[Bug target/99977] arm: ICE with __sync_bool_compare_and_swap and -mcpu=cortex-m23

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99977

--- Comment #10 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-9 branch has been updated by Alex Coplan
<acoplan@gcc.gnu.org>:

https://gcc.gnu.org/g:f037a81fcd1ac5d5adddfb204e1c07bdd2bffbbe

commit r9-10006-gf037a81fcd1ac5d5adddfb204e1c07bdd2bffbbe
Author: Alex Coplan <alex.coplan@arm.com>
Date:   Wed Apr 6 11:16:10 2022 +0100

    arm: Fix ICEs with compare-and-swap and -march=armv8-m.base [PR99977]

    The PR shows two ICEs with __sync_bool_compare_and_swap and
    -mcpu=cortex-m23 (equivalently, -march=armv8-m.base): one in LRA and one
    later on, after the CAS insn is split.

    The LRA ICE occurs because the
    @atomic_compare_and_swap<CCSI:arch><SIDI:mode>_1 pattern attempts to tie
    two output operands together (operands 0 and 1 in the third
    alternative). LRA can't handle this, since it doesn't make sense for an
    insn to assign to the same operand twice.

    The later (post-splitting) ICE occurs because the expansion of the
    cbranchsi4_scratch insn doesn't quite go according to plan. As it
    stands, arm_split_compare_and_swap calls gen_cbranchsi4_scratch,
    attempting to pass a register (neg_bval) to use as a scratch register.
    However, since the RTL template has a match_scratch here,
    gen_cbranchsi4_scratch ignores this argument and produces a scratch rtx.
    Since this is all happening after RA, this is doomed to fail (and we get
    an ICE about the insn not matching its constraints).

    It seems that the motivation for the choice of constraints in the
    atomic_compare_and_swap pattern comes from an attempt to satisfy the
    constraints of the cbranchsi4_scratch insn. This insn requires the
    scratch register to be the same as the input register in the case that
    we use a larger negative immediate (one that satisfies J, but not L).

    Of course, as noted above, LRA refuses to assign two output operands to
    the same register, so this was never going to work.

    The solution I'm proposing here is to collapse the alternatives to the
    CAS insn (allowing the two output register operands to be matched to
    different registers) and to ensure that the constraints for
    cbranchsi4_scratch are met in arm_split_compare_and_swap. We do this by
    inserting a move to ensure the source and destination registers match if
    necessary (i.e. in the case of large negative immediates).

    Another notable change here is that we only do:

      emit_move_insn (neg_bval, const1_rtx);

    for non-negative immediates. This is because the ADDS instruction used in
    the negative case suffices to leave a suitable value in neg_bval: if the
    operands compare equal, we don't take the branch (so neg_bval will be
    set by the load exclusive). Otherwise, the ADDS will leave a nonzero
    value in neg_bval, which will correctly signal that the CAS has failed
    when it is later negated.

    gcc/ChangeLog:

            PR target/99977
            * config/arm/arm.c (arm_split_compare_and_swap): Fix up codegen
            with negative immediates: ensure we expand cbranchsi4_scratch
            correctly and ensure we satisfy its constraints.
            * config/arm/sync.md
            (@atomic_compare_and_swap<CCSI:arch><NARROW:mode>_1): Don't
            attempt to tie two output operands together with constraints;
            collapse two alternatives.
            (@atomic_compare_and_swap<CCSI:arch><SIDI:mode>_1): Likewise.
            * config/arm/thumb1.md (cbranchsi4_neg_late): New.

    gcc/testsuite/ChangeLog:

            PR target/99977
            * gcc.target/arm/pr99977.c: New test.