On 09/08/2016 02:59 PM, Jason Merrill wrote:
On Thu, Sep 8, 2016 at 11:55 AM, Joseph Myers <jos...@codesourcery.com> wrote:
> On Thu, 8 Sep 2016, Jason Merrill wrote:
>> Various places in GCC use negate, bit-and and compare to test whether
>> an integer is a power of 2, but I think it would be clearer for this
>> test to be wrapped in a function.
> (x & -x) == x is also true for 0.  Whatever the correct function semantics
> for 0, the comment needs to reflect them.
Yep, I was just realizing that, too.  This much larger patch introduces:

 least_bit_hwi: (x & -x)
 pow2_or_zerop: (x & -x) == x
 pow2p_hwi: x && x == (x & -x)
 ctz_or_zero: floor_log2 (x & -x)

and replaces these patterns accordingly.  I'm not at all attached to the names.

Tested x86_64-pc-linux-gnu.


commit 4b76501b72f3953ac57cb077b4e25a90afb6d9a9
Author: Jason Merrill <ja...@redhat.com>
Date:   Thu Sep 8 13:10:12 2016 -0400

            Add inline functions for various bitwise operations.

            * hwint.h (least_bit_hwi, pow2_or_zerop, pow2p_hwi, ctz_or_zero):
            * hwint.c (exact_log2): Use pow2p_hwi.
            (ctz_hwi, ffs_hwi): Use least_bit_hwi.
            * alias.c (memrefs_conflict_p): Use pow2_or_zerop.
            * builtins.c (get_object_alignment_2, get_object_alignment)
            (get_pointer_alignment, fold_builtin_atomic_always_lock_free): Use
            * calls.c (compute_argument_addresses, store_one_arg): Use
            * cfgexpand.c (expand_one_stack_var_at): Use least_bit_hwi.
            * combine.c (force_to_mode): Use least_bit_hwi.
            * emit-rtl.c (set_mem_attributes_minus_bitpos, adjust_address_1):
            Use least_bit_hwi.
            * expmed.c (synth_mult, expand_divmod): Use ctz_or_zero, ctz_hwi.
            (init_expmed_one_conv): Use pow2p_hwi.
            * fold-const.c (round_up_loc, round_down_loc): Use pow2_or_zerop.
            (fold_binary_loc): Use pow2p_hwi.
            * function.c (assign_parm_find_stack_rtl): Use least_bit_hwi.
            * gimple-fold.c (gimple_fold_builtin_memory_op): Use pow2p_hwi.
            * gimple-ssa-strength-reduction.c (replace_ref): Use least_bit_hwi.
            * hsa-gen.c (gen_hsa_addr_with_align, hsa_bitmemref_alignment):
            Use least_bit_hwi.
            * ipa-cp.c (ipcp_alignment_lattice::meet_with_1): Use least_bit_hwi.
            * ipa-prop.c (ipa_modify_call_arguments): Use least_bit_hwi.
            * omp-low.c (oacc_loop_fixed_partitions)
            (oacc_loop_auto_partitions): Use least_bit_hwi.
            * rtlanal.c (nonzero_bits1): Use ctz_or_zero.
            * stor-layout.c (place_field): Use least_bit_hwi.
            * tree-pretty-print.c (dump_generic_node): Use pow2p_hwi.
            * tree-sra.c (build_ref_for_offset): Use least_bit_hwi.
            * tree-ssa-ccp.c (ccp_finalize): Use least_bit_hwi.
            * tree-ssa-math-opts.c (bswap_replace): Use least_bit_hwi.
            * tree-ssa-strlen.c (handle_builtin_memcmp): Use pow2p_hwi.
            * tree-vect-data-refs.c (vect_analyze_group_access_1)
            (vect_grouped_store_supported, vect_grouped_load_supported)
            (vect_permute_load_chain, vect_shift_permute_load_chain)
            (vect_transform_grouped_load): Use pow2p_hwi.
            * tree-vect-generic.c (expand_vector_divmod): Use ctz_or_zero.
            * tree-vect-patterns.c (vect_recog_divmod_pattern): Use ctz_or_zero.
            * tree-vect-stmts.c (vectorizable_mask_load_store): Use
            * tsan.c (instrument_expr): Use least_bit_hwi.
            * var-tracking.c (negative_power_of_two_p): Use pow2_or_zerop.
I was briefly worried about some of the expmed changes, but managed to convince myself they were correct.

Ok for the trunk.


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