[PING][PATCH 2/2] arm: Add support for MVE Tail-Predicated Low Overhead Loops

[Kyrylo Tkachov]

Hi Stam,

> -----Original Message-----
> From: Stam Markianos-Wright <Stam.Markianos-Wright@arm.com>
> Sent: Wednesday, September 6, 2023 6:19 PM
> To: gcc-patches@gcc.gnu.org
> Cc: Kyrylo Tkachov <Kyrylo.Tkachov@arm.com>; Richard Earnshaw
> <Richard.Earnshaw@arm.com>
> Subject: [PING][PATCH 2/2] arm: Add support for MVE Tail-Predicated Low
> Overhead Loops
> 
> Hi all,
> 
> This is the 2/2 patch that contains the functional changes needed
> for MVE Tail Predicated Low Overhead Loops.  See my previous email
> for a general introduction of MVE LOLs.
> 
> This support is added through the already existing loop-doloop
> mechanisms that are used for non-MVE dls/le looping.
> 
> Mid-end changes are:
> 
> 1) Relax the loop-doloop mechanism in the mid-end to allow for
>     decrement numbers other that -1 and for `count` to be an
>     rtx containing a simple REG (which in this case will contain
>     the number of elements to be processed), rather
>     than an expression for calculating the number of iterations.
> 2) Added a new df utility function: `df_bb_regno_only_def_find` that
>     will return the DEF of a REG if it is DEF-ed only once within the
>     basic block.
> 
> And many things in the backend to implement the above optimisation:
> 
> 3)  Implement the `arm_predict_doloop_p` target hook to instruct the
>      mid-end about Low Overhead Loops (MVE or not), as well as
>      `arm_loop_unroll_adjust` which will prevent unrolling of any loops
>      that are valid for becoming MVE Tail_Predicated Low Overhead Loops
>      (unrolling can transform a loop in ways that invalidate the dlstp/
>      letp tranformation logic and the benefit of the dlstp/letp loop
>      would be considerably higher than that of unrolling)
> 4)  Appropriate changes to the define_expand of doloop_end, new
>      patterns for dlstp and letp, new iterators,  unspecs, etc.
> 5) `arm_mve_loop_valid_for_dlstp` and a number of checking functions:
>     * `arm_mve_dlstp_check_dec_counter`
>     * `arm_mve_dlstp_check_inc_counter`
>     * `arm_mve_check_reg_origin_is_num_elems`
>     * `arm_mve_check_df_chain_back_for_implic_predic`
>     * `arm_mve_check_df_chain_fwd_for_implic_predic_impact`
>     This all, in smoe way or another, are running checks on the loop
>     structure in order to determine if the loop is valid for dlstp/letp
>     transformation.
> 6) `arm_attempt_dlstp_transform`: (called from the define_expand of
>      doloop_end) this function re-checks for the loop's suitability for
>      dlstp/letp transformation and then implements it, if possible.
> 7) Various utility functions:
>     *`arm_mve_get_vctp_lanes` to map
>     from vctp unspecs to number of lanes, and `arm_get_required_vpr_reg`
>     to check an insn to see if it requires the VPR or not.
>     * `arm_mve_get_loop_vctp`
>     * `arm_mve_get_vctp_lanes`
>     * `arm_emit_mve_unpredicated_insn_to_seq`
>     * `arm_get_required_vpr_reg`
>     * `arm_get_required_vpr_reg_param`
>     * `arm_get_required_vpr_reg_ret_val`
>     * `arm_mve_is_across_vector_insn`
>     * `arm_is_mve_load_store_insn`
>     * `arm_mve_vec_insn_is_predicated_with_this_predicate`
>     * `arm_mve_vec_insn_is_unpredicated_or_uses_other_predicate`
> 
> No regressions on arm-none-eabi with various targets and on
> aarch64-none-elf. Thoughts on getting this into trunk?

The arm parts look sensible but we'd need review for the df-core.h and df-core.cc changes.
Maybe Jeff can help or can recommend someone to take a look?
Thanks,
Kyrill

> 
> Thank you,
> Stam Markianos-Wright
> 
> gcc/ChangeLog:
> 
>      * config/arm/arm-protos.h (arm_target_insn_ok_for_lob): Rename to...
>      (arm_target_bb_ok_for_lob): ...this
>      (arm_attempt_dlstp_transform): New.
>      * config/arm/arm.cc (TARGET_LOOP_UNROLL_ADJUST): New.
>      (TARGET_PREDICT_DOLOOP_P): New.
>      (arm_block_set_vect):
>      (arm_target_insn_ok_for_lob): Rename from arm_target_insn_ok_for_lob.
>      (arm_target_bb_ok_for_lob): New.
>      (arm_mve_get_vctp_lanes): New.
>      (arm_get_required_vpr_reg): New.
>      (arm_get_required_vpr_reg_param): New.
>      (arm_get_required_vpr_reg_ret_val): New.
>      (arm_mve_get_loop_vctp): New.
>      (arm_mve_vec_insn_is_unpredicated_or_uses_other_predicate): New.
>      (arm_mve_vec_insn_is_predicated_with_this_predicate): New.
>      (arm_mve_check_df_chain_back_for_implic_predic): New.
>      (arm_mve_check_df_chain_fwd_for_implic_predic_impact): New.
>      (arm_mve_check_reg_origin_is_num_elems): New.
>      (arm_mve_dlstp_check_inc_counter): New.
>      (arm_mve_dlstp_check_dec_counter): New.
>      (arm_mve_loop_valid_for_dlstp): New.
>      (arm_mve_is_across_vector_insn): New.
>      (arm_is_mve_load_store_insn): New.
>      (arm_predict_doloop_p): New.
>      (arm_loop_unroll_adjust): New.
>      (arm_emit_mve_unpredicated_insn_to_seq): New.
>      (arm_attempt_dlstp_transform): New.
>          * config/arm/iterators.md (DLSTP): New.
>          (mode1): Add DLSTP mappings.
>          * config/arm/mve.md (*predicated_doloop_end_internal): New.
>          (dlstp<mode1>_insn): New.
>          * config/arm/thumb2.md (doloop_end): Update for MVE LOLs.
>          * config/arm/unspecs.md: New unspecs.
>      * df-core.cc (df_bb_regno_only_def_find): New.
>      * df.h (df_bb_regno_only_def_find): New.
>          * loop-doloop.cc (doloop_condition_get): Relax conditions.
>          (doloop_optimize): Add support for elementwise LoLs.
> 
> gcc/testsuite/ChangeLog:
> 
>          * gcc.target/arm/lob.h: Update framework.
>          * gcc.target/arm/lob1.c: Likewise.
>          * gcc.target/arm/lob6.c: Likewise.
>      * gcc.target/arm/mve/dlstp-compile-asm.c: New test.
>      * gcc.target/arm/mve/dlstp-int16x8.c: New test.
>      * gcc.target/arm/mve/dlstp-int32x4.c: New test.
>      * gcc.target/arm/mve/dlstp-int64x2.c: New test.
>      * gcc.target/arm/mve/dlstp-int8x16.c: New test.
>      * gcc.target/arm/mve/dlstp-invalid-asm.c: New test.