fold-const: Fix A <= 0 ? A : -A folding [PR95810]

We folded A <= 0 ? A : -A into -ABS (A), which is for signed integral types
incorrect - can invoke on INT_MIN UB twice, once on ABS and once on its
negation.

The following patch fixes it by instead folding it to (type)-ABSU (A).

2020-06-24  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/95810
	* fold-const.c (fold_cond_expr_with_comparison): Optimize
	A <= 0 ? A : -A into (type)-absu(A) rather than -abs(A).

	* gcc.dg/ubsan/pr95810.c: New test.

(cherry picked from commit 01e10b0ee77e82cb331414c569e02dc7a2c4999e)

gcc/fold-const.c

@@ -5770,8 +5770,22 @@ fold_cond_expr_with_comparison (location_t loc, tree type,
       case LT_EXPR:
 	if (TYPE_UNSIGNED (TREE_TYPE (arg1)))
 	  break;
-	tem = fold_build1_loc (loc, ABS_EXPR, TREE_TYPE (arg1), arg1);
-	return negate_expr (fold_convert_loc (loc, type, tem));
+	if (ANY_INTEGRAL_TYPE_P (TREE_TYPE (arg1))
+	    && !TYPE_OVERFLOW_WRAPS (TREE_TYPE (arg1)))
+	  {
+	    /* A <= 0 ? A : -A for A INT_MIN is valid, but -abs(INT_MIN)
+	       is not, invokes UB both in abs and in the negation of it.
+	       So, use ABSU_EXPR instead.  */
+	    tree utype = unsigned_type_for (TREE_TYPE (arg1));
+	    tem = fold_build1_loc (loc, ABSU_EXPR, utype, arg1);
+	    tem = negate_expr (tem);
+	    return fold_convert_loc (loc, type, tem);
+	  }
+	else
+	  {
+	    tem = fold_build1_loc (loc, ABS_EXPR, TREE_TYPE (arg1), arg1);
+	    return negate_expr (fold_convert_loc (loc, type, tem));
+	  }
       default:
 	gcc_assert (TREE_CODE_CLASS (comp_code) == tcc_comparison);
 	break;

gcc/testsuite/gcc.dg/ubsan/pr95810.c

+/* PR middle-end/95810 */
+/* { dg-do run } */
+/* { dg-options "-fsanitize=undefined -fno-sanitize-recover=undefined" } */
+
+int
+main ()
+{
+  int x = -__INT_MAX__ - 1;
+  x = (x <= 0 ? x : -x);
+  if (x != -__INT_MAX__ - 1)
+    __builtin_abort ();
+  return 0;
+}