[PATCH] Fix gimplification of const var initialization from COND_EXPR (PR c++/80129)

[Jakub Jelinek <jakub@redhat.com>]

Hi!

For non-gimple reg types var = x ? y : z; is gimplified as
x ? (var = y) : (var = z);.  The problem is that if var is TREE_READONLY,
we can gimplify one of those assignments by promoting them to TREE_STATIC
(if not addressable) and making the initializer into their DECL_INITIAL.
That doesn't work well, because then we conditionally store into that and
during optimizations, as it is a TREE_READONLY var with DECL_INITIAL, just
use that initializer for optimizations.

The following patch fixes that by clearing TREE_READONLY flag if we are
going to store it more than once.

Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

2017-03-21  Jakub Jelinek  <jakub@redhat.com>

	PR c++/80129
	* gimplify.c (gimplify_modify_expr_rhs) <case COND_EXPR>: Clear
	TREE_READONLY on result if writing it more than once.

	* g++.dg/torture/pr80129.C: New test.

--- gcc/gimplify.c.jj	2017-03-21 07:56:55.000000000 +0100
+++ gcc/gimplify.c	2017-03-21 13:37:45.555612652 +0100
@@ -5098,6 +5098,13 @@ gimplify_modify_expr_rhs (tree *expr_p,
 	      if (ret != GS_ERROR)
 		ret = GS_OK;
 
+	      /* If we are going to write RESULT more than once, clear
+		 TREE_READONLY flag, otherwise we might gimplify it
+		 incorrectly.  */
+	      if (DECL_P (result)
+		  && TREE_TYPE (TREE_OPERAND (cond, 1)) != void_type_node
+		  && TREE_TYPE (TREE_OPERAND (cond, 2)) != void_type_node)
+		TREE_READONLY (result) = 0;
 	      if (TREE_TYPE (TREE_OPERAND (cond, 1)) != void_type_node)
 		TREE_OPERAND (cond, 1)
 		  = build2 (code, void_type_node, result,
--- gcc/testsuite/g++.dg/torture/pr80129.C.jj	2017-03-21 13:40:04.179852313 +0100
+++ gcc/testsuite/g++.dg/torture/pr80129.C	2017-03-21 13:41:32.121735570 +0100
@@ -0,0 +1,14 @@
+// PR c++/80129
+// { dg-do run }
+// { dg-options "-std=c++11" }
+
+struct A { bool a; int b; };
+
+int
+main ()
+{
+  bool c = false;
+  const A x = c ? A {true, 1} : A {false, 0};
+  if (x.a)
+    __builtin_abort ();
+}

	Jakub