Jakub Jelinek <ja...@redhat.com> writes:
> Hi!
>
> As the following self-test testcase shows, wi::shifted_mask sometimes
> doesn't create canonicalized wide_ints, which then fail to compare equal
> to canonicalized wide_ints with the same value.
> In particular, wi::mask (128, false, 128) gives { -1 } with len 1 and prec
> 128,
> while wi::shifted_mask (0, 128, false, 128) gives { -1, -1 } with len 2
> and prec 128.
> The problem is that the code is written with the assumption that there are
> 3 bit blocks (or 2 if start is 0), but doesn't consider the possibility
> where there are 2 bit blocks (or 1 if start is 0) where the highest block
> isn't present. In that case, there is the optional block of negate ? 0 : -1
> elts, followed by just one elt (either one from the if (shift) or just
> negate ? -1 : 0) and the rest is implicit sign-extension.
> Only if end < prec there is 1 or more bits above it that have different bit
> value and so we need to emit all the elts till end and then one more elt.
>
> if (end == prec) would work too, because we have:
> if (width > prec - start)
> width = prec - start;
> unsigned int end = start + width;
> so end is guaranteed to be end <= prec, dunno what is preferred.
>
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
>
> 2022-07-01 Jakub Jelinek <ja...@redhat.com>
>
> PR middle-end/106144
> * wide-int.cc (wi::shifted_mask): If end >= prec, return right after
> emitting element for shift or if shift is 0 first element after start.
> (wide_int_cc_tests): Add tests for equivalency of wi::mask and
> wi::shifted_mask with 0 start.
OK, thanks, but could you also remove the "end < prec" condition from:
else if (end < prec)
val[i++] = negate ? -1 : 0;
Richard
> --- gcc/wide-int.cc.jj 2022-01-11 23:11:23.592273263 +0100
> +++ gcc/wide-int.cc 2022-06-30 20:41:25.506292687 +0200
> @@ -842,6 +842,13 @@ wi::shifted_mask (HOST_WIDE_INT *val, un
> val[i++] = negate ? block : ~block;
> }
>
> + if (end >= prec)
> + {
> + if (!shift)
> + val[i++] = negate ? 0 : -1;
> + return i;
> + }
> +
> while (i < end / HOST_BITS_PER_WIDE_INT)
> /* 1111111 */
> val[i++] = negate ? 0 : -1;
> @@ -2583,6 +2590,10 @@ wide_int_cc_tests ()
> run_all_wide_int_tests <widest_int> ();
> test_overflow ();
> test_round_for_mask ();
> + ASSERT_EQ (wi::mask (128, false, 128),
> + wi::shifted_mask (0, 128, false, 128));
> + ASSERT_EQ (wi::mask (128, true, 128),
> + wi::shifted_mask (0, 128, true, 128));
> }
>
> } // namespace selftest
>
> Jakub
