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? Thanks, Richard.
