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?

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.