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Subject: [Bug middle-end/113205] [14 Regression] internal compiler error: in
 backward_pass, at tree-vect-slp.cc:5346 since r14-3220
Date: Sat, 24 Feb 2024 11:58:36 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D113205

--- Comment #13 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The trunk branch has been updated by Richard Sandiford <rsandifo@gcc.gnu.or=
g>:

https://gcc.gnu.org/g:0394ae31e832c5303f3b4aad9c66710a30c097f0

commit r14-9165-g0394ae31e832c5303f3b4aad9c66710a30c097f0
Author: Richard Sandiford <richard.sandiford@arm.com>
Date:   Sat Feb 24 11:58:22 2024 +0000

    vect: Tighten check for impossible SLP layouts [PR113205]

    During its forward pass, the SLP layout code tries to calculate
    the cost of a layout change on an incoming edge.  This is taken
    as the minimum of two costs: one in which the source partition
    keeps its current layout (chosen earlier during the pass) and
    one in which the source partition switches to the new layout.
    The latter can sometimes be arranged by the backward pass.

    If only one of the costs is valid, the other cost was ignored.
    But the PR shows that this is not safe.  If the source partition
    has layout 0 (the normal layout), we have to be prepared to handle
    the case in which that ends up being the only valid layout.

    Other code already accounts for this restriction, e.g. see
    the code starting with:

        /* Reject the layout if it would make layout 0 impossible
           for later partitions.  This amounts to testing that the
           target supports reversing the layout change on edges
           to later partitions.

    gcc/
            PR tree-optimization/113205
            * tree-vect-slp.cc (vect_optimize_slp_pass::forward_cost): Reje=
ct
            the proposed layout if it does not allow a source partition with
            layout 2 to keep that layout.

    gcc/testsuite/
            PR tree-optimization/113205
            * gcc.dg/torture/pr113205.c: New test.=