[PATCH] Fix PR81097

[Richard Biener]

The following fixes PR81097 but not eventually more latent issues
in transforming ~x "back" to -x - 1.  I want a testcase before
fiddling more with this.  The following follows the pattern
of previous fixes to this function, making sure to do negation
in 'type'.

Bootstrapped and tested on x86_64-unknown-linux-gnu, applied to trunk.

Richard.

2017-06-20  Richard Biener  <rguent...@suse.de>

        PR middle-end/81097
        * fold-const.c (split_tree): Fold to type before negating.

        * c-c++-common/ubsan/pr81097.c: New testcase.

Index: gcc/fold-const.c
===================================================================
--- gcc/fold-const.c    (revision 249397)
+++ gcc/fold-const.c    (working copy)
@@ -853,9 +853,9 @@ split_tree (location_t loc, tree in, tre
           && code == PLUS_EXPR)
     {
       /* -X - 1 is folded to ~X, undo that here.  Do _not_ do this
-         when IN is constant.  */
-      *minus_litp = build_one_cst (TREE_TYPE (in));
-      var = negate_expr (TREE_OPERAND (in, 0));
+         when IN is constant.  Convert to TYPE before negating.  */
+      *minus_litp = build_one_cst (type);
+      var = negate_expr (fold_convert_loc (loc, type, TREE_OPERAND (in, 0)));
     }
   else
     var = in;
Index: gcc/testsuite/c-c++-common/ubsan/pr81097.c
===================================================================
--- gcc/testsuite/c-c++-common/ubsan/pr81097.c  (nonexistent)
+++ gcc/testsuite/c-c++-common/ubsan/pr81097.c  (working copy)
@@ -0,0 +1,12 @@
+/* { dg-do run } */
+/* { dg-options "-fsanitize=undefined -fsanitize-undefined-trap-on-error" } */
+
+unsigned int a = 3309568;
+unsigned int b = -1204857327;
+short c = -10871;
+short x;
+int main()
+{
+  x = ((short)(~a) | ~c) +  ((short)(~b) | ~c);
+  return 0;
+}