Emit static constexpr member as available_externally definition

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

This is a recommit of r311857 that was reverted in r311898 because
an assert was hit when building Chromium.
We have to take into account that the GlobalVariable may be first
created with a different type than the initializer. This can
happen for example when the variable is a struct with tail padding
while the initializer does not have padding. In such case, the
variable needs to be destroyed an replaced with a new one with the
type of the initializer.

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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@312512 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 68 insertions, 0 deletions

diff --git a/test/CodeGenCXX/cxx11-extern-constexpr.cpp b/test/CodeGenCXX/cxx11-extern-constexpr.cpp
new file mode 100644
index 0000000000..6c520038e9
--- /dev/null
+++ b/test/CodeGenCXX/cxx11-extern-constexpr.cpp
@@ -0,0 +1,68 @@
+// 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 PODWithInit {
+  int g = 42;
+  char h = 43;
+};
+struct CreatePOD {
+  // Deferred initialization of the structure here requires changing
+  // the type of the global variable: the initializer list does not include
+  // the tail padding.
+  // CXX11: @_ZN9CreatePOD3podE = available_externally constant { i32, i8 } { i32 42, i8 43 },
+  static constexpr PODWithInit pod{};
+};
+const int *p_pod = &CreatePOD::pod.g;
+
+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);
+}