[Bug rtl-optimization/91838] [8/9 Regression] incorrect use of shr and shrx to shift by 64, missed optimization of vector shift

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91838

--- Comment #13 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-9 branch has been updated by Tamar Christina
<tnfchris@gcc.gnu.org>:

https://gcc.gnu.org/g:f6e9ae4da8f040ab2ef2eb37d0fb4da6f823bf81

commit r9-8210-gf6e9ae4da8f040ab2ef2eb37d0fb4da6f823bf81
Author: Tamar Christina <tamar.christina@arm.com>
Date:   Tue Feb 11 10:50:12 2020 +0000

    middle-end: Fix logical shift truncation (PR rtl-optimization/91838) (gcc-9
backport)

    This fixes a fall-out from a patch I had submitted two years ago which
started
    allowing simplify-rtx to fold logical right shifts by offsets a followed by
b
    into >> (a + b).

    However this can generate inefficient code when the resulting shift count
ends
    up being the same as the size of the shift mode.  This will create some
    undefined behavior on most platforms.

    This patch changes to code to truncate to 0 if the shift amount goes out of
    range.  Before my older patch this used to happen in combine when it saw
the
    two shifts.  However since we combine them here combine never gets a chance
to
    truncate them.

    The issue mostly affects GCC 8 and 9 since on 10 the back-end knows how to
deal
    with this shift constant but it's better to do the right thing in
simplify-rtx.

    Note that this doesn't take care of the Arithmetic shift where you could
replace
    the constant with MODE_BITS (mode) - 1, but that's not a regression so
punting it.

    gcc/ChangeLog:

        Backport from mainline
        2020-01-31  Tamar Christina  <tamar.christina@arm.com>

        PR rtl-optimization/91838
        * simplify-rtx.c (simplify_binary_operation_1): Update LSHIFTRT case
        to truncate if allowed or reject combination.

    gcc/testsuite/ChangeLog:

        Backport from mainline
        2020-01-31  Tamar Christina  <tamar.christina@arm.com>
                    Jakub Jelinek  <jakub@redhat.com>

        PR rtl-optimization/91838
        * g++.dg/opt/pr91838.C: New test.