[Bug middle-end/106144] wide_int shifted_mask() and mask() do not agree

["cvs-commit at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106144

--- Comment #7 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-10 branch has been updated by Jakub Jelinek
<jakub@gcc.gnu.org>:

https://gcc.gnu.org/g:6e7bf9bc3e51d2d0d0df3d810cb20624594573cf

commit r10-11334-g6e7bf9bc3e51d2d0d0df3d810cb20624594573cf
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Fri Jul 1 11:17:41 2022 +0200

    wide-int: Fix up wi::shifted_mask [PR106144]

    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.

    2022-07-01  Jakub Jelinek  <jakub@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.

    (cherry picked from commit e52592073f6df3d7a3acd9f0436dcc32a8b7493d)