.Foo*, align 4
+  store i32 %y, i32* %y.addr, align 4
+  call void @llvm.dbg.declare(metadata i32* %y.addr, metadata !34, metadata !DIExpression()), !dbg !35
+  store i32 %x, i32* %x.addr, align 4
+  call void @llvm.dbg.declare(metadata i32* %x.addr, metadata !36, metadata !DIExpression()), !dbg !35
+  store %struct.Foo* %this, %struct.Foo** %this.addr, align 4
+  call void @llvm.dbg.declare(metadata %struct.Foo** %this.addr, metadata !37, metadata !DIExpression()), !dbg !39
+  %this1 = load %struct.Foo*, %struct.Foo** %this.addr, align 4
+  %X = getelementptr inbounds %struct.Foo, %struct.Foo* %this1, i32 0, i32 0, !dbg !35
+  %0 = load i32, i32* %x.addr, align 4, !dbg !35
+  store i32 %0, i32* %X, align 4, !dbg !35
+  %Y = getelementptr inbounds %struct.Foo, %struct.Foo* %this1, i32 0, i32 1, !dbg !35
+  %1 = load i32, i32* %y.addr, align 4, !dbg !35
+  store i32 %1, i32* %Y, align 4, !dbg !35
+  ret %struct.Foo* %this1, !dbg !35
+}
+
+; Function Attrs: noinline nounwind optnone
+define linkonce_odr x86_thiscallcc i32 @"\01?method@Foo@@QAEHXZ"(%struct.Foo* %this) #2 comdat align 2 !dbg !40 {
+entry:
+  %this.addr = alloca %struct.Foo*, align 4
+  store %struct.Foo* %this, %struct.Foo** %this.addr, align 4
+  call void @llvm.dbg.declare(metadata %struct.Foo** %this.addr, metadata !41, metadata !DIExpression()), !dbg !42
+  %this1 = load %struct.Foo*, %struct.Foo** %this.addr, align 4
+  %X = getelementptr inbounds %struct.Foo, %struct.Foo* %this1, i32 0, i32 0, !dbg !43
+  %0 = load i32, i32* %X, align 4, !dbg !43
+  %Y = getelementptr inbounds %struct.Foo, %struct.Foo* %this1, i32 0, i32 1, !dbg !43
+  %1 = load i32, i32* %Y, align 4, !dbg !43
+  %add = add nsw i32 %0, %1, !dbg !43
+  ret i32 %add, !dbg !43
+}
+
+attributes #0 = { noinline norecurse nounwind optnone "correctly-rounded-divide-sqrt-fp-math"="false" "disable-tail-calls"="false" "less-precise-fpmad"="false" "no-frame-pointer-elim"="false" "no-infs-fp-math"="false" "no-jump-tables"="false" "no-nans-fp-math"="false" "no-signed-zeros-fp-math"="false" "no-trapping-math"="false" "stack-protector-buffer-size"="8" "target-features"="+x87" "unsafe-fp-math"="false" "use-soft-float"="false" }
+attributes #1 = { nounwind readnone speculatable }
+attributes #2 = { noinline nounwind optnone "correctly-rounded-divide-sqrt-fp-math"="false" "disable-tail-calls"="false" "less-precise-fpmad"="false" "no-frame-pointer-elim"="false" "no-infs-fp-math"="false" "no-jump-tables"="false" "no-nans-fp-math"="false" "no-signed-zeros-fp-math"="false" "no-trapping-math"="false" "stack-protector-buffer-size"="8" "target-features"="+x87" "unsafe-fp-math"="false" "use-soft-float"="false" }
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4, !5, !6}
+!llvm.ident = !{!7}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: !1, producer: "clang version 6.0.0 ", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !2)
+!1 = !DIFile(filename: "<stdin>", directory: "D:\5Csrc\5Cllvmbuild\5Cclang\5CDebug\5Cx86", checksumkind: CSK_MD5, checksum: "6279449503d9075c38e615e8387667c3")
+!2 = !{}
+!3 = !{i32 1, !"NumRegisterParameters", i32 0}
+!4 = !{i32 2, !"CodeView", i32 1}
+!5 = !{i32 2, !"Debug Info Version", i32 3}
+!6 = !{i32 1, !"wchar_size", i32 2}
+!7 = !{!"clang version 6.0.0 "}
+!8 = distinct !DISubprogram(name: "main", scope: !9, file: !9, line: 8, type: !10, isLocal: false, isDefinition: true, scopeLine: 8, flags: DIFlagPrototyped, isOptimized: false, unit: !0, variables: !2)
+!9 = !DIFile(filename: "foo.cpp", directory: "D:\5Csrc\5Cllvmbuild\5Cclang\5CDebug\5Cx86", checksumkind: CSK_MD5, checksum: "6279449503d9075c38e615e8387667c3")
+!10 = !DISubroutineType(types: !11)
+!11 = !{!12, !12, !13}
+!12 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!13 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !14, size: 32)
+!14 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !15, size: 32)
+!15 = !DIBasicType(name: "char", size: 8, encoding: DW_ATE_signed_char)
+!16 = !DILocalVariable(name: "argv", arg: 2, scope: !8, file: !9, line: 8, type: !13)
+!17 = !DILocation(line: 8, scope: !8)
+!18 = !DILocalVariable(name: "argc", arg: 1, scope: !8, file: !9, line: 8, type: !12)
+!19 = !DILocalVariable(name: "F", scope: !8, file: !9, line: 9, type: !20)
+!20 = distinct !DICompositeType(tag: DW_TAG_structure_type, name: "Foo", file: !9, line: 1, size: 64, elements: !21, identifier: ".?AUFoo@@")
+!21 = !{!22, !23, !24, !28}
+!22 = !DIDerivedType(tag: DW_TAG_member, name: "X", scope: !20, file: !9, line: 4, baseType: !12, size: 32)
+!23 = !DIDerivedType(tag: DW_TAG_member, name: "Y", scope: !20, file: !9, line: 5, baseType: !12, size: 32, offset: 32)
+!24 = !DISubprogram(name: "Foo", scope: !20, file: !9, line: 2, type: !25, isLocal: false, isDefinition: false, scopeLine: 2, flags: DIFlagPrototyped, isOptimized: false)
+!25 = !DISubroutineType(cc: DW_CC_BORLAND_thiscall, types: !26)
+!26 = !{null, !27, !12, !12}
+!27 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !20, size: 32, flags: DIFlagArtificial | DIFlagObjectPointer)
+!28 = !DISubprogram(name: "method", linkageName: "\01?method@Foo@@QAEHXZ", scope: !20, file: !9, line: 3, type: !29, isLocal: false, isDefinition: false, scopeLine: 3, flags: DIFlagPrototyped, isOptimized: false)
+!29 = !DISubroutineType(cc: DW_CC_BORLAND_thiscall, types: !30)
+!30 = !{!12, !27}
+!31 = !DILocation(line: 9, scope: !8)
+!32 = !DILocation(line: 10, scope: !8)
+!33 = distinct !DISubprogram(name: "Foo", linkageName: "\01??0Foo@@QAE@HH@Z", scope: !20, file: !9, line: 2, type: !25, isLocal: false, isDefinition: true, scopeLine: 2, flags: DIFlagPrototyped, isOptimized: false, unit: !0, declaration: !24, variables: !2)
+!34 = !DILocalVariable(name: "y", arg: 3, scope: !33, file: !9, line: 2, type: !12)
+!35 = !DILocation(line: 2, scope: !33)
+!36 = !DILocalVariable(name: "x", arg: 2, scope: !33, file: !9, line: 2, type: !12)
+!37 = !DILocalVariable(name: "this", arg: 1, scope: !33, type: !38, flags: DIFlagArtificial | DIFlagObjectPointer)
+!38 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !20, size: 32)
+!39 = !DILocation(line: 0, scope: !33)
+!40 = distinct !DISubprogram(name: "method", linkageName: "\01?method@Foo@@QAEHXZ", scope: !20, file: !9, line: 3, type: !29, isLocal: false, isDefinition: true, scopeLine: 3, flags: DIFlagPrototyped, isOptimized: false, unit: !0, declaration: !28, variables: !2)
+!41 = !DILocalVariable(name: "this", arg: 1, scope: !40, type: !38, flags: DIFlagArtificial | DIFlagObjectPointer)
+!42 = !DILocation(line: 0, scope: !40)
+!43 = !DILocation(line: 3, scope: !40)
+
+
+; YAML: --- !COFF
+; YAML: header:
+; YAML:   Machine:         IMAGE_FILE_MACHINE_I386
+; YAML:   Characteristics: [  ]
+; YAML: sections:
+; YAML:   - Name:            '.debug$T'
+; YAML:     Characteristics: [ IMAGE_SCN_CNT_INITIALIZED_DATA, IMAGE_SCN_MEM_DISCARDABLE, IMAGE_SCN_MEM_READ ]
+; YAML:     Alignment:       4
+; YAML:     Types:
+; YAML:       - Kind:            LF_POINTER
+; YAML:         Pointer:
+; YAML:           ReferentType:    1136
+; YAML:           Attrs:           32778
+; YAML:       - Kind:            LF_ARGLIST
+; YAML:         ArgList:
+; YAML:           ArgIndices:      [ 116, 4096 ]
+; YAML:       - Kind:            LF_PROCEDURE
+; YAML:         Procedure:
+; YAML:           ReturnType:      116
+; YAML:           CallConv:        NearC
+; YAML:           Options:         [ None ]
+; YAML:           ParameterCount:  2
+; YAML:           ArgumentList:    4097
+; YAML:       - Kind:            LF_FUNC_ID
+; YAML:         FuncId:
+; YAML:           ParentScope:     0
+; YAML:           FunctionType:    4098
+; YAML:           Name:            main
+; YAML:       - Kind:            LF_STRUCTURE
+; YAML:         Class:
+; YAML:           MemberCount:     0
+; YAML:           Options:         [ None, ForwardReference, HasUniqueName ]
+; YAML:           FieldList:       0
+; YAML:           Name:            Foo
+; YAML:           UniqueName:      '.?AUFoo@@'
+; YAML:           DerivationList:  0
+; YAML:           VTableShape:     0
+; YAML:           Size:            0
+; YAML:       - Kind:            LF_POINTER
+; YAML:         Pointer:
+; YAML:           ReferentType:    4100
+; YAML:           Attrs:           32778
+; YAML:       - Kind:            LF_ARGLIST
+; YAML:         ArgList:
+; YAML:           ArgIndices:      [ 116, 116 ]
+; YAML:       - Kind:            LF_MFUNCTION
+; YAML:         MemberFunction:
+; YAML:           ReturnType:      3
+; YAML:           ClassType:       4100
+; YAML:           ThisType:        4101
+; YAML:           CallConv:        ThisCall
+; YAML:           Options:         [ None ]
+; YAML:           ParameterCount:  2
+; YAML:           ArgumentList:    4102
+; YAML:           ThisPointerAdjustment: 0
+; YAML:       - Kind:            LF_ARGLIST
+; YAML:         ArgList:
+; YAML:           ArgIndices:      [  ]
+; YAML:       - Kind:            LF_MFUNCTION
+; YAML:         MemberFunction:
+; YAML:           ReturnType:      116
+; YAML:           ClassType:       4100
+; YAML:           ThisType:        4101
+; YAML:           CallConv:        ThisCall
+; YAML:           Options:         [ None ]
+; YAML:           ParameterCount:  0
+; YAML:           ArgumentList:    4104
+; YAML:           ThisPointerAdjustment: 0
+; YAML:       - Kind:            LF_FIELDLIST
+; YAML:         FieldList:
+; YAML:           - Kind:            LF_MEMBER
+; YAML:             DataMember:
+; YAML:               Attrs:           3
+; YAML:               Type:            116
+; YAML:               FieldOffset:     0
+; YAML:               Name:            X
+; YAML:           - Kind:            LF_MEMBER
+; YAML:             DataMember:
+; YAML:               Attrs:           3
+; YAML:               Type:            116
+; YAML:               FieldOffset:     4
+; YAML:               Name:            Y
+; YAML:           - Kind:            LF_ONEMETHOD
+; YAML:             OneMethod:
+; YAML:               Type:            4103
+; YAML:               Attrs:           3
+; YAML:               VFTableOffset:   -1
+; YAML:               Name:            Foo
+; YAML:           - Kind:            LF_ONEMETHOD
+; YAML:             OneMethod:
+; YAML:               Type:            4105
+; YAML:               Attrs:           3
+; YAML:               VFTableOffset:   -1
+; YAML:               Name:            method
+; YAML:       - Kind:            LF_STRUCTURE
+; YAML:         Class:
+; YAML:           MemberCount:     4
+; YAML:           Options:         [ None, HasUniqueName ]
+; YAML:           FieldList:       4106
+; YAML:           Name:            Foo
+; YAML:           UniqueName:      '.?AUFoo@@'
+; YAML:           DerivationList:  0
+; YAML:           VTableShape:     0
+; YAML:           Size:            8
+; YAML:       - Kind:            LF_STRING_ID
+; YAML:         StringId:
+; YAML:           Id:              0
+; YAML:           String:          'D:\src\llvmbuild\clang\Debug\x86\foo.cpp'
+; YAML:       - Kind:            LF_UDT_SRC_LINE
+; YAML:         UdtSourceLine:
+; YAML:           UDT:             4107
+; YAML:           SourceFile:      4108
+; YAML:           LineNumber:      1
+; YAML:       - Kind:            LF_MFUNC_ID
+; YAML:         MemberFuncId:
+; YAML:           ClassType:       4100
+; YAML:           FunctionType:    4103
+; YAML:           Name:            Foo
+; YAML:       - Kind:            LF_MFUNC_ID
+; YAML:         MemberFuncId:
+; YAML:           ClassType:       4100
+; YAML:           FunctionType:    4105
+; YAML:           Name:            method
+; YAML:   - Name:            '.debug$H'
+; YAML:     Characteristics: [ IMAGE_SCN_CNT_INITIALIZED_DATA, IMAGE_SCN_MEM_DISCARDABLE, IMAGE_SCN_MEM_READ ]
+; YAML:     Alignment:       4
+; YAML:     GlobalHashes:
+; YAML:       Version:         0
+; YAML:       HashAlgorithm:   0
+; YAML:       HashValues:
+; YAML:         - 9E56666824DC4B12E25261D4E09E6E9DA0F4EE31
+; YAML:         - FDEC3D2D96287486127C66070B248ED52E421F55
+; YAML:         - 074AE5CC2D68AF9F0A3BEF23993968F7FD82CA84
+; YAML:         - BF0439C1A64C9070C6A6ADB0A34D21DAD0FFC3E9
+; YAML:         - CF1B3AD4A96BA628E6556FD28A222FBBEBBE140E
+; YAML:         - EC50195BFE148C0DC6A87A59D49CA1D9B146DB86
+; YAML:         - 123C8BA63AD23386897AB6D814A9932F03846156
+; YAML:         - 0F135243878289B83835BC2DB9EE25A1D4D0DA2B
+; YAML:         - 9069CA78E7450A285173431B3E52C5C25299E473
+; YAML:         - ADA6E11350E9F2069D4689E3646C90D67B28DA62
+; YAML:         - BD535FA9877A4DD123840AF849F3B0110EEB1D7A
+; YAML:         - 8044F70193FE40B71867158C5E50F0467485FA99
+; YAML:         - 558606D57A76D125B705FC6DD18EEE3C1C0C4C09
+; YAML:         - A64A018D9EB1EB8015917925662C8508D81CDA68
+; YAML:         - 51E89AD9992AC6F11F9E3F1665F41C53BDA8AFC4
+; YAML:        - 4F1C3BCA73099EF3466AAC99CC4951767DF890F5
+; ...

llvm/test/DebugInfo/COFF/globals.ll

 ; OBJ:       DataOffset: ?last@@3HA+0x0
 ; OBJ:       Type: int (0x74)
 ; OBJ:       DisplayName: last
 ; OBJ:       LinkageName: ?last@@3HA
 ; OBJ:     }
 ; OBJ:   ]
 ; OBJ: ]
 ; OBJ: CodeViewDebugInfo [
-; OBJ:   Section: .debug$S (8)
+; OBJ:   Section: .debug$S
 ; OBJ:   Magic: 0x4
 ; OBJ:   Subsection [
 ; OBJ:     SubSectionType: Symbols (0xF1)
 ; OBJ:     GlobalData {
 ; OBJ:       DataOffset: ?comdat@?$A@X@@2HB+0x0
 ; OBJ:       Type: const int (0x1000)
 ; OBJ:       DisplayName: comdat
 ; OBJ:       LinkageName: ?comdat@?$A@X@@2HB