On Wed, 16 Nov 2016, Richard Biener wrote:
On Wed, 16 Nov 2016, Marc Glisse wrote:
On Wed, 16 Nov 2016, Richard Biener wrote:
On Wed, 16 Nov 2016, Marc Glisse wrote:
On Wed, 16 Nov 2016, Richard Biener wrote:
I am testing the following to avoid undefined behavior when negating
a multiplication (basically extending a previous fix to properly handle
negative power of two).
Bootstrap / regtest running on x86_64-unknown-linux-gnu.
Richard.
2016-11-16 Richard Biener <rguent...@suse.de>
PR middle-end/78305
* fold-const.c (negate_expr_p): Fix multiplication case.
* gcc.dg/torture/pr78305.c: New testcase.
Index: gcc/fold-const.c
===================================================================
--- gcc/fold-const.c (revision 242471)
+++ gcc/fold-const.c (working copy)
@@ -450,13 +450,15 @@ negate_expr_p (tree t)
if (TYPE_UNSIGNED (type))
break;
/* INT_MIN/n * n doesn't overflow while negating one operand it
does
- if n is a power of two. */
+ if n is a power of (minus) two. */
if n is (minus) a power of two.
if n is a divisor of INT_MIN.
n is a divisor of INT_MIN is correct.
if (INTEGRAL_TYPE_P (TREE_TYPE (t))
&& ! TYPE_OVERFLOW_WRAPS (TREE_TYPE (t))
&& ! ((TREE_CODE (TREE_OPERAND (t, 0)) == INTEGER_CST
- && ! integer_pow2p (TREE_OPERAND (t, 0)))
+ && (wi::popcount (TREE_OPERAND (t, 0))
+ != 1 + wi::neg_p (TREE_OPERAND (t, 0), SIGNED)))
Is that supposed to test for (possibly negated) powers of 2? I don't see
it.
For -2, aka 0b11...110, popcount is 31 != 1 + 1.
It's supposed to test for a power of two with the sign-bit ORed in.
I believe those are which, when multiplied with some number can
yield INT_MIN. That is, we look for a test that ensures that there
exists no number when multiplied with X yields INT_MIN.
The first sentence about ORing the sign bit sounds strange (except for a
sign-magnitude representation). With 2's complement, INT_MIN is -2^31, the
divisors are the 2^k and -(2^k). -2 * 2^30 yields INT_MIN, but your test
misses -2 as a possible divisor. On the other hand, 0b100...001 (aka -INT_MAX)
is not a divisor of INT_MIN but your test says the reverse.
Yeah, but it handled the testcase ;) So I guess the easiest would be
to check integer_pow2p (abs (TREE_OPERAND (t, 0)) then, thus
wi::popcount (wi::abs (TREE_OPERAND (t, 0))) == 1?
Looks good to me, thanks.
--
Marc Glisse
