The clang half of r150794: after the construction of a global or static const

variable ends, if the variable has a trivial destructor and no mutable
subobjects then emit an llvm.invariant.start call for it. globalopt knows to
make the variable const when evaluating this.

llvm-svn: 150798

2 files changed, 77 insertions, 1 deletions

diff --git a/clang/lib/CodeGen/CGDeclCXX.cpp b/clang/lib/CodeGen/CGDeclCXX.cpp
index 189760821b7..75bb7dedd90 100644
--- a/clang/lib/CodeGen/CGDeclCXX.cpp
+++ b/clang/lib/CodeGen/CGDeclCXX.cpp
@@ -101,6 +101,19 @@ static void EmitDeclDestroy(CodeGenFunction &CGF, const VarDecl &D,
   CGF.EmitCXXGlobalDtorRegistration(function, argument);
 }
 
+/// Emit code to cause the variable at the given address to be considered as
+/// constant from this point onwards.
+static void EmitDeclInvariant(CodeGenFunction &CGF, llvm::Constant *Addr) {
+  // Grab the llvm.invariant.start intrinsic.
+  llvm::Intrinsic::ID InvStartID = llvm::Intrinsic::invariant_start;
+  llvm::Constant *InvariantStart = CGF.CGM.getIntrinsic(InvStartID);
+
+  // Emit a call, with size -1 signifying the whole object.
+  llvm::Value *Args[2] = { llvm::ConstantInt::getSigned(CGF.Int64Ty, -1),
+                           llvm::ConstantExpr::getBitCast(Addr, CGF.Int8PtrTy)};
+  CGF.Builder.CreateCall(InvariantStart, Args);
+}
+
 void CodeGenFunction::EmitCXXGlobalVarDeclInit(const VarDecl &D,
                                                llvm::Constant *DeclPtr,
                                                bool PerformInit) {
@@ -111,7 +124,10 @@ void CodeGenFunction::EmitCXXGlobalVarDeclInit(const VarDecl &D,
   if (!T->isReferenceType()) {
     if (PerformInit)
       EmitDeclInit(*this, D, DeclPtr);
-    EmitDeclDestroy(*this, D, DeclPtr);
+    if (CGM.isTypeConstant(D.getType(), true))
+      EmitDeclInvariant(*this, DeclPtr);
+    else
+      EmitDeclDestroy(*this, D, DeclPtr);
     return;
   }
 
diff --git a/clang/test/CodeGenCXX/init-invariant.cpp b/clang/test/CodeGenCXX/init-invariant.cpp
new file mode 100644
index 00000000000..da17303b2d4
--- /dev/null
+++ b/clang/test/CodeGenCXX/init-invariant.cpp
@@ -0,0 +1,60 @@
+// RUN: %clang_cc1 -triple i686-linux-gnu -emit-llvm %s -o - | FileCheck %s
+
+// Check that we add an llvm.invariant.start to mark when a global becomes
+// read-only. If globalopt can fold the initializer, it will then mark the
+// variable as constant.
+
+struct A {
+  A() : n(42) {}
+  int n;
+};
+
+// CHECK: @a = global {{.*}} zeroinitializer
+extern const A a = A();
+
+struct B {
+  B() : n(76) {}
+  mutable int n;
+};
+
+// CHECK: @b = global {{.*}} zeroinitializer
+extern const B b = B();
+
+struct C {
+  C() : n(81) {}
+  ~C();
+  int n;
+};
+
+// CHECK: @c = global {{.*}} zeroinitializer
+extern const C c = C();
+
+int f() { return 5; }
+// CHECK: @d = global i32 0
+extern const int d = f();
+
+void e() {
+  static const A a = A();
+}
+
+// CHECK: define internal void @__cxx_global_var_init
+// CHECK: call void @_ZN1AC1Ev({{.*}}* @a)
+// CHECK-NEXT: call {{.*}}@llvm.invariant.start(i64 -1, i8* bitcast ({{.*}} @a to i8*))
+
+// CHECK: define internal void @__cxx_global_var_init
+// CHECK: call void @_ZN1BC1Ev({{.*}}* @b)
+// CHECK-NOT: call {{.*}}@llvm.invariant.start(i64 -1, i8* bitcast ({{.*}} @b to i8*))
+
+// CHECK: define internal void @__cxx_global_var_init
+// CHECK: call void @_ZN1CC1Ev({{.*}}* @c)
+// CHECK-NOT: call {{.*}}@llvm.invariant.start(i64 -1, i8* bitcast ({{.*}} @c to i8*))
+
+// CHECK: define internal void @__cxx_global_var_init
+// CHECK: call i32 @_Z1fv(
+// CHECK: store {{.*}}, i32* @d
+// CHECK: call {{.*}}@llvm.invariant.start(i64 -1, i8* bitcast ({{.*}} @d to i8*))
+
+// CHECK: define void @_Z1ev(
+// CHECK: call void @_ZN1AC1Ev(%struct.A* @_ZZ1evE1a)
+// CHECK: call {{.*}}@llvm.invariant.start(i64 -1, i8* bitcast ({{.*}} @_ZZ1evE1a to i8*))
+// CHECK-NOT: llvm.invariant.end