Re: Add support for fully-predicated loops

[Christophe Lyon <christophe dot lyon at linaro dot org>]

Hi Richard,


On 7 January 2018 at 18:08, James Greenhalgh <james.greenhalgh@arm.com> wrote:
> On Mon, Dec 18, 2017 at 07:40:00PM +0000, Jeff Law wrote:
>> On 11/17/2017 07:56 AM, Richard Sandiford wrote:
>> > This patch adds support for using a single fully-predicated loop instead
>> > of a vector loop and a scalar tail.  An SVE WHILELO instruction generates
>> > the predicate for each iteration of the loop, given the current scalar
>> > iv value and the loop bound.  This operation is wrapped up in a new internal
>> > function called WHILE_ULT.  E.g.:
>> >
>> >    WHILE_ULT (0, 3, { 0, 0, 0, 0}) -> { 1, 1, 1, 0 }
>> >    WHILE_ULT (UINT_MAX - 1, UINT_MAX, { 0, 0, 0, 0 }) -> { 1, 0, 0, 0 }
>> >
>> > The third WHILE_ULT argument is needed to make the operation
>> > unambiguous: without it, WHILE_ULT (0, 3) for one vector type would
>> > seem equivalent to WHILE_ULT (0, 3) for another, even if the types have
>> > different numbers of elements.
>> >
>> > Note that the patch uses "mask" and "fully-masked" instead of
>> > "predicate" and "fully-predicated", to follow existing GCC terminology.
>> >
>> > This patch just handles the simple cases, punting for things like
>> > reductions and live-out values.  Later patches remove most of these
>> > restrictions.
>> >
>> > Tested on aarch64-linux-gnu (with and without SVE), x86_64-linux-gnu
>> > and powerpc64le-linux-gnu.  OK to install?
>> >
>> > Richard
>> >
>> >
>> > 2017-11-17  Richard Sandiford  <richard.sandiford@linaro.org>
>> >         Alan Hayward  <alan.hayward@arm.com>
>> >         David Sherwood  <david.sherwood@arm.com>
>> >
>> > gcc/
>> >     * optabs.def (while_ult_optab): New optab.
>> >     * doc/md.texi (while_ult@var{m}@var{n}): Document.
>> >     * internal-fn.def (WHILE_ULT): New internal function.
>> >     * internal-fn.h (direct_internal_fn_supported_p): New override
>> >     that takes two types as argument.
>> >     * internal-fn.c (while_direct): New macro.
>> >     (expand_while_optab_fn): New function.
>> >     (convert_optab_supported_p): Likewise.
>> >     (direct_while_optab_supported_p): New macro.
>> >     * wide-int.h (wi::udiv_ceil): New function.
>> >     * tree-vectorizer.h (rgroup_masks): New structure.
>> >     (vec_loop_masks): New typedef.
>> >     (_loop_vec_info): Add masks, mask_compare_type, can_fully_mask_p
>> >     and fully_masked_p.
>> >     (LOOP_VINFO_CAN_FULLY_MASK_P, LOOP_VINFO_FULLY_MASKED_P)
>> >     (LOOP_VINFO_MASKS, LOOP_VINFO_MASK_COMPARE_TYPE): New macros.
>> >     (vect_max_vf): New function.
>> >     (slpeel_make_loop_iterate_ntimes): Delete.
>> >     (vect_set_loop_condition, vect_get_loop_mask_type, vect_gen_while)
>> >     (vect_halve_mask_nunits, vect_double_mask_nunits): Declare.
>> >     )vect_record_loop_mask, vect_get_loop_mask): Likewise.
>> >     * tree-vect-loop-manip.c: Include tree-ssa-loop-niter.h,
>> >     internal-fn.h, stor-layout.h and optabs-query.h.
>> >     (vect_set_loop_mask): New function.
>> >     (add_preheader_seq): Likewise.
>> >     (add_header_seq): Likewise.
>> >     (vect_maybe_permute_loop_masks): Likewise.
>> >     (vect_set_loop_masks_directly): Likewise.
>> >     (vect_set_loop_condition_masked): Likewise.
>> >     (vect_set_loop_condition_unmasked): New function, split out from
>> >     slpeel_make_loop_iterate_ntimes.
>> >     (slpeel_make_loop_iterate_ntimes): Rename to..
>> >     (vect_set_loop_condition): ...this.  Use vect_set_loop_condition_masked
>> >     for fully-masked loops and vect_set_loop_condition_unmasked otherwise.
>> >     (vect_do_peeling): Update call accordingly.
>> >     (vect_gen_vector_loop_niters): Use VF as the step for fully-masked
>> >     loops.
>> >     * tree-vect-loop.c (_loop_vec_info::_loop_vec_info): Initialize
>> >     mask_compare_type, can_fully_mask_p and fully_masked_p.
>> >     (release_vec_loop_masks): New function.
>> >     (_loop_vec_info): Use it to free the loop masks.
>> >     (can_produce_all_loop_masks_p): New function.
>> >     (vect_get_max_nscalars_per_iter): Likewise.
>> >     (vect_verify_full_masking): Likewise.
>> >     (vect_analyze_loop_2): Save LOOP_VINFO_CAN_FULLY_MASK_P around
>> >     retries, and free the mask rgroups before retrying.  Check loop-wide
>> >     reasons for disallowing fully-masked loops.  Make the final decision
>> >     about whether use a fully-masked loop or not.
>> >     (vect_estimate_min_profitable_iters): Do not assume that peeling
>> >     for the number of iterations will be needed for fully-masked loops.
>> >     (vectorizable_reduction): Disable fully-masked loops.
>> >     (vectorizable_live_operation): Likewise.
>> >     (vect_halve_mask_nunits): New function.
>> >     (vect_double_mask_nunits): Likewise.
>> >     (vect_record_loop_mask): Likewise.
>> >     (vect_get_loop_mask): Likewise.
>> >     (vect_transform_loop): Handle the case in which the final loop
>> >     iteration might handle a partial vector.  Call vect_set_loop_condition
>> >     instead of slpeel_make_loop_iterate_ntimes.
>> >     * tree-vect-stmts.c: Include tree-ssa-loop-niter.h and gimple-fold.h.
>> >     (check_load_store_masking): New function.
>> >     (prepare_load_store_mask): Likewise.
>> >     (vectorizable_store): Handle fully-masked loops.
>> >     (vectorizable_load): Likewise.
>> >     (supportable_widening_operation): Use vect_halve_mask_nunits for
>> >     booleans.
>> >     (supportable_narrowing_operation): Likewise vect_double_mask_nunits.
>> >     (vect_gen_while): New function.
>> >     * config/aarch64/aarch64.md (umax<mode>3): New expander.
>> >     (aarch64_uqdec<mode>): New insn.
>> >     * config/aarch64/aarch64-sve.md (<perm_optab>_<mode>)
>> >     (*aarch64_sve_<perm_insn><perm_hilo><mode>): New predicate patterns.
>> >
>> > gcc/testsuite/
>> >     * gcc.dg/tree-ssa/cunroll-10.c: Disable vectorization.
>> >     * gcc.dg/tree-ssa/peel1.c: Likewise.
>> >     * gcc.dg/vect/vect-load-lanes-peeling-1.c: Remove XFAIL for
>> >     variable-length vectors.
>> >     * gcc.target/aarch64/sve_vcond_6.c: XFAIL test for AND.
>> >     * gcc.target/aarch64/sve_vec_bool_cmp_1.c: Expect BIC instead of NOT.
>> >     * gcc.target/aarch64/sve_slp_1.c: Check for a fully-masked loop.
>> >     * gcc.target/aarch64/sve_slp_2.c: Likewise.
>> >     * gcc.target/aarch64/sve_slp_3.c: Likewise.
>> >     * gcc.target/aarch64/sve_slp_4.c: Likewise.
>> >     * gcc.target/aarch64/sve_slp_6.c: Likewise.
>> >     * gcc.target/aarch64/sve_slp_8.c: New test.
>> >     * gcc.target/aarch64/sve_slp_8_run.c: Likewise.
>> >     * gcc.target/aarch64/sve_slp_9.c: Likewise.
>> >     * gcc.target/aarch64/sve_slp_9_run.c: Likewise.
>> >     * gcc.target/aarch64/sve_slp_10.c: Likewise.
>> >     * gcc.target/aarch64/sve_slp_10_run.c: Likewise.
>> >     * gcc.target/aarch64/sve_slp_11.c: Likewise.
>> >     * gcc.target/aarch64/sve_slp_11_run.c: Likewise.
>> >     * gcc.target/aarch64/sve_slp_12.c: Likewise.
>> >     * gcc.target/aarch64/sve_slp_12_run.c: Likewise.
>> >     * gcc.target/aarch64/sve_ld1r_2.c: Likewise.
>> >     * gcc.target/aarch64/sve_ld1r_2_run.c: Likewise.
>> >     * gcc.target/aarch64/sve_while_1.c: Likewise.
>> >     * gcc.target/aarch64/sve_while_2.c: Likewise.
>> >     * gcc.target/aarch64/sve_while_3.c: Likewise.
>> >     * gcc.target/aarch64/sve_while_4.c: Likewise.
>> Like other SVE related patches, I haven't looked at the aarch64 specific
>> bits, just the generic bits.
>>
>> Sadly, I'm totally lost on this one....   I understand at a 30000ft
>> level what you're trying to do and many of the low level primitives made
>> sense.  But I wasn't able to go from those primitives to the higher
>> level implementation details, even though the higher level
>> implementation details didn't seem all that large.
>>
>> I trust your judgment on this stuff.
>>
>> OK for the trunk.
>
> The AArch64 bits are OK.
>

As I reported in https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83846,
I've noticed that one of the new tests (aarch64/sve/while_4.c)
fails when using -mabi=ilp32

Christophe

> Thanks,
> James
>