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From: "cvs-commit at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/108840] Aarch64 doesn't optimize away shift counter
 masking
Date: Wed, 19 Apr 2023 08:35:50 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D108840

--- Comment #4 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Kyrylo Tkachov <ktkachov@gcc.gnu.org>:

https://gcc.gnu.org/g:136330bf637b50a4f10ace017a4316541386b9c0

commit r14-62-g136330bf637b50a4f10ace017a4316541386b9c0
Author: Kyrylo Tkachov <kyrylo.tkachov@arm.com>
Date:   Wed Apr 19 09:34:40 2023 +0100

    aarch64: PR target/108840 Simplify register shift RTX costs and elimina=
te
shift amount masking

    In this PR we fail to eliminate explicit &31 operations for variable sh=
ifts
such as in:
    void
    bar (int x[3], int y)
    {
      x[0] <<=3D (y & 31);
      x[1] <<=3D (y & 31);
      x[2] <<=3D (y & 31);
    }

    This is rejected by RTX costs that end up giving too high a cost for:
    (set (reg:SI 96)
        (ashift:SI (reg:SI 98)
            (subreg:QI (and:SI (reg:SI 99)
                    (const_int 31 [0x1f])) 0)))

    There is code to handle the AND-31 case in rtx costs, but it gets confu=
sed
by the subreg.
    It's easy enough to fix by looking inside the subreg when costing the
expression.
    While doing that I noticed that the ASHIFT case and the other shift-like
cases are almost identical
    and we should just merge them. This code will only be used for valid in=
sns
anyway, so the code after this
    patch should do the Right Thing (TM) for all such shift cases.

    With this patch there are no more "and wn, wn, 31" instructions left in=
 the
testcase.

    Bootstrapped and tested on aarch64-none-linux-gnu.

            PR target/108840

    gcc/ChangeLog:

            * config/aarch64/aarch64.cc (aarch64_rtx_costs): Merge ASHIFT a=
nd
            ROTATE, ROTATERT, LSHIFTRT, ASHIFTRT cases.  Handle subregs in =
op1.

    gcc/testsuite/ChangeLog:

            * gcc.target/aarch64/pr108840.c: New test.=