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

command-line option which defaults off.

llvm-svn: 96649

clang/include/clang/CodeGen/CodeGenOptions.h

@@ -53,6 +53,8 @@ public:
   unsigned UnwindTables      : 1; /// Emit unwind tables.
   unsigned VerifyModule      : 1; /// Control whether the module should be run
                                   /// through the LLVM Verifier.
+  unsigned CXXCtorDtorAliases: 1; /// Emit complete ctors/dtors as linker
+                                  /// aliases to base ctors when possible.
 
   /// The code model to use (-mcmodel).
   std::string CodeModel;
@@ -101,6 +103,7 @@ public:
     UnrollLoops = 0;
     UnwindTables = 0;
     VerifyModule = 1;
+    CXXCtorDtorAliases = 0;
 
     Inlining = NoInlining;
     RelocationModel = "pic";

clang/include/clang/Driver/CC1Options.td

@@ -143,6 +143,8 @@ def mrelocation_model : Separate<"-mrelocation-model">,
   HelpText<"The relocation model to use">;
 def munwind_tables : Flag<"-munwind-tables">,
   HelpText<"Generate unwinding tables for all functions">;
+def mconstructor_aliases : Flag<"-mconstructor-aliases">,
+  HelpText<"Emit complete constructors and destructors as aliases when possible">;
 def O : Joined<"-O">, HelpText<"Optimization level">;
 def Os : Flag<"-Os">, HelpText<"Optimize for size">;
 

clang/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)));
 }

clang/lib/CodeGen/CodeGenModule.cpp

@@ -316,24 +316,20 @@ GetLinkageForFunction(ASTContext &Context, const FunctionDecl *FD,
   return CodeGenModule::GVA_CXXInline;
 }
 
-/// SetFunctionDefinitionAttributes - Set attributes for a global.
+llvm::GlobalValue::LinkageTypes
-///
+CodeGenModule::getFunctionLinkage(const FunctionDecl *D) {
-/// FIXME: This is currently only done for aliases and functions, but not for
-/// variables (these details are set in EmitGlobalVarDefinition for variables).
-void CodeGenModule::SetFunctionDefinitionAttributes(const FunctionDecl *D,
-                                                    llvm::GlobalValue *GV) {
   GVALinkage Linkage = GetLinkageForFunction(getContext(), D, Features);
 
   if (Linkage == GVA_Internal) {
-    GV->setLinkage(llvm::Function::InternalLinkage);
+    return llvm::Function::InternalLinkage;
   } else if (D->hasAttr<DLLExportAttr>()) {
-    GV->setLinkage(llvm::Function::DLLExportLinkage);
+    return llvm::Function::DLLExportLinkage;
   } else if (D->hasAttr<WeakAttr>()) {
-    GV->setLinkage(llvm::Function::WeakAnyLinkage);
+    return llvm::Function::WeakAnyLinkage;
   } else if (Linkage == GVA_C99Inline) {
     // In C99 mode, 'inline' functions are guaranteed to have a strong
     // definition somewhere else, so we can use available_externally linkage.
-    GV->setLinkage(llvm::Function::AvailableExternallyLinkage);
+    return llvm::Function::AvailableExternallyLinkage;
   } else if (Linkage == GVA_CXXInline || Linkage == GVA_TemplateInstantiation) {
     // In C++, the compiler has to emit a definition in every translation unit
     // that references the function.  We should use linkonce_odr because
@@ -341,13 +337,22 @@ void CodeGenModule::SetFunctionDefinitionAttributes(const FunctionDecl *D,
     // don't need to codegen it.  b) if the function persists, it needs to be
     // merged with other definitions. c) C++ has the ODR, so we know the
     // definition is dependable.
-    GV->setLinkage(llvm::Function::LinkOnceODRLinkage);
+    return llvm::Function::LinkOnceODRLinkage;
   } else {
     assert(Linkage == GVA_StrongExternal);
     // Otherwise, we have strong external linkage.
-    GV->setLinkage(llvm::Function::ExternalLinkage);
+    return llvm::Function::ExternalLinkage;
+  }
 }
 
+
+/// SetFunctionDefinitionAttributes - Set attributes for a global.
+///
+/// FIXME: This is currently only done for aliases and functions, but not for
+/// variables (these details are set in EmitGlobalVarDefinition for variables).
+void CodeGenModule::SetFunctionDefinitionAttributes(const FunctionDecl *D,
+                                                    llvm::GlobalValue *GV) {
+  GV->setLinkage(getFunctionLinkage(D));
   SetCommonAttributes(D, GV);
 }
 

clang/lib/CodeGen/CodeGenModule.h

@@ -206,6 +206,19 @@ public:
   /// GlobalValue.
   void setGlobalVisibility(llvm::GlobalValue *GV, const Decl *D) const;
 
+  llvm::Constant *GetAddrOfGlobal(GlobalDecl GD) {
+    if (isa<CXXConstructorDecl>(GD.getDecl()))
+      return GetAddrOfCXXConstructor(cast<CXXConstructorDecl>(GD.getDecl()),
+                                     GD.getCtorType());
+    else if (isa<CXXDestructorDecl>(GD.getDecl()))
+      return GetAddrOfCXXDestructor(cast<CXXDestructorDecl>(GD.getDecl()),
+                                     GD.getDtorType());
+    else if (isa<FunctionDecl>(GD.getDecl()))
+      return GetAddrOfFunction(GD);
+    else
+      return GetAddrOfGlobalVar(cast<VarDecl>(GD.getDecl()));
+  }
+
   /// GetAddrOfGlobalVar - Return the llvm::Constant for the address of the
   /// given global variable.  If Ty is non-null and if the global doesn't exist,
   /// then it will be greated with the specified type instead of whatever the
@@ -291,12 +304,12 @@ public:
 
   /// GetAddrOfCXXConstructor - Return the address of the constructor of the
   /// given type.
-  llvm::Function *GetAddrOfCXXConstructor(const CXXConstructorDecl *D,
+  llvm::GlobalValue *GetAddrOfCXXConstructor(const CXXConstructorDecl *D,
                                              CXXCtorType Type);
 
   /// GetAddrOfCXXDestructor - Return the address of the constructor of the
   /// given type.
-  llvm::Function *GetAddrOfCXXDestructor(const CXXDestructorDecl *D,
+  llvm::GlobalValue *GetAddrOfCXXDestructor(const CXXDestructorDecl *D,
                                             CXXDtorType Type);
 
   /// getBuiltinLibFunction - Given a builtin id for a function like
@@ -417,6 +430,9 @@ public:
     GVA_TemplateInstantiation
   };
 
+  llvm::GlobalVariable::LinkageTypes
+  getFunctionLinkage(const FunctionDecl *FD);
+
   /// getVtableLinkage - Return the appropriate linkage for the vtable, VTT,
   /// and type information of the given class.
   static llvm::GlobalVariable::LinkageTypes 
@@ -468,6 +484,8 @@ private:
 
   // C++ related functions.
 
+  bool TryEmitDefinitionAsAlias(GlobalDecl Alias, GlobalDecl Target);
+
   void EmitNamespace(const NamespaceDecl *D);
   void EmitLinkageSpec(const LinkageSpecDecl *D);
 

clang/lib/Frontend/CompilerInvocation.cpp

@@ -180,6 +180,8 @@ static void CodeGenOptsToArgs(const CodeGenOptions &Opts,
     Res.push_back("-mrelocation-model");
     Res.push_back(Opts.RelocationModel);
   }
+  if (Opts.CXXCtorDtorAliases)
+    Res.push_back("-mconstructor-aliases");
   if (!Opts.VerifyModule)
     Res.push_back("-disable-llvm-verifier");
 }
@@ -789,6 +791,7 @@ static void ParseCodeGenArgs(CodeGenOptions &Opts, ArgList &Args,
   Opts.SoftFloat = Args.hasArg(OPT_msoft_float);
   Opts.UnwindTables = Args.hasArg(OPT_munwind_tables);
   Opts.RelocationModel = getLastArgValue(Args, OPT_mrelocation_model, "pic");
+  Opts.CXXCtorDtorAliases = Args.hasArg(OPT_mconstructor_aliases);
 
   Opts.MainFileName = getLastArgValue(Args, OPT_main_file_name);
   Opts.VerifyModule = !Args.hasArg(OPT_disable_llvm_verifier);

clang/test/CodeGenCXX/virtual-bases.cpp

@@ -1,10 +1,10 @@
-// RUN: %clang_cc1 -emit-llvm %s -o - -triple=x86_64-apple-darwin10 | FileCheck %s
+// RUN: %clang_cc1 -emit-llvm %s -o - -triple=x86_64-apple-darwin10 -mconstructor-aliases | FileCheck %s
 
 struct A { 
   A();
 };
 
-// CHECK: define void @_ZN1AC1Ev(%struct.A* %this)
+// CHECK: @_ZN1AC1Ev = alias {{.*}} @_ZN1AC2Ev
 // CHECK: define void @_ZN1AC2Ev(%struct.A* %this)
 A::A() { }
 

clang/test/CodeGenCXX/virtual-destructor-calls.cpp

@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -emit-llvm %s -o - -triple=x86_64-apple-darwin10 | FileCheck %s
+// RUN: %clang_cc1 -emit-llvm %s -o - -triple=x86_64-apple-darwin10 -mconstructor-aliases | FileCheck %s
 
 struct A {
   virtual ~A();
@@ -8,9 +8,8 @@ struct B : A {
   virtual ~B();
 };
 
-// Complete dtor: just defers to base dtor because there are no vbases.
+// Complete dtor: just an alias because there are no virtual bases.
-// CHECK: define void @_ZN1BD1Ev
+// CHECK: @_ZN1BD1Ev = alias {{.*}} @_ZN1BD2Ev
-// CHECK: call void @_ZN1BD2Ev
 
 // Deleting dtor: defers to the complete dtor.
 // CHECK: define void @_ZN1BD0Ev

clang/test/CodeGenCXX/vtable-pointer-initialization.cpp

@@ -19,14 +19,14 @@ struct A : Base {
   Field field;
 };
 
-// CHECK: define void @_ZN1AC1Ev(
+// CHECK: define void @_ZN1AC2Ev(
 // CHECK: call void @_ZN4BaseC2Ev(
 // CHECK: store i8** getelementptr inbounds ([3 x i8*]* @_ZTV1A, i64 0, i64 2)
 // CHECK: call void @_ZN5FieldC1Ev(
 // CHECK: ret void
 A::A() { }
 
-// CHECK: define void @_ZN1AD1Ev(
+// CHECK: define void @_ZN1AD2Ev(
 // CHECK: store i8** getelementptr inbounds ([3 x i8*]* @_ZTV1A, i64 0, i64 2)
 // CHECK: call void @_ZN5FieldD1Ev(
 // CHECK: call void @_ZN4BaseD2Ev(