On Tue, 7 Jan 2020, Jakub Jelinek wrote:
> Hi!
>
> In x * x = ((x & -2) + (x & 1))^2 = ((x^2) & -4) + 2 * (x & -2) * (x & 1) +
> (x & 1)^2
> the first two terms are divisible by 4 and the last one is 0 or 1, so
> (x * x) % 4U <= 1
> and thus bit ccp can assume he second least significant bit of any power of
> two is
> 0.
>
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
OK.
Thanks,
Richard.
> 2020-01-07 Jakub Jelinek <ja...@redhat.com>
>
> PR tree-optimization/93156
> * tree-ssa-ccp.c (bit_value_binop): For x * x note that the second
> least significant bit is always clear.
>
> * gcc.dg/tree-ssa/pr93156.c: New test.
>
> --- gcc/tree-ssa-ccp.c.jj 2020-01-01 12:15:48.026609558 +0100
> +++ gcc/tree-ssa-ccp.c 2020-01-06 15:39:16.807109578 +0100
> @@ -1650,6 +1650,17 @@ bit_value_binop (enum tree_code code, tr
> TYPE_SIGN (TREE_TYPE (rhs2)), TYPE_PRECISION (TREE_TYPE
> (rhs2)),
> value_to_wide_int (r2val), r2val.mask);
>
> + /* (x * x) & 2 == 0. */
> + if (code == MULT_EXPR && rhs1 == rhs2 && TYPE_PRECISION (type) > 1)
> + {
> + widest_int m = 2;
> + if (wi::sext (mask, TYPE_PRECISION (type)) != -1)
> + value = wi::bit_and_not (value, m);
> + else
> + value = 0;
> + mask = wi::bit_and_not (mask, m);
> + }
> +
> if (wi::sext (mask, TYPE_PRECISION (type)) != -1)
> {
> val.lattice_val = CONSTANT;
> --- gcc/testsuite/gcc.dg/tree-ssa/pr93156.c.jj 2020-01-06
> 15:26:57.750286597 +0100
> +++ gcc/testsuite/gcc.dg/tree-ssa/pr93156.c 2020-01-06 15:40:32.987957791
> +0100
> @@ -0,0 +1,23 @@
> +/* PR tree-optimization/93156 */
> +/* { dg-do compile } */
> +/* { dg-options "-O2 -fdump-tree-optimized" } */
> +/* { dg-final { scan-tree-dump-times "return 0;" 3 "optimized" } } */
> +
> +int
> +foo (int x)
> +{
> + return (x * x) & 2;
> +}
> +
> +unsigned long long
> +bar (unsigned long long x)
> +{
> + return (x * x) & 2;
> +}
> +
> +int
> +baz (int x)
> +{
> + x &= -2;
> + return (x * x) & 3;
> +}
>
> Jakub
>
>
--
Richard Biener <rguent...@suse.de>
SUSE Software Solutions Germany GmbH, Maxfeldstrasse 5, 90409 Nuernberg,
Germany; GF: Felix Imendörffer; HRB 36809 (AG Nuernberg)
