On Tue, 31 Aug 2021, Jakub Jelinek wrote:
> Hi!
>
> As mentioned in the PR, this hunk is guarded with !wi::neg_p (r1val | r1mask,
> sgn)
> which means if sgn is UNSIGNED, it is always true, but r1val | r1mask in
> widest_int is still sign-extended. That means wi::clz (arg) returns 0,
> wi::get_precision (arg) returns some very large number
> (WIDE_INT_MAX_PRECISION, on x86_64 576 bits) and width is 64, so we end up
> with lzcount of -512 where the code afterwards expects a non-negative
> lzcount. For arg without the sign bit set the code works right, those
> numbers are zero extended and so wi::clz must return wi::get_precision (arg)
> - width
> plus number of leading zero bits within the width precision.
> The patch fixes it by handling the sign-extension specially, either it could
> be done through wi::neg_p (arg) check, but lzcount == 0 works identically.
>
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
OK.
> 2021-08-31 Jakub Jelinek <ja...@redhat.com>
>
> PR tree-optimization/102134
> * tree-ssa-ccp.c (bit_value_binop) <case RSHIFT_EXPR>: If sgn is
> UNSIGNED and r1val | r1mask has MSB set, ensure lzcount doesn't
> become negative.
>
> * gcc.c-torture/execute/pr102134.c: New test.
>
> --- gcc/tree-ssa-ccp.c.jj 2021-08-30 08:36:11.302515439 +0200
> +++ gcc/tree-ssa-ccp.c 2021-08-30 22:49:21.957503630 +0200
> @@ -1695,7 +1695,8 @@ bit_value_binop (enum tree_code code, si
> /* Logical right shift, or zero sign bit. */
> widest_int arg = r1val | r1mask;
> int lzcount = wi::clz (arg);
> - lzcount -= wi::get_precision (arg) - width;
> + if (lzcount)
> + lzcount -= wi::get_precision (arg) - width;
> widest_int tmp = wi::mask <widest_int> (width, false);
> tmp = wi::lrshift (tmp, lzcount);
> tmp = wi::lrshift (tmp, wi::bit_and_not (r2val, r2mask));
> --- gcc/testsuite/gcc.c-torture/execute/pr102134.c.jj 2021-08-30
> 22:47:41.115920522 +0200
> +++ gcc/testsuite/gcc.c-torture/execute/pr102134.c 2021-08-30
> 22:47:20.811205820 +0200
> @@ -0,0 +1,23 @@
> +/* PR tree-optimization/102134 */
> +
> +typedef unsigned long long u64;
> +
> +u64 g;
> +
> +void
> +foo (u64 a, u64 b, u64 c, u64 *r)
> +{
> + b *= b;
> + u64 x = a && ((b >> (c & 63)) | ((b << (c & 63)) & g));
> + *r = x + a;
> +}
> +
> +int
> +main ()
> +{
> + u64 x;
> + foo (1, 3000, 0, &x);
> + if (x != 2)
> + __builtin_abort ();
> + return 0;
> +}
>
> Jakub
>
>
--
Richard Biener <rguent...@suse.de>
SUSE Software Solutions Germany GmbH, Maxfeldstrasse 5, 90409 Nuernberg,
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