Re: [PATCH 2/4]AArch64: add new tuning param and attribute for enabling conditional early clobber

[Richard Sandiford <richard.sandiford@arm.com>]

Tamar Christina <tamar.christina@arm.com> writes:
> Hi All,
>
> This adds a new tuning parameter EARLY_CLOBBER_SVE_PRED_DEST for AArch64 to
> allow us to conditionally enable the early clobber alternatives based on the
> tuning models.
>
> Bootstrapped Regtested on aarch64-none-linux-gnu and no issues.
>
> Ok for master?
>
> Thanks,
> Tamar
>
> gcc/ChangeLog:
>
> 	* config/aarch64/aarch64-tuning-flags.def
> 	(EARLY_CLOBBER_SVE_PRED_DEST): New.
> 	* config/aarch64/aarch64.h (TARGET_SVE_PRED_CLOBBER): New.
> 	* config/aarch64/aarch64.md (pred_clobber): New.
> 	(arch_enabled): Use it.
>
> ---
> diff --git a/gcc/config/aarch64/aarch64-tuning-flags.def b/gcc/config/aarch64/aarch64-tuning-flags.def
> index d5bcaebce770f0b217aac783063d39135f754c77..49fbad3ff28bc82b25c61ac501ccf533ec4b4c3f 100644
> --- a/gcc/config/aarch64/aarch64-tuning-flags.def
> +++ b/gcc/config/aarch64/aarch64-tuning-flags.def
> @@ -48,4 +48,8 @@ AARCH64_EXTRA_TUNING_OPTION ("avoid_cross_loop_fma", AVOID_CROSS_LOOP_FMA)
>  
>  AARCH64_EXTRA_TUNING_OPTION ("fully_pipelined_fma", FULLY_PIPELINED_FMA)
>  
> +/* Enable is the target prefers to use a fresh register for predicate outputs
> +   rather than re-use an input predicate register.  */
> +AARCH64_EXTRA_TUNING_OPTION ("early_clobber_sve_pred_dest", EARLY_CLOBBER_SVE_PRED_DEST)

Sorry for the bike-shedding, but how about something like "avoid_pred_rmw"?
(I'm open to other suggestions.)  Just looking for something that describes
either the architecture or the end result that we want to achieve.
And preferable something fairly short :)

avoid_* would be consistent with the existing "avoid_cross_loop_fma".

> +
>  #undef AARCH64_EXTRA_TUNING_OPTION
> diff --git a/gcc/config/aarch64/aarch64.h b/gcc/config/aarch64/aarch64.h
> index bbf11faaf4b4340956094a983f8b0dc2649b2d27..76a18dd511f40ebb58ed12d56b46c74084ba7c3c 100644
> --- a/gcc/config/aarch64/aarch64.h
> +++ b/gcc/config/aarch64/aarch64.h
> @@ -495,6 +495,11 @@ constexpr auto AARCH64_FL_DEFAULT_ISA_MODE = AARCH64_FL_SM_OFF;
>      enabled through +gcs.  */
>  #define TARGET_GCS (AARCH64_ISA_GCS)
>  
> +/*  Prefer different predicate registers for the output of a predicated operation over
> +    re-using an existing input predicate.  */
> +#define TARGET_SVE_PRED_CLOBBER (TARGET_SVE \
> +				 && (aarch64_tune_params.extra_tuning_flags \
> +				     & AARCH64_EXTRA_TUNE_EARLY_CLOBBER_SVE_PRED_DEST))
>  
>  /* Standard register usage.  */
>  
> diff --git a/gcc/config/aarch64/aarch64.md b/gcc/config/aarch64/aarch64.md
> index dbde066f7478bec51a8703b017ea553aa98be309..1ecd1a2812969504bd5114a53473b478c5ddba82 100644
> --- a/gcc/config/aarch64/aarch64.md
> +++ b/gcc/config/aarch64/aarch64.md
> @@ -445,6 +445,10 @@ (define_enum_attr "arch" "arches" (const_string "any"))
>  ;; target-independent code.
>  (define_attr "is_call" "no,yes" (const_string "no"))
>  
> +;; Indicates whether we want to enable the pattern with an optional early
> +;; clobber for SVE predicates.
> +(define_attr "pred_clobber" "no,yes" (const_string "no"))
> +
>  ;; [For compatibility with Arm in pipeline models]
>  ;; Attribute that specifies whether or not the instruction touches fp
>  ;; registers.
> @@ -461,7 +465,8 @@ (define_attr "fp" "no,yes"
>  (define_attr "arch_enabled" "no,yes"
>    (if_then_else
>      (ior
> -	(eq_attr "arch" "any")
> +	(and (eq_attr "arch" "any")
> +	     (eq_attr "pred_clobber" "no"))
>  
>  	(and (eq_attr "arch" "rcpc8_4")
>  	     (match_test "AARCH64_ISA_RCPC8_4"))
> @@ -488,7 +493,10 @@ (define_attr "arch_enabled" "no,yes"
>  	     (match_test "TARGET_SVE"))
>  
>  	(and (eq_attr "arch" "sme")
> -	     (match_test "TARGET_SME")))
> +	     (match_test "TARGET_SME"))
> +
> +	(and (eq_attr "pred_clobber" "yes")
> +	     (match_test "TARGET_SVE_PRED_CLOBBER")))

IMO it'd be bettero handle pred_clobber separately from arch, as a new
top-level AND:

  (and
    (ior
      (eq_attr "pred_clobber" "no")
      (match_test "!TARGET_..."))
    (ior
      ...existing arch tests...))

Thanks,
Richard