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.

1 files changed, 7 insertions, 1 deletions

diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
index 3506b0f..9fad3cb 100644
--- a/gcc/cp/call.cc
+++ b/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);