[Bug rtl-optimization/108681] [13 Regression] gcc hangs compiling opencv/channels_combine.cpp for aarch64

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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108681

--- Comment #8 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The trunk branch has been updated by Richard Sandiford <rsandifo@gcc.gnu.org>:

https://gcc.gnu.org/g:3cac06d84f334705ed0bce12fbc3a4cec4a8fd3b

commit r13-5972-g3cac06d84f334705ed0bce12fbc3a4cec4a8fd3b
Author: Richard Sandiford <richard.sandiford@arm.com>
Date:   Mon Feb 13 21:13:59 2023 +0000

    lra: Replace subregs in bare uses & clobbers [PR108681]

    In this PR we had a write to one vector of a 4-vector tuple.
    The vector had mode V1DI, and the target doesn't provide V1DI
    moves, so this was converted into:

        (clobber (subreg:V1DI (reg/v:V4x1DI 92 [ b ]) 24))

    followed by a DImode move.  (The clobber isn't really necessary
    or helpful for a single word, but would be for wider moves.)

    The subreg in the clobber survived until after RA:

        (clobber (subreg:V1DI (reg/v:V4x1DI 34 v2 [orig:92 b ] [92]) 24))

    IMO this isn't well-formed.  If a subreg of a hard register simplifies
    to a hard register, it should be replaced by the hard register.  If the
    subreg doesn't simplify, then target-independent code can't be sure
    which parts of the register are affected and which aren't.  A clobber
    of such a subreg isn't useful and (again IMO) should just be removed.
    Conversely, a use of such a subreg is effectively a use of the whole
    inner register.

    LRA has code to simplify subregs of hard registers, but it didn't
    handle bare uses and clobbers.  The patch extends it to do that.

    One question was whether the final_p argument to alter_subregs
    should be true or false.  True is IMO dangerous, since it forces
    replacements that might not be valid from a dataflow perspective,
    and uses and clobbers only exist for dataflow.  As said above,
    I think the correct way of handling a failed simplification would
    be to delete clobbers and replace uses of subregs with uses of
    the inner register.  But I didn't want to write untested code
    to do that.

    In the PR, the clobber caused an infinite loop in DCE, because
    of a disagreement about what effect the clobber had.  But for
    the reasons above, I think that was GIGO rather than a bug in
    DF or DCE.

    gcc/
            PR rtl-optimization/108681
            * lra-spills.cc (lra_final_code_change): Extend subreg replacement
            code to handle bare uses and clobbers.

    gcc/testsuite/
            PR rtl-optimization/108681
            * gcc.target/aarch64/pr108681.c: New test.