c++: reduce temporaries in ?:

When the sides of ?: are class prvalues, we wrap the COND_EXPR in a
TARGET_EXPR so that both sides will initialize the same temporary.  But in
this case we were stripping the outer TARGET_EXPR and conditionally creating
different temporaries, unnecessarily using extra stack.  The
recently added TARGET_EXPR_NO_ELIDE flag avoids this.

gcc/cp/ChangeLog:

	* call.cc (build_conditional_expr): Set TARGET_EXPR_NO_ELIDE on the
	outer TARGET_EXPR.

gcc/testsuite/ChangeLog:

	* g++.dg/tree-ssa/cond-temp1.C: New test.

gcc/cp/call.cc

@@ -6009,7 +6009,13 @@ build_conditional_expr (const op_location_t &loc,
 	 but now we sometimes wrap them in NOP_EXPRs so the test would
 	 fail.  */
       if (CLASS_TYPE_P (TREE_TYPE (result)))
-	result = get_target_expr (result, complain);
+	{
+	  result = get_target_expr (result, complain);
+	  /* Tell gimplify_modify_expr_rhs not to strip this in
+	     assignment context: we want both arms to initialize
+	     the same temporary.  */
+	  TARGET_EXPR_NO_ELIDE (result) = true;
+	}
       /* If this expression is an rvalue, but might be mistaken for an
 	 lvalue, we must add a NON_LVALUE_EXPR.  */
       result = rvalue (result);

gcc/testsuite/g++.dg/tree-ssa/cond-temp1.C

@@ -0,0 +1,16 @@
+// Test that the ?: only creates one temporary.
+// { dg-additional-options "-fdump-tree-gimple" }
+// { dg-final { scan-tree-dump-times "struct A" 2 "gimple" } }
+
+struct A
+{
+  int i;
+  A(int);
+};
+
+bool b;
+int main()
+{
+  A a = 1;
+  a = b ? A(2) : A(3);
+}