Re-introduce the ctor/dtor alias optimization, this time hidden behind a

command-line option which defaults off.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@96649 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 104 insertions, 7 deletions

diff --git a/lib/CodeGen/CGCXX.cpp b/lib/CodeGen/CGCXX.cpp
index c4c52aac03..e84b68104b 100644
--- a/lib/CodeGen/CGCXX.cpp
+++ b/lib/CodeGen/CGCXX.cpp
@@ -22,21 +22,96 @@
 #include "clang/AST/DeclCXX.h"
 #include "clang/AST/DeclObjC.h"
 #include "clang/AST/StmtCXX.h"
+#include "clang/CodeGen/CodeGenOptions.h"
 #include "llvm/ADT/StringExtras.h"
 using namespace clang;
 using namespace CodeGen;
 
+/// Try to emit a definition as a global alias for another definition.
+bool CodeGenModule::TryEmitDefinitionAsAlias(GlobalDecl AliasDecl,
+                                             GlobalDecl TargetDecl) {
+  if (!getCodeGenOpts().CXXCtorDtorAliases)
+    return true;
+
+  // Find the referrent.
+  llvm::GlobalValue *Ref = cast<llvm::GlobalValue>(GetAddrOfGlobal(TargetDecl));
+
+  // Look for an existing entry.
+  const char *MangledName = getMangledName(AliasDecl);
+  llvm::GlobalValue *&Entry = GlobalDeclMap[MangledName];
+  if (Entry) {
+    assert(Entry->isDeclaration() && "definition already exists for alias");
+    assert(Entry->getType() == Ref->getType() &&
+           "declaration exists with different type");
+  }
+
+  // The alias will use the linkage of the referrent.  If we can't
+  // support aliases with that linkage, fail.
+  llvm::GlobalValue::LinkageTypes Linkage
+    = getFunctionLinkage(cast<FunctionDecl>(AliasDecl.getDecl()));
+
+  switch (Linkage) {
+  // We can definitely emit aliases to definitions with external linkage.
+  case llvm::GlobalValue::ExternalLinkage:
+  case llvm::GlobalValue::ExternalWeakLinkage:
+    break;
+
+  // Same with local linkage.
+  case llvm::GlobalValue::InternalLinkage:
+  case llvm::GlobalValue::PrivateLinkage:
+  case llvm::GlobalValue::LinkerPrivateLinkage:
+    break;
+
+  // We should try to support linkonce linkages.
+  case llvm::GlobalValue::LinkOnceAnyLinkage:
+  case llvm::GlobalValue::LinkOnceODRLinkage:
+    return true;
+
+  // Other linkages will probably never be supported.
+  default:
+    return true;
+  }
+
+  // Create the alias with no name.
+  llvm::GlobalAlias *Alias = 
+    new llvm::GlobalAlias(Ref->getType(), Linkage, "", Ref, &getModule());
+
+  // Switch any previous uses to the alias and continue.
+  if (Entry) {
+    Entry->replaceAllUsesWith(Alias);
+    Entry->eraseFromParent();
+  }
+  Entry = Alias;
+
+  // Finally, set up the alias with its proper name and attributes.
+  Alias->setName(MangledName);
+  SetCommonAttributes(AliasDecl.getDecl(), Alias);
+
+  return false;
+}
 
 
 void CodeGenModule::EmitCXXConstructors(const CXXConstructorDecl *D) {
+  // The constructor used for constructing this as a complete class;
+  // constucts the virtual bases, then calls the base constructor.
   EmitGlobal(GlobalDecl(D, Ctor_Complete));
+
+  // The constructor used for constructing this as a base class;
+  // ignores virtual bases.
   EmitGlobal(GlobalDecl(D, Ctor_Base));
 }
 
 void CodeGenModule::EmitCXXConstructor(const CXXConstructorDecl *D,
                                        CXXCtorType Type) {
+  // The complete constructor is equivalent to the base constructor
+  // for classes with no virtual bases.  Try to emit it as an alias.
+  if (Type == Ctor_Complete &&
+      !D->getParent()->getNumVBases() &&
+      !TryEmitDefinitionAsAlias(GlobalDecl(D, Ctor_Complete),
+                                GlobalDecl(D, Ctor_Base)))
+    return;
 
-  llvm::Function *Fn = GetAddrOfCXXConstructor(D, Type);
+  llvm::Function *Fn = cast<llvm::Function>(GetAddrOfCXXConstructor(D, Type));
 
   CodeGenFunction(*this).GenerateCode(GlobalDecl(D, Type), Fn);
 
@@ -44,15 +119,17 @@ void CodeGenModule::EmitCXXConstructor(const CXXConstructorDecl *D,
   SetLLVMFunctionAttributesForDefinition(D, Fn);
 }
 
-llvm::Function *
+llvm::GlobalValue *
 CodeGenModule::GetAddrOfCXXConstructor(const CXXConstructorDecl *D,
                                        CXXCtorType Type) {
+  const char *Name = getMangledCXXCtorName(D, Type);
+  if (llvm::GlobalValue *V = GlobalDeclMap[Name])
+    return V;
+
   const FunctionProtoType *FPT = D->getType()->getAs<FunctionProtoType>();
   const llvm::FunctionType *FTy =
     getTypes().GetFunctionType(getTypes().getFunctionInfo(D, Type), 
                                FPT->isVariadic());
-
-  const char *Name = getMangledCXXCtorName(D, Type);
   return cast<llvm::Function>(
                       GetOrCreateLLVMFunction(Name, FTy, GlobalDecl(D, Type)));
 }
@@ -67,15 +144,32 @@ const char *CodeGenModule::getMangledCXXCtorName(const CXXConstructorDecl *D,
 }
 
 void CodeGenModule::EmitCXXDestructors(const CXXDestructorDecl *D) {
+  // The destructor in a virtual table is always a 'deleting'
+  // destructor, which calls the complete destructor and then uses the
+  // appropriate operator delete.
   if (D->isVirtual())
     EmitGlobal(GlobalDecl(D, Dtor_Deleting));
+
+  // The destructor used for destructing this as a most-derived class;
+  // call the base destructor and then destructs any virtual bases.
   EmitGlobal(GlobalDecl(D, Dtor_Complete));
+
+  // The destructor used for destructing this as a base class; ignores
+  // virtual bases.
   EmitGlobal(GlobalDecl(D, Dtor_Base));
 }
 
 void CodeGenModule::EmitCXXDestructor(const CXXDestructorDecl *D,
                                       CXXDtorType Type) {
-  llvm::Function *Fn = GetAddrOfCXXDestructor(D, Type);
+  // The complete destructor is equivalent to the base destructor for
+  // classes with no virtual bases, so try to emit it as an alias.
+  if (Type == Dtor_Complete &&
+      !D->getParent()->getNumVBases() &&
+      !TryEmitDefinitionAsAlias(GlobalDecl(D, Dtor_Complete),
+                                GlobalDecl(D, Dtor_Base)))
+    return;
+
+  llvm::Function *Fn = cast<llvm::Function>(GetAddrOfCXXDestructor(D, Type));
 
   CodeGenFunction(*this).GenerateCode(GlobalDecl(D, Type), Fn);
 
@@ -83,13 +177,16 @@ void CodeGenModule::EmitCXXDestructor(const CXXDestructorDecl *D,
   SetLLVMFunctionAttributesForDefinition(D, Fn);
 }
 
-llvm::Function *
+llvm::GlobalValue *
 CodeGenModule::GetAddrOfCXXDestructor(const CXXDestructorDecl *D,
                                       CXXDtorType Type) {
+  const char *Name = getMangledCXXDtorName(D, Type);
+  if (llvm::GlobalValue *V = GlobalDeclMap[Name])
+    return V;
+
   const llvm::FunctionType *FTy =
     getTypes().GetFunctionType(getTypes().getFunctionInfo(D, Type), false);
 
-  const char *Name = getMangledCXXDtorName(D, Type);
   return cast<llvm::Function>(
                       GetOrCreateLLVMFunction(Name, FTy, GlobalDecl(D, Type)));
 }