Emit static constexpr member as available_externally definition

By exposing the constant initializer, the optimizer can fold many
of these constructs.

Differential Revision: https://reviews.llvm.org/D34992

llvm-svn: 311857

clang/lib/CodeGen/CodeGenModule.cpp

@@ -2437,6 +2437,28 @@ CodeGenModule::GetOrCreateLLVMGlobal(StringRef MangledName,
         D->getType().isConstant(Context) &&
         isExternallyVisible(D->getLinkageAndVisibility().getLinkage()))
       GV->setSection(".cp.rodata");
+
+    // Check if we a have a const declaration with an initializer, we may be
+    // able to emit it as available_externally to expose it's value to the
+    // optimizer.
+    if (Context.getLangOpts().CPlusPlus && GV->hasExternalLinkage() &&
+        D->getType().isConstQualified() && !GV->hasInitializer() &&
+        !D->hasDefinition() && D->hasInit() && !D->hasAttr<DLLImportAttr>()) {
+      const auto *Record =
+          Context.getBaseElementType(D->getType())->getAsCXXRecordDecl();
+      bool HasMutableFields = Record && Record->hasMutableFields();
+      if (!HasMutableFields) {
+        const VarDecl *InitDecl;
+        const Expr *InitExpr = D->getAnyInitializer(InitDecl);
+        if (InitExpr) {
+          GV->setConstant(true);
+          GV->setLinkage(llvm::GlobalValue::AvailableExternallyLinkage);
+          ConstantEmitter emitter(*this);
+          GV->setInitializer(emitter.tryEmitForInitializer(*InitDecl));
+          emitter.finalize(GV);
+        }
+      }
+    }
   }
 
   auto ExpectedAS =

clang/test/CodeGenCXX/cxx11-extern-constexpr.cpp

@@ -0,0 +1,55 @@
+// RUN: %clang_cc1 -std=c++11 %s -emit-llvm -o - -triple x86_64-linux-gnu | FileCheck %s --check-prefix=CHECK --check-prefix=CXX11
+// RUN: %clang_cc1 -std=c++1z %s -emit-llvm -o - -triple x86_64-linux-gnu | FileCheck %s --check-prefix=CHECK --check-prefix=CXX17
+
+struct A {
+  static const int Foo = 123;
+};
+// CHECK: @_ZN1A3FooE = constant i32 123, align 4
+const int *p = &A::Foo; // emit available_externally
+const int A::Foo;       // convert to full definition
+
+struct Bar {
+  int b;
+};
+
+struct MutableBar {
+  mutable int b;
+};
+
+struct Foo {
+  // CXX11: @_ZN3Foo21ConstexprStaticMemberE = available_externally constant i32 42,
+  // CXX17: @_ZN3Foo21ConstexprStaticMemberE = linkonce_odr constant i32 42,
+  static constexpr int ConstexprStaticMember = 42;
+  // CHECK: @_ZN3Foo17ConstStaticMemberE = available_externally constant i32 43,
+  static const int ConstStaticMember = 43;
+
+  // CXX11: @_ZN3Foo23ConstStaticStructMemberE = available_externally constant %struct.Bar { i32 44 },
+  // CXX17: @_ZN3Foo23ConstStaticStructMemberE = linkonce_odr constant %struct.Bar { i32 44 },
+  static constexpr Bar ConstStaticStructMember = {44};
+
+  // CXX11: @_ZN3Foo34ConstexprStaticMutableStructMemberE = external global %struct.MutableBar,
+  // CXX17: @_ZN3Foo34ConstexprStaticMutableStructMemberE = linkonce_odr global %struct.MutableBar { i32 45 },
+  static constexpr MutableBar ConstexprStaticMutableStructMember = {45};
+};
+// CHECK: @_ZL15ConstStaticexpr = internal constant i32 46,
+static constexpr int ConstStaticexpr = 46;
+// CHECK: @_ZL9ConstExpr = internal constant i32 46, align 4
+static const int ConstExpr = 46;
+
+// CHECK: @_ZL21ConstexprStaticStruct = internal constant %struct.Bar { i32 47 },
+static constexpr Bar ConstexprStaticStruct = {47};
+
+// CHECK: @_ZL28ConstexprStaticMutableStruct = internal global %struct.MutableBar { i32 48 },
+static constexpr MutableBar ConstexprStaticMutableStruct = {48};
+
+void use(const int &);
+void foo() {
+  use(Foo::ConstexprStaticMember);
+  use(Foo::ConstStaticMember);
+  use(Foo::ConstStaticStructMember.b);
+  use(Foo::ConstexprStaticMutableStructMember.b);
+  use(ConstStaticexpr);
+  use(ConstExpr);
+  use(ConstexprStaticStruct.b);
+  use(ConstexprStaticMutableStruct.b);
+}