Hi!
As the following testcase shows, the (-A) - B -> (-B) - A optimization can't
be done the way it is if the negation of A is performed in type with
wrapping behavior while the subtraction is done in signed type (with the
same precision), as if A is (unsigned) INT_MIN, then (int) -(unsigned) INT_MIN
is INT_MIN and INT_MIN - B is different from (-B) - INT_MIN.
The reason we can see this is because we check that arg0 is NEGATE_EXPR, but
arg0 is STRIP_NOPS from op0. If the NEGATE_EXPR is already done in signed
type, then it would be already UB if A was INT_MIN and so we can safely do
it.
Whether we perform the subtraction in the unsigned type or just don't
optimize I think doesn't matter that much, at least the only spot during
x86_64-linux and i686-linux bootstraps/regtests this new condition triggered
was the new testcase, nothing else. So if you instead prefer to punt, I can
tweak the patch, move the negated condition to the if above it.
Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
2017-12-14 Jakub Jelinek <ja...@redhat.com>
PR tree-optimization/83269
* fold-const.c (fold_binary_loc): Perform (-A) - B -> (-B) - A
subtraction in arg0's type if type is signed and arg0 is unsigned.
Formatting fix.
* gcc.c-torture/execute/pr83269.c: New test.
--- gcc/fold-const.c.jj 2017-12-08 00:50:27.000000000 +0100
+++ gcc/fold-const.c 2017-12-14 17:42:31.221398170 +0100
@@ -9098,8 +9098,8 @@ expr_not_equal_to (tree t, const wide_in
return NULL_TREE. */
tree
-fold_binary_loc (location_t loc,
- enum tree_code code, tree type, tree op0, tree op1)
+fold_binary_loc (location_t loc, enum tree_code code, tree type,
+ tree op0, tree op1)
{
enum tree_code_class kind = TREE_CODE_CLASS (code);
tree arg0, arg1, tem;
@@ -9770,10 +9770,34 @@ fold_binary_loc (location_t loc,
/* (-A) - B -> (-B) - A where B is easily negated and we can swap. */
if (TREE_CODE (arg0) == NEGATE_EXPR
&& negate_expr_p (op1))
- return fold_build2_loc (loc, MINUS_EXPR, type,
- negate_expr (op1),
- fold_convert_loc (loc, type,
- TREE_OPERAND (arg0, 0)));
+ {
+ /* If arg0 is e.g. unsigned int and type is int, then we need to
+ perform the subtraction in arg0's type, because if A is
+ INT_MIN at runtime, the original expression can be well defined
+ while the latter is not. See PR83269. */
+ if (ANY_INTEGRAL_TYPE_P (type)
+ && TYPE_OVERFLOW_UNDEFINED (type)
+ && ANY_INTEGRAL_TYPE_P (TREE_TYPE (arg0))
+ && !TYPE_OVERFLOW_UNDEFINED (TREE_TYPE (arg0)))
+ {
+ /* Don't do this when sanitizing, as by doing the subtraction
+ in unsigned type we won't notice if the original program
+ has been buggy. */
+ if (!TYPE_OVERFLOW_SANITIZED (type))
+ {
+ tem = fold_build2_loc (loc, MINUS_EXPR, TREE_TYPE (arg0),
+ fold_convert_loc (loc,
+ TREE_TYPE (arg0),
+ negate_expr (op1)),
+ TREE_OPERAND (arg0, 0));
+ return fold_convert_loc (loc, type, tem);
+ }
+ }
+ else
+ return fold_build2_loc (loc, MINUS_EXPR, type, negate_expr (op1),
+ fold_convert_loc (loc, type,
+ TREE_OPERAND (arg0, 0)));
+ }
/* Fold __complex__ ( x, 0 ) - __complex__ ( 0, y ) to
__complex__ ( x, -y ). This is not the same for SNaNs or if
--- gcc/testsuite/gcc.c-torture/execute/pr83269.c.jj 2017-12-14
17:43:24.534710997 +0100
+++ gcc/testsuite/gcc.c-torture/execute/pr83269.c 2017-12-14
17:43:10.000000000 +0100
@@ -0,0 +1,14 @@
+/* PR tree-optimization/83269 */
+
+int
+main ()
+{
+#if __SIZEOF_INT__ == 4 && __SIZEOF_LONG_LONG__ > 4 && __CHAR_BIT__ == 8
+ volatile unsigned char a = 1;
+ long long b = 0x80000000L;
+ int c = -((int)(-b) - (-0x7fffffff * a));
+ if (c != 1)
+ __builtin_abort ();
+#endif
+ return 0;
+}
Jakub
