rsmith created this revision.
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rsmith requested review of this revision.
As far as I can determine, the only case we need to be careful about is
when we would generate a reference to the symbol and not emit any
corresponding definition, and that only happens when generating an
available_externally body of the corresponding complete structor
variant. Disable all complete -> base forwarding in that case for
structors of final classes.
See https://github.com/itanium-cxx-abi/cxx-abi/issues/104 for more
background.
Repository:
rG LLVM Github Monorepo
https://reviews.llvm.org/D86494
Files:
clang/lib/CodeGen/CGClass.cpp
clang/lib/CodeGen/CodeGenModule.h
clang/lib/CodeGen/ItaniumCXXABI.cpp
clang/test/CodeGenCXX/final-structor-delegation.cpp
Index: clang/test/CodeGenCXX/final-structor-delegation.cpp
===================================================================
--- /dev/null
+++ clang/test/CodeGenCXX/final-structor-delegation.cpp
@@ -0,0 +1,56 @@
+// RUN: %clang_cc1 -triple x86_64-linux-gnu -std=c++20 -emit-llvm %s -O2 -disable-llvm-passes -o /tmp/noalias
+// RUN: %clang_cc1 -triple x86_64-linux-gnu -std=c++20 -emit-llvm %s -O2 -disable-llvm-passes -mconstructor-aliases -o /tmp/alias
+// RUN: %clang_cc1 -triple x86_64-linux-gnu -std=c++20 -emit-llvm %s -O2 -disable-llvm-passes -o - | FileCheck %s --check-prefixes=CHECK,NOALIAS
+// RUN: %clang_cc1 -triple x86_64-linux-gnu -std=c++20 -emit-llvm %s -O2 -disable-llvm-passes -mconstructor-aliases -o - | FileCheck %s --check-prefixes=CHECK,ALIAS
+
+extern "C" void f(), g();
+
+// Non-final class: we can assume that the base subobject variants of structors
+// exist whenever the complete object variants exist.
+template <typename T> struct A {
+ A() { f(); }
+ ~A() { g(); }
+};
+extern template struct A<int>;
+// CHECK-LABEL: define {{.*}}init
+// NOALIAS: call void @_ZN1AIiEC1Ev({{.*}} @a)
+// NOALIAS: call {{.*}}cxa_atexit{{.*}} @_ZN1AIiED1Ev {{.*}} @a
+// ALIAS: call void @_ZN1AIiEC2Ev({{.*}} @a)
+// ALIAS: call {{.*}}cxa_atexit{{.*}} @_ZN1AIiED2Ev {{.*}} @a
+A<int> a;
+
+// NOALIAS-LABEL: define available_externally {{.*}} @_ZN1AIiEC1Ev(
+// NOALIAS-NOT: call void @f(
+// NOALIAS: call void @_ZN1AIiEC2Ev(
+// NOALIAS-NOT: call void @f(
+// NOALIAS: }
+
+// NOALIAS-LABEL: define available_externally {{.*}} @_ZN1AIiED1Ev(
+// NOALIAS-NOT: call void @g(
+// NOALIAS: call void @_ZN1AIiED2Ev(
+// NOALIAS-NOT: call void @g(
+// NOALIAS: }
+
+// Final class: we can't assume that the base subobject variants exist, because
+// GCC 10 doesn't emit them (even though the ABI requires them).
+template <typename T> struct B final {
+ B() { f(); }
+ ~B() { g(); }
+};
+extern template struct B<int>;
+// CHECK-LABEL: define {{.*}}init
+// CHECK: call void @_ZN1BIiEC1Ev({{.*}} @b)
+// CHECK: call {{.*}}cxa_atexit{{.*}} @_ZN1BIiED1Ev {{.*}} @b
+B<int> b;
+
+// CHECK-LABEL: define available_externally {{.*}} @_ZN1BIiEC1Ev(
+// CHECK-NOT: call void @_ZN1BIiEC2Ev(
+// CHECK: call void @f(
+// CHECK-NOT: call void @_ZN1BIiEC2Ev(
+// CHECK: }
+
+// CHECK-LABEL: define available_externally {{.*}} @_ZN1BIiED1Ev(
+// CHECK-NOT: call void @_ZN1BIiED2Ev(
+// CHECK: call void @g(
+// CHECK-NOT: call void @_ZN1BIiED2Ev(
+// CHECK: }
Index: clang/lib/CodeGen/ItaniumCXXABI.cpp
===================================================================
--- clang/lib/CodeGen/ItaniumCXXABI.cpp
+++ clang/lib/CodeGen/ItaniumCXXABI.cpp
@@ -1594,6 +1594,7 @@
// The constructor used for constructing this as a base class;
// ignores virtual bases.
+ // FIXME: Skip this for final classes once the ABI permits us to.
CGM.EmitGlobal(GlobalDecl(D, Ctor_Base));
// The constructor used for constructing this as a complete class;
@@ -1626,6 +1627,7 @@
void ItaniumCXXABI::EmitCXXDestructors(const CXXDestructorDecl *D) {
// The destructor used for destructing this as a base class; ignores
// virtual bases.
+ // FIXME: Skip this for final classes once the ABI permits us to.
CGM.EmitGlobal(GlobalDecl(D, Dtor_Base));
// The destructor used for destructing this as a most-derived class;
@@ -4050,6 +4052,12 @@
const auto *CD = cast<CXXConstructorDecl>(MD);
AliasDecl = GlobalDecl(CD, Ctor_Complete);
}
+
+ // If we don't know that the base class version will definitely exist, we
+ // can't emit a reference to it.
+ if (!CGM.isBaseStructorEmitted(AliasDecl))
+ return StructorCodegen::Emit;
+
llvm::GlobalValue::LinkageTypes Linkage = CGM.getFunctionLinkage(AliasDecl);
if (llvm::GlobalValue::isDiscardableIfUnused(Linkage))
Index: clang/lib/CodeGen/CodeGenModule.h
===================================================================
--- clang/lib/CodeGen/CodeGenModule.h
+++ clang/lib/CodeGen/CodeGenModule.h
@@ -710,6 +710,11 @@
CtorList &getGlobalCtors() { return GlobalCtors; }
CtorList &getGlobalDtors() { return GlobalDtors; }
+ /// For a GlobalDecl representing a complete-object constructor or destructor
+ /// that we will emit, determine whether we know that the base subobject
+ /// constructor or destructor will definitely be emitted if referenced.
+ bool isBaseStructorEmitted(GlobalDecl GD);
+
/// getTBAATypeInfo - Get metadata used to describe accesses to objects of
/// the given type.
llvm::MDNode *getTBAATypeInfo(QualType QTy);
Index: clang/lib/CodeGen/CGClass.cpp
===================================================================
--- clang/lib/CodeGen/CGClass.cpp
+++ clang/lib/CodeGen/CGClass.cpp
@@ -722,6 +722,25 @@
pushEHDestroy(dtorKind, LHS.getAddress(*this), FieldType);
}
+bool CodeGenModule::isBaseStructorEmitted(GlobalDecl GD) {
+ assert((isa<CXXConstructorDecl>(GD.getDecl()) &&
+ GD.getCtorType() == Ctor_Complete) ||
+ (isa<CXXDestructorDecl>(GD.getDecl()) &&
+ GD.getDtorType() == Dtor_Complete));
+ // For a non-final class, the base structor version is always emitted
+ // whenever the complete version is emitted.
+ if (!cast<CXXMethodDecl>(GD.getDecl())->getParent()->isEffectivelyFinal())
+ return true;
+ // For a final class, the ABI still requires the base structor to be emitted
+ // with the complete object structor, but GCC 10 does not do this. Don't emit
+ // a reference from an available_externally complete object structor to a
+ // potentially-non-existent base subobject structor.
+ //
+ // See https://github.com/itanium-cxx-abi/cxx-abi/issues/104.
+ return !llvm::GlobalValue::isAvailableExternallyLinkage(
+ getFunctionLinkage(GD));
+}
+
/// Checks whether the given constructor is a valid subject for the
/// complete-to-base constructor delegation optimization, i.e.
/// emitting the complete constructor as a simple call to the base
@@ -849,7 +868,8 @@
// Before we go any further, try the complete->base constructor
// delegation optimization.
if (CtorType == Ctor_Complete && IsConstructorDelegationValid(Ctor) &&
- CGM.getTarget().getCXXABI().hasConstructorVariants()) {
+ CGM.getTarget().getCXXABI().hasConstructorVariants() &&
+ CGM.isBaseStructorEmitted(CurGD)) {
EmitDelegateCXXConstructorCall(Ctor, Ctor_Base, Args, Ctor->getEndLoc());
return;
}
@@ -1502,7 +1522,7 @@
// Enter the cleanup scopes for virtual bases.
EnterDtorCleanups(Dtor, Dtor_Complete);
- if (!isTryBody) {
+ if (!isTryBody && CGM.isBaseStructorEmitted(CurGD)) {
QualType ThisTy = Dtor->getThisObjectType();
EmitCXXDestructorCall(Dtor, Dtor_Base, /*ForVirtualBase=*/false,
/*Delegating=*/false, LoadCXXThisAddress(), ThisTy);
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