[PDB] Fill in "Parent" and "End" fields of scope-like symbol records

Summary:
There are a variety of records that open scopes: function scopes, block
scopes, and inlined call site scopes. These symbol records contain
Parent and End fields with the offsets of other symbol records. The End
field contains the offset of the matching S_END or S_INLINESITE_END
record. The Parent field contains the offset of the parent record, or 0
if this is a top-level scope (i.e. a function).

With this change, `llvm-pdbutil pretty -all` no longer crashes on PDBs
produced by LLD. I haven't tried a real debugger yet.

Reviewers: zturner, ruiu

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D34898

llvm-svn: 307278

7 files changed, 945 insertions, 7 deletions

diff --git a/lld/COFF/PDB.cpp b/lld/COFF/PDB.cpp
index c9842cfd1b9..85d5678ef1a 100644
--- a/lld/COFF/PDB.cpp
+++ b/lld/COFF/PDB.cpp
@@ -176,6 +176,70 @@ static MutableArrayRef<uint8_t> copySymbolForPdb(const CVSymbol &Sym,
   return NewData;
 }
 
+/// Return true if this symbol opens a scope. This implies that the symbol has
+/// "parent" and "end" fields, which contain the offset of the S_END or
+/// S_INLINESITE_END record.
+static bool symbolOpensScope(SymbolKind Kind) {
+  switch (Kind) {
+  case SymbolKind::S_GPROC32:
+  case SymbolKind::S_LPROC32:
+  case SymbolKind::S_LPROC32_ID:
+  case SymbolKind::S_GPROC32_ID:
+  case SymbolKind::S_BLOCK32:
+  case SymbolKind::S_SEPCODE:
+  case SymbolKind::S_THUNK32:
+  case SymbolKind::S_INLINESITE:
+  case SymbolKind::S_INLINESITE2:
+    return true;
+  default:
+    break;
+  }
+  return false;
+}
+
+static bool symbolEndsScope(SymbolKind Kind) {
+  switch (Kind) {
+  case SymbolKind::S_END:
+  case SymbolKind::S_PROC_ID_END:
+  case SymbolKind::S_INLINESITE_END:
+    return true;
+  default:
+    break;
+  }
+  return false;
+}
+
+struct ScopeRecord {
+  ulittle32_t PtrParent;
+  ulittle32_t PtrEnd;
+};
+
+struct SymbolScope {
+  ScopeRecord *OpeningRecord;
+  uint32_t ScopeOffset;
+};
+
+static void scopeStackOpen(SmallVectorImpl<SymbolScope> &Stack,
+                           uint32_t CurOffset, CVSymbol &Sym) {
+  assert(symbolOpensScope(Sym.kind()));
+  SymbolScope S;
+  S.ScopeOffset = CurOffset;
+  S.OpeningRecord = const_cast<ScopeRecord *>(
+      reinterpret_cast<const ScopeRecord *>(Sym.content().data()));
+  S.OpeningRecord->PtrParent = Stack.empty() ? 0 : Stack.back().ScopeOffset;
+  Stack.push_back(S);
+}
+
+static void scopeStackClose(SmallVectorImpl<SymbolScope> &Stack,
+                            uint32_t CurOffset, ObjectFile *File) {
+  if (Stack.empty()) {
+    warn("symbol scopes are not balanced in " + File->getName());
+    return;
+  }
+  SymbolScope S = Stack.pop_back_val();
+  S.OpeningRecord->PtrEnd = CurOffset;
+}
+
 static void mergeSymbolRecords(BumpPtrAllocator &Alloc, ObjectFile *File,
                                ArrayRef<TypeIndex> TypeIndexMap,
                                BinaryStreamRef SymData) {
@@ -184,6 +248,7 @@ static void mergeSymbolRecords(BumpPtrAllocator &Alloc, ObjectFile *File,
   CVSymbolArray Syms;
   BinaryStreamReader Reader(SymData);
   ExitOnErr(Reader.readArray(Syms, Reader.getLength()));
+  SmallVector<SymbolScope, 4> Scopes;
   for (const CVSymbol &Sym : Syms) {
     // Discover type index references in the record. Skip it if we don't know
     // where they are.
@@ -202,11 +267,15 @@ static void mergeSymbolRecords(BumpPtrAllocator &Alloc, ObjectFile *File,
     if (!remapTypesInSymbolRecord(File, Contents, TypeIndexMap, TypeRefs))
       continue;
 
-    // FIXME: Fill in "Parent" and "End" fields by maintaining a stack of
-    // scopes.
+    // Fill in "Parent" and "End" fields by maintaining a stack of scopes.
+    CVSymbol NewSym(Sym.kind(), NewData);
+    if (symbolOpensScope(Sym.kind()))
+      scopeStackOpen(Scopes, File->ModuleDBI->getNextSymbolOffset(), NewSym);
+    else if (symbolEndsScope(Sym.kind()))
+      scopeStackClose(Scopes, File->ModuleDBI->getNextSymbolOffset(), File);
 
     // Add the symbol to the module.
-    File->ModuleDBI->addSymbol(CVSymbol(Sym.kind(), NewData));
+    File->ModuleDBI->addSymbol(NewSym);
   }
 }
 
diff --git a/lld/test/COFF/Inputs/pdb-scopes-a.yaml b/lld/test/COFF/Inputs/pdb-scopes-a.yaml
new file mode 100644
index 00000000000..e9f4484c706
--- /dev/null
+++ b/lld/test/COFF/Inputs/pdb-scopes-a.yaml
@@ -0,0 +1,425 @@
+--- !COFF
+header:          
+  Machine:         IMAGE_FILE_MACHINE_AMD64
+  Characteristics: [  ]
+sections:        
+  - Name:            .drectve
+    Characteristics: [ IMAGE_SCN_LNK_INFO, IMAGE_SCN_LNK_REMOVE ]
+    Alignment:       1
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+  - Name:            '.debug$S'
+    Characteristics: [ IMAGE_SCN_CNT_INITIALIZED_DATA, IMAGE_SCN_MEM_DISCARDABLE, IMAGE_SCN_MEM_READ ]
+    Alignment:       1
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+            ScopeEndSym:     
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diff --git a/lld/test/COFF/Inputs/pdb-scopes-b.yaml b/lld/test/COFF/Inputs/pdb-scopes-b.yaml
new file mode 100644
index 00000000000..2839bf7e353
--- /dev/null
+++ b/lld/test/COFF/Inputs/pdb-scopes-b.yaml
@@ -0,0 +1,365 @@
+--- !COFF
+header:          
+  Machine:         IMAGE_FILE_MACHINE_AMD64
+  Characteristics: [  ]
+sections:        
+  - Name:            .drectve
+    Characteristics: [ IMAGE_SCN_LNK_INFO, IMAGE_SCN_LNK_REMOVE ]
+    Alignment:       1
+    SectionData:     2020202F44454641554C544C49423A224C4942434D5422202F44454641554C544C49423A224F4C444E414D45532220
+  - Name:            '.debug$S'
+    Characteristics: [ IMAGE_SCN_CNT_INITIALIZED_DATA, IMAGE_SCN_MEM_DISCARDABLE, IMAGE_SCN_MEM_READ ]
+    Alignment:       1
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+      - !Symbols
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+            Checksum:        8E8C92DB46478902EBEAEBFCFF15A6E0
+      - !StringTable
+        Strings:         
+          - 'c:\src\llvm-project\build\b.c'
+      - !Symbols
+        Records:         
+          - Kind:            S_BUILDINFO
+            BuildInfoSym:    
+              BuildId:         4110
+    Relocations:     
+      - VirtualAddress:  152
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+        Type:            IMAGE_REL_AMD64_SECREL
+      - VirtualAddress:  156
+        SymbolName:      f
+        Type:            IMAGE_REL_AMD64_SECTION
+      - VirtualAddress:  223
+        SymbolName:      f
+        Type:            IMAGE_REL_AMD64_SECREL
+      - VirtualAddress:  227
+        SymbolName:      f
+        Type:            IMAGE_REL_AMD64_SECTION
+      - VirtualAddress:  266
+        SymbolName:      f
+        Type:            IMAGE_REL_AMD64_SECREL
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+        SymbolName:      f
+        Type:            IMAGE_REL_AMD64_SECTION
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+        SymbolName:      f
+        Type:            IMAGE_REL_AMD64_SECREL
+      - VirtualAddress:  312
+        SymbolName:      f
+        Type:            IMAGE_REL_AMD64_SECTION
+  - Name:            '.debug$T'
+    Characteristics: [ IMAGE_SCN_CNT_INITIALIZED_DATA, IMAGE_SCN_MEM_DISCARDABLE, IMAGE_SCN_MEM_READ ]
+    Alignment:       1
+    Types:           
+      - Kind:            LF_ARGLIST
+        ArgList:         
+          ArgIndices:      [ 0 ]
+      - Kind:            LF_PROCEDURE
+        Procedure:       
+          ReturnType:      3
+          CallConv:        NearC
+          Options:         [ None ]
+          ParameterCount:  0
+          ArgumentList:    4096
+      - Kind:            LF_POINTER
+        Pointer:         
+          ReferentType:    4097
+          Attrs:           65548
+      - Kind:            LF_ARGLIST
+        ArgList:         
+          ArgIndices:      [ 116 ]
+      - Kind:            LF_PROCEDURE
+        Procedure:       
+          ReturnType:      3
+          CallConv:        NearC
+          Options:         [ None ]
+          ParameterCount:  1
+          ArgumentList:    4099
+      - Kind:            LF_FUNC_ID
+        FuncId:          
+          ParentScope:     0
+          FunctionType:    4100
+          Name:            f
+      - Kind:            LF_FUNC_ID
+        FuncId:          
+          ParentScope:     0
+          FunctionType:    4097
+          Name:            g
+      - Kind:            LF_STRING_ID
+        StringId:        
+          Id:              0
+          String:          'C:\src\llvm-project\build'
+      - Kind:            LF_STRING_ID
+        StringId:        
+          Id:              0
+          String:          'C:\PROGRA~2\MICROS~1.0\VC\Bin\amd64\cl.exe'
+      - Kind:            LF_STRING_ID
+        StringId:        
+          Id:              0
+          String:          '-c -Z7 -MT -IC:\PROGRA~2\MICROS~1.0\VC\include -IC:\PROGRA~2\MICROS~1.0\VC\atlmfc\include -IC:\PROGRA~2\WI3CF2~1\10\include\10.0.14393.0\ucrt -IC:\PROGRA~2\WI3CF2~1\10\include\10.0.14393.0\shared -IC:\PROGRA~2\WI3CF2~1\10\include\10.0.14393.0\um'
+      - Kind:            LF_SUBSTR_LIST
+        StringList:      
+          StringIndices:   [ 4105 ]
+      - Kind:            LF_STRING_ID
+        StringId:        
+          Id:              4106
+          String:          ' -IC:\PROGRA~2\WI3CF2~1\10\include\10.0.14393.0\winrt -TC -X'
+      - Kind:            LF_STRING_ID
+        StringId:        
+          Id:              0
+          String:          b.c
+      - Kind:            LF_STRING_ID
+        StringId:        
+          Id:              0
+          String:          'C:\src\llvm-project\build\vc140.pdb'
+      - Kind:            LF_BUILDINFO
+        BuildInfo:       
+          ArgIndices:      [ 4103, 4104, 4108, 4109, 4107 ]
+  - Name:            '.text$mn'
+    Characteristics: [ IMAGE_SCN_CNT_CODE, IMAGE_SCN_MEM_EXECUTE, IMAGE_SCN_MEM_READ ]
+    Alignment:       16
+    SectionData:     894C24084883EC38837C24400074168B44244083C003894424208B4C2420E800000000EB148B44244083C004894424248B4C2424E8000000004883C438C3
+    Relocations:     
+      - VirtualAddress:  31
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+        Type:            IMAGE_REL_AMD64_REL32
+      - VirtualAddress:  53
+        SymbolName:      g
+        Type:            IMAGE_REL_AMD64_REL32
+  - Name:            .xdata
+    Characteristics: [ IMAGE_SCN_CNT_INITIALIZED_DATA, IMAGE_SCN_MEM_READ ]
+    Alignment:       4
+    SectionData:     '0108010008620000'
+  - Name:            .pdata
+    Characteristics: [ IMAGE_SCN_CNT_INITIALIZED_DATA, IMAGE_SCN_MEM_READ ]
+    Alignment:       4
+    SectionData:     '000000003E00000000000000'
+    Relocations:     
+      - VirtualAddress:  0
+        SymbolName:      '$LN5'
+        Type:            IMAGE_REL_AMD64_ADDR32NB
+      - VirtualAddress:  4
+        SymbolName:      '$LN5'
+        Type:            IMAGE_REL_AMD64_ADDR32NB
+      - VirtualAddress:  8
+        SymbolName:      '$unwind$f'
+        Type:            IMAGE_REL_AMD64_ADDR32NB
+symbols:         
+  - Name:            .drectve
+    Value:           0
+    SectionNumber:   1
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_STATIC
+    SectionDefinition: 
+      Length:          47
+      NumberOfRelocations: 0
+      NumberOfLinenumbers: 0
+      CheckSum:        0
+      Number:          0
+  - Name:            '.debug$S'
+    Value:           0
+    SectionNumber:   2
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_STATIC
+    SectionDefinition: 
+      Length:          484
+      NumberOfRelocations: 8
+      NumberOfLinenumbers: 0
+      CheckSum:        0
+      Number:          0
+  - Name:            '.debug$T'
+    Value:           0
+    SectionNumber:   3
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_STATIC
+    SectionDefinition: 
+      Length:          616
+      NumberOfRelocations: 0
+      NumberOfLinenumbers: 0
+      CheckSum:        0
+      Number:          0
+  - Name:            '.text$mn'
+    Value:           0
+    SectionNumber:   4
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_STATIC
+    SectionDefinition: 
+      Length:          62
+      NumberOfRelocations: 2
+      NumberOfLinenumbers: 0
+      CheckSum:        3841032836
+      Number:          0
+  - Name:            g
+    Value:           0
+    SectionNumber:   0
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_FUNCTION
+    StorageClass:    IMAGE_SYM_CLASS_EXTERNAL
+  - Name:            f
+    Value:           0
+    SectionNumber:   4
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_FUNCTION
+    StorageClass:    IMAGE_SYM_CLASS_EXTERNAL
+  - Name:            '$LN5'
+    Value:           0
+    SectionNumber:   4
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_LABEL
+  - Name:            .xdata
+    Value:           0
+    SectionNumber:   5
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_STATIC
+    SectionDefinition: 
+      Length:          8
+      NumberOfRelocations: 0
+      NumberOfLinenumbers: 0
+      CheckSum:        3137252093
+      Number:          0
+  - Name:            '$unwind$f'
+    Value:           0
+    SectionNumber:   5
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_STATIC
+  - Name:            .pdata
+    Value:           0
+    SectionNumber:   6
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_STATIC
+    SectionDefinition: 
+      Length:          12
+      NumberOfRelocations: 3
+      NumberOfLinenumbers: 0
+      CheckSum:        2420588879
+      Number:          0
+  - Name:            '$pdata$f'
+    Value:           0
+    SectionNumber:   6
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_STATIC
+...
diff --git a/lld/test/COFF/pdb-comdat.test b/lld/test/COFF/pdb-comdat.test
index a7b5c401ab9..ea691aec87a 100644
--- a/lld/test/COFF/pdb-comdat.test
+++ b/lld/test/COFF/pdb-comdat.test
@@ -47,7 +47,7 @@ CHECK:         machine = intel x86-x64, Ver = Microsoft (R) Optimizing Compiler,
 CHECK:         frontend = 19.0.24215.1, backend = 19.0.24215.1
 CHECK:         flags = security checks | hot patchable
 CHECK:   120 | S_GPROC32_ID [size = 44] `main`
-CHECK:         parent = 0, end = 0, addr = 0002:0000, code size = 24
+CHECK:         parent = 0, end = 196, addr = 0002:0000, code size = 24
 CHECK:         debug start = 4, debug end = 19, flags = none
 CHECK:   164 | S_FRAMEPROC [size = 32]
 CHECK:         size = 40, padding size = 0, offset to padding = 0
@@ -58,7 +58,7 @@ CHECK:   200 | S_GDATA32 [size = 24] `global`
 CHECK:         type = 0x0074 (int), addr = 0000:0000
 CHECK:   224 | S_BUILDINFO [size = 8] BuildId = `4106`
 CHECK:   232 | S_GPROC32_ID [size = 44] `foo`
-CHECK:         parent = 0, end = 0, addr = 0002:0032, code size = 15
+CHECK:         parent = 0, end = 308, addr = 0002:0032, code size = 15
 CHECK:         debug start = 0, debug end = 14, flags = none
 CHECK:   276 | S_FRAMEPROC [size = 32]
 CHECK:         size = 0, padding size = 0, offset to padding = 0
@@ -72,7 +72,7 @@ CHECK:       machine = intel x86-x64, Ver = Microsoft (R) Optimizing Compiler, l
 CHECK:       frontend = 19.0.24215.1, backend = 19.0.24215.1
 CHECK:       flags = security checks | hot patchable
 CHECK:   120 | S_GPROC32_ID [size = 44] `bar`
-CHECK:       parent = 0, end = 0, addr = 0002:0048, code size = 14
+CHECK:       parent = 0, end = 196, addr = 0002:0048, code size = 14
 CHECK:       debug start = 4, debug end = 9, flags = none
 CHECK:   164 | S_FRAMEPROC [size = 32]
 CHECK:       size = 40, padding size = 0, offset to padding = 0
diff --git a/lld/test/COFF/pdb-scopes.test b/lld/test/COFF/pdb-scopes.test
new file mode 100644
index 00000000000..81db2386589
--- /dev/null
+++ b/lld/test/COFF/pdb-scopes.test
@@ -0,0 +1,75 @@
+Consider this program:
+
+$ cat a.c
+void g(int x) {}
+void f(int x);
+int main(int argc) {
+  if (argc) {
+    int x = 42;
+    f(x);
+  } else {
+    int y = 13;
+    f(y);
+  }
+}
+
+$ cat b.c
+extern void g();
+void f(int x) {
+  if (x) {
+    int y = x + 3;
+    g(y);
+  } else {
+    int w = x + 4;
+    g(w);
+  }
+}
+
+This program is interesting because there are two TUs, and each TU has nested
+scopes. Make sure we get the right parent and end offsets.
+
+RUN: yaml2obj %S/Inputs/pdb-scopes-a.yaml -o %t-a.obj
+RUN: yaml2obj %S/Inputs/pdb-scopes-b.yaml -o %t-b.obj
+RUN: lld-link %t-a.obj %t-b.obj -debug -entry:main -nodefaultlib -out:%t.exe -pdb:%t.pdb
+RUN: llvm-pdbutil dump -symbols %t.pdb | FileCheck %s
+
+CHECK-LABEL: Mod 0000 | `C:\src\llvm-project\build\tools\lld\test\COFF\Output\pdb-scopes.test.tmp-a.obj`:
+CHECK: 104 | S_GPROC32_ID [size = 44] `g`
+CHECK:       parent = 0, end = 196, addr = 0002:0000, code size = 5
+CHECK:       debug start = 4, debug end = 4, flags = none
+CHECK: 180 | S_REGREL32 [size = 16] `x`
+CHECK: 196 | S_END [size = 4]
+CHECK: 200 | S_GPROC32_ID [size = 44] `main`
+CHECK:       parent = 0, end = 384, addr = 0002:0016, code size = 58
+CHECK:       debug start = 8, debug end = 53, flags = none
+CHECK: 276 | S_REGREL32 [size = 20] `argc`
+CHECK: 296 | S_BLOCK32 [size = 24] ``
+CHECK:       parent = 200, end = 336
+CHECK:       code size = 17, addr = 0002:0031
+CHECK: 320 | S_REGREL32 [size = 16] `x`
+CHECK: 336 | S_END [size = 4]
+CHECK: 340 | S_BLOCK32 [size = 24] ``
+CHECK:       parent = 200, end = 380
+CHECK:       code size = 17, addr = 0002:0050
+CHECK: 364 | S_REGREL32 [size = 16] `y`
+CHECK: 380 | S_END [size = 4]
+CHECK: 384 | S_END [size = 4]
+
+CHECK-LABEL: Mod 0001 | `C:\src\llvm-project\build\tools\lld\test\COFF\Output\pdb-scopes.test.tmp-b.obj`:
+CHECK: 104 | S_GPROC32_ID [size = 44] `f`
+CHECK:       parent = 0, end = 284, addr = 0002:0080, code size = 62
+CHECK:       debug start = 8, debug end = 57, flags = none
+CHECK: 180 | S_REGREL32 [size = 16] `x`
+CHECK: 196 | S_BLOCK32 [size = 24] ``
+CHECK:       parent = 104, end = 236
+CHECK:       code size = 20, addr = 0002:0095
+CHECK: 220 | S_REGREL32 [size = 16] `y`
+CHECK: 236 | S_END [size = 4]
+CHECK: 240 | S_BLOCK32 [size = 24] ``
+CHECK:       parent = 104, end = 280
+CHECK:       code size = 20, addr = 0002:0117
+CHECK: 264 | S_REGREL32 [size = 16] `w`
+CHECK: 280 | S_END [size = 4]
+CHECK: 284 | S_END [size = 4]
+
+CHECK-LABEL: Mod 0002 | `* Linker *`:
diff --git a/lld/test/COFF/pdb-symbol-types.yaml b/lld/test/COFF/pdb-symbol-types.yaml
index eceb434f0d0..8abbc365b34 100644
--- a/lld/test/COFF/pdb-symbol-types.yaml
+++ b/lld/test/COFF/pdb-symbol-types.yaml
@@ -22,7 +22,7 @@
 # CHECK:         frontend = 19.0.24215.1, backend = 19.0.24215.1
 # CHECK:         flags = security checks | hot patchable
 # CHECK:   116 | S_GPROC32_ID [size = 44] `main`
-# CHECK:         parent = 0, end = 0, addr = 0002:0000, code size = 7
+# CHECK:         parent = 0, end = 192, addr = 0002:0000, code size = 7
 # CHECK:         debug start = 0, debug end = 6, flags = none
 # CHECK:   160 | S_FRAMEPROC [size = 32]
 # CHECK:         size = 0, padding size = 0, offset to padding = 0
diff --git a/llvm/include/llvm/DebugInfo/PDB/Native/DbiModuleDescriptorBuilder.h b/llvm/include/llvm/DebugInfo/PDB/Native/DbiModuleDescriptorBuilder.h
index e4cb1b24e30..64b195034f5 100644
--- a/llvm/include/llvm/DebugInfo/PDB/Native/DbiModuleDescriptorBuilder.h
+++ b/llvm/include/llvm/DebugInfo/PDB/Native/DbiModuleDescriptorBuilder.h
@@ -68,6 +68,10 @@ public:
 
   uint32_t calculateSerializedLength() const;
 
+  /// Return the offset within the module symbol stream of the next symbol
+  /// record passed to addSymbol. Add four to account for the signature.
+  uint32_t getNextSymbolOffset() const { return SymbolByteSize + 4; }
+
   void finalize();
   Error finalizeMsfLayout();