Patch to optimize away copy constructor call when

appropriate.

llvm-svn: 78267

clang/lib/CodeGen/CGCXX.cpp

@@ -256,6 +256,19 @@ CodeGenFunction::EmitCXXConstructExpr(llvm::Value *Dest,
   if (RD->hasTrivialConstructor())
     return;
 
+  // Code gen optimization to eliminate copy constructor and return 
+  // its first argument instead.
+  const CXXConstructorDecl *CDecl = E->getConstructor();
+  if (E->getNumArgs() == 1 &&
+      CDecl->isCopyConstructor(getContext())) {
+    CXXConstructExpr::const_arg_iterator i = E->arg_begin();
+    const Expr *SubExpr = (*i);
+    // FIXME. Any other cases can be optimized away?
+    if (isa<CallExpr>(SubExpr) || isa<CXXTemporaryObjectExpr>(SubExpr)) {
+      EmitAggExpr(SubExpr, Dest, false);
+      return;
+    }
+  }
   // Call the constructor.
   EmitCXXConstructorCall(E->getConstructor(), Ctor_Complete, Dest, 
                          E->arg_begin(), E->arg_end());

clang/test/CodeGenCXX/copy-constructor-elim.cpp

@@ -0,0 +1,31 @@
+// RUN: clang-cc -emit-llvm -o %t %s &&
+// RUN: grep "_ZN1CC1ERK1C" %t | count 0
+
+extern "C" int printf(...);
+
+
+struct C {
+	C() : iC(6) {printf("C()\n"); }
+	C(const C& c) { printf("C(const C& c)\n"); }
+	int iC;
+};
+
+C foo() {
+  return C();
+};
+
+class X { // ...
+public: 
+	X(int) {}
+	X(const X&, int i = 1, int j = 2, C c = foo()) {
+		printf("X(const X&, %d, %d, %d)\n", i, j, c.iC);
+	}
+};
+
+int main()
+{
+	X a(1);
+	X b(a, 2);
+	X c = b;
+	X d(a, 5, 6);
+}