[Bug tree-optimization/102124] [11/12 Regression] -ftree-loop-vectorize Causing Data To Lose Sign Bit on AARCH64 Platform

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102124

--- Comment #9 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-11 branch has been updated by Jakub Jelinek
<jakub@gcc.gnu.org>:

https://gcc.gnu.org/g:051040f0642cfd002d31f655a70aef50e6f44d25

commit r11-8948-g051040f0642cfd002d31f655a70aef50e6f44d25
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Wed Sep 1 13:30:51 2021 +0200

    vectorizer: Fix up vectorization using WIDEN_MINUS_EXPR [PR102124]

    The following testcase is miscompiled on aarch64-linux at -O3 since the
    introduction of WIDEN_MINUS_EXPR.
    The problem is if the inner type (half_type) is unsigned and the result
    type in which the subtraction is performed (type) has precision more than
    twice as larger as the inner type's precision.
    For other widening operations like WIDEN_{PLUS,MULT}_EXPR, if half_type
    is unsigned, the addition/multiplication result in itype is also unsigned
    and needs to be zero-extended to type.
    But subtraction is special, even when half_type is unsigned, the
subtraction
    behaves as signed (also regardless of whether the result type is signed or
    unsigned), 0xfeU - 0xffU is -1 or 0xffffffffU, not 0x0000ffff.

    I think it is better not to use mixed signedness of types in
    WIDEN_MINUS_EXPR (have unsigned vector of operands and signed result
    vector), so this patch instead adds another cast to make sure we always
    sign-extend the result from itype to type if type is wider than itype.

    2021-09-01  Jakub Jelinek  <jakub@redhat.com>

            PR tree-optimization/102124
            * tree-vect-patterns.c (vect_recog_widen_op_pattern): For ORIG_CODE
            MINUS_EXPR, if itype is unsigned with smaller precision than type,
            add an extra cast to signed variant of itype to ensure
sign-extension.

            * gcc.dg/torture/pr102124.c: New test.

    (cherry picked from commit bea07159d1d4c9a61c8f7097e9f88c2b206b1b2f)