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.

diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
index 3506b0fcfbb5b9cbe5f9109daf77b68807117a25..9fad3cb950b59693efead3cba6a20b08cc75804f 100644 (file)
--- 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);

diff --git a/gcc/testsuite/g++.dg/tree-ssa/cond-temp1.C b/gcc/testsuite/g++.dg/tree-ssa/cond-temp1.C
new file mode 100644 (file)
index 0000000..b156358
--- /dev/null
+++ b/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);
+}