[Bug target/115406] [15 Regression] wrong code with vector compare at -O0 with -mavx512f since r15-920-gb6c6d5abf0d31c

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115406

--- Comment #9 from GCC 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:348d890c287a7ec4c88d3082ae6105537bd39398

commit r15-2161-g348d890c287a7ec4c88d3082ae6105537bd39398
Author: Richard Sandiford <richard.sandiford@arm.com>
Date:   Fri Jul 19 19:09:37 2024 +0100

    Treat boolean vector elements as 0/-1 [PR115406]

    Previously we built vector boolean constants using 1 for true
    elements and 0 for false elements.  This matches the predicates
    produced by SVE's PTRUE instruction, but leads to a miscompilation
    on AVX, where all bits of a boolean element should be set.

    One option for RTL would be to make this target-configurable.
    But that isn't really possible at the tree level, where vectors
    should work in a more target-independent way.  (There is currently
    no way to create a "generic" packed boolean vector, but never say
    never :))  And, if we were going to pick a generic behaviour,
    it would make sense to use 0/-1 rather than 0/1, for consistency
    with integer vectors.

    Both behaviours should work with SVE on read, since SVE ignores
    the upper bits in each predicate element.  And the choice shouldn't
    make much difference for RTL, since all SVE predicate modes are
    expressed as vectors of BI, rather than of multi-bit booleans.

    I suspect there might be some fallout from this change on SVE.
    But I think we should at least give it a go, and see whether any
    fallout provides a strong counterargument against the approach.

    gcc/
            PR middle-end/115406
            * fold-const.cc (native_encode_vector_part): For vector booleans,
            check whether an element is nonzero and, if so, set all of the
            correspending bits in the target image.
            * simplify-rtx.cc (native_encode_rtx): Likewise.

    gcc/testsuite/
            PR middle-end/115406
            * gcc.dg/torture/pr115406.c: New test.