From a7b041748d97ac0671e1d59233be0f9a7653ecd8 Mon Sep 17 00:00:00 2001
From: Zachary Turner <zturner@google.com>
Date: Mon, 28 Aug 2017 18:49:04 +0000
Subject: [CodeView] Don't output S_UDT symbols for forward decls.

S_UDT symbols are the debugger's "index" for all the structs,
typedefs, classes, and enums in a program.  If any of those
structs/classes don't have a complete declaration, or if there
is a typedef to something that doesn't have a complete definition,
then emitting the S_UDT is unhelpful because it doesn't give
the debugger enough information to do anything useful.  On the
other hand, it results in a huge size blow-up in the resulting
PDB, which is exacerbated by an order of magnitude when linking
with /DEBUG:FASTLINK.

With this patch, we drop S_UDT records for types that refer either
directly or indirectly (e.g. through a typedef, pointer, etc) to
a class/struct/union/enum without a complete definition.  This
brings us about 50% of the way towards parity with /DEBUG:FASTLINK
PDBs generated from cl-compiled object files.

Differential Revision: https://reviews.llvm.org/D37162

llvm-svn: 311904
---
 llvm/lib/CodeGen/AsmPrinter/CodeViewDebug.cpp | 41 ++++++++++++++++++++-------
 1 file changed, 30 insertions(+), 11 deletions(-)

(limited to 'llvm/lib/CodeGen/AsmPrinter/CodeViewDebug.cpp')

diff --git a/llvm/lib/CodeGen/AsmPrinter/CodeViewDebug.cpp b/llvm/lib/CodeGen/AsmPrinter/CodeViewDebug.cpp
index 4346803..8879f6b1 100644
--- a/llvm/lib/CodeGen/AsmPrinter/CodeViewDebug.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/CodeViewDebug.cpp
@@ -1099,10 +1099,25 @@ void CodeViewDebug::beginFunctionImpl(const MachineFunction *MF) {
   }
 }
 
-void CodeViewDebug::addToUDTs(const DIType *Ty, TypeIndex TI) {
+static bool shouldEmitUdt(const DIType *T) {
+  while (true) {
+    if (!T || T->isForwardDecl())
+      return false;
+
+    const DIDerivedType *DT = dyn_cast<DIDerivedType>(T);
+    if (!DT)
+      return true;
+    T = DT->getBaseType().resolve();
+  }
+  return true;
+}
+
+void CodeViewDebug::addToUDTs(const DIType *Ty) {
   // Don't record empty UDTs.
   if (Ty->getName().empty())
     return;
+  if (!shouldEmitUdt(Ty))
+    return;
 
   SmallVector<StringRef, 5> QualifiedNameComponents;
   const DISubprogram *ClosestSubprogram = getQualifiedNameComponents(
@@ -1111,10 +1126,11 @@ void CodeViewDebug::addToUDTs(const DIType *Ty, TypeIndex TI) {
   std::string FullyQualifiedName =
       getQualifiedName(QualifiedNameComponents, getPrettyScopeName(Ty));
 
-  if (ClosestSubprogram == nullptr)
-    GlobalUDTs.emplace_back(std::move(FullyQualifiedName), TI);
-  else if (ClosestSubprogram == CurrentSubprogram)
-    LocalUDTs.emplace_back(std::move(FullyQualifiedName), TI);
+  if (ClosestSubprogram == nullptr) {
+    GlobalUDTs.emplace_back(std::move(FullyQualifiedName), Ty);
+  } else if (ClosestSubprogram == CurrentSubprogram) {
+    LocalUDTs.emplace_back(std::move(FullyQualifiedName), Ty);
+  }
 
   // TODO: What if the ClosestSubprogram is neither null or the current
   // subprogram?  Currently, the UDT just gets dropped on the floor.
@@ -1172,7 +1188,7 @@ TypeIndex CodeViewDebug::lowerTypeAlias(const DIDerivedType *Ty) {
   TypeIndex UnderlyingTypeIndex = getTypeIndex(UnderlyingTypeRef);
   StringRef TypeName = Ty->getName();
 
-  addToUDTs(Ty, UnderlyingTypeIndex);
+  addToUDTs(Ty);
 
   if (UnderlyingTypeIndex == TypeIndex(SimpleTypeKind::Int32Long) &&
       TypeName == "HRESULT")
@@ -1764,7 +1780,7 @@ TypeIndex CodeViewDebug::lowerCompleteTypeClass(const DICompositeType *Ty) {
     TypeTable.writeKnownType(USLR);
   }
 
-  addToUDTs(Ty, ClassTI);
+  addToUDTs(Ty);
 
   return ClassTI;
 }
@@ -1803,7 +1819,7 @@ TypeIndex CodeViewDebug::lowerCompleteTypeUnion(const DICompositeType *Ty) {
   UdtSourceLineRecord USLR(UnionTI, SIRI, Ty->getLine());
   TypeTable.writeKnownType(USLR);
 
-  addToUDTs(Ty, UnionTI);
+  addToUDTs(Ty);
 
   return UnionTI;
 }
@@ -2203,8 +2219,11 @@ void CodeViewDebug::endCVSubsection(MCSymbol *EndLabel) {
 }
 
 void CodeViewDebug::emitDebugInfoForUDTs(
-    ArrayRef<std::pair<std::string, TypeIndex>> UDTs) {
-  for (const std::pair<std::string, codeview::TypeIndex> &UDT : UDTs) {
+    ArrayRef<std::pair<std::string, const DIType *>> UDTs) {
+  for (const auto &UDT : UDTs) {
+    const DIType *T = UDT.second;
+    assert(shouldEmitUdt(T));
+
     MCSymbol *UDTRecordBegin = MMI->getContext().createTempSymbol(),
              *UDTRecordEnd = MMI->getContext().createTempSymbol();
     OS.AddComment("Record length");
@@ -2215,7 +2234,7 @@ void CodeViewDebug::emitDebugInfoForUDTs(
     OS.EmitIntValue(unsigned(SymbolKind::S_UDT), 2);
 
     OS.AddComment("Type");
-    OS.EmitIntValue(UDT.second.getIndex(), 4);
+    OS.EmitIntValue(getCompleteTypeIndex(T).getIndex(), 4);
 
     emitNullTerminatedSymbolName(OS, UDT.first);
     OS.EmitLabel(UDTRecordEnd);
-- 
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