[Bug c/102989] Implement C2x's n2763 (_BitInt)

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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102989

--- Comment #97 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jakub Jelinek <jakub@gcc.gnu.org>:

https://gcc.gnu.org/g:a9d6c7fbeb374365058ffe2b9815d2b4b7193d38

commit r14-3746-ga9d6c7fbeb374365058ffe2b9815d2b4b7193d38
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Wed Sep 6 17:27:41 2023 +0200

    _BitInt lowering support [PR102989]

    The following patch adds a new bitintlower lowering pass which lowers most
    operations on medium _BitInt into operations on corresponding integer
types,
    large _BitInt into straight line code operating on 2 or more limbs and
    finally huge _BitInt into a loop plus optional straight line code.

    As the only supported architecture is little-endian, the lowering only
    supports little-endian for now, because it would be impossible to test it
    all for big-endian.  Rest is written with any endian support in mind, but
    of course only little-endian has been actually tested.
    I hope it is ok to add big-endian support to the lowering pass
incrementally
    later when first big-endian target shows with the backend support.
    There are 2 possibilities of adding such support, one would be minimal one,
    just tweak limb_access function and perhaps one or two other spots and
    transform there the indexes from little endian (index 0 is least
significant)
    to big endian for just the memory access.  Advantage is I think
maintainance
    costs, disadvantage is that the loops will still iterate from 0 to some
number
    of limbs and we'd rely on IVOPTs or something similar changing it later if
    needed.  Or we could make those indexes endian related everywhere, though
    I'm afraid that would be several hundreds of changes.

    For switches indexed by large/huge _BitInt the patch invokes what the
switch
    lowering pass does (but only on those specific switches, not all of them);
    the switch lowering breaks the switches into clusters and none of the
clusters
    can have a range which doesn't fit into 64-bit UWHI, everything else will
be
    turned into a tree of comparisons.  For clusters normally emitted as
smaller
    switches, because we already have a guarantee that the low .. high range is
    at most 64 bits, the patch forces subtraction of the low and turns it into
    a 64-bit switch.  This is done before the actual pass starts.
    Similarly, we cancel lowering of certain constructs like ABS_EXPR,
ABSU_EXPR,
    MIN_EXPR, MAX_EXPR and COND_EXPR and turn those back to simpler comparisons
    etc., so that fewer operations need to be lowered later.

    2023-09-06  Jakub Jelinek  <jakub@redhat.com>

            PR c/102989
            * Makefile.in (OBJS): Add gimple-lower-bitint.o.
            * passes.def: Add pass_lower_bitint after pass_lower_complex and
            pass_lower_bitint_O0 after pass_lower_complex_O0.
            * tree-pass.h (PROP_gimple_lbitint): Define.
            (make_pass_lower_bitint_O0, make_pass_lower_bitint): Declare.
            * gimple-lower-bitint.h: New file.
            * tree-ssa-live.h (struct _var_map): Add bitint member.
            (init_var_map): Adjust declaration.
            (region_contains_p): Handle map->bitint like map->outofssa_p.
            * tree-ssa-live.cc (init_var_map): Add BITINT argument, initialize
            map->bitint and set map->outofssa_p to false if it is non-NULL.
            * tree-ssa-coalesce.cc: Include gimple-lower-bitint.h.
            (build_ssa_conflict_graph): Call build_bitint_stmt_ssa_conflicts if
            map->bitint.
            (create_coalesce_list_for_region): For map->bitint ignore SSA_NAMEs
            not in that bitmap, and allow res without default def.
            (compute_optimized_partition_bases): In map->bitint mode try hard
to
            coalesce any SSA_NAMEs with the same size.
            (coalesce_bitint): New function.
            (coalesce_ssa_name): In map->bitint mode, or map->bitmap into
            used_in_copies and call coalesce_bitint.
            * gimple-lower-bitint.cc: New file.