On November 16, 2016 5:22:17 PM GMT+01:00, Marc Glisse <marc.gli...@inria.fr> wrote: >On Wed, 16 Nov 2016, Michael Matz wrote: > >> Hi, >> >> On Wed, 16 Nov 2016, Marc Glisse wrote: >> >>>>> 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. >> >> An integer X is a power of two if and only if >> X & -X == 0 (&& X != 0 if you want to exclude zero) >> which also nicely handles positive and negative numbers at the same >time. >> No need for popcounts or abs. > >There are bit tricks to test for powers of 2, but X & -X == 0 doesn't >quite work (X & -X == X is closer, but needs a tweak for negative >numbers). We could use >wi::pow2_p (wi::abs (TREE_OPERAND (t, 0))) >adding a new function pow2_p so it remains readable and we reduce the >risk >of using the wrong bit trick...
Tree_pow2p uses wi::popcount Richard.
