Hi Alfie,
> -----Original Message-----
> From: Alfie Richards <[email protected]>
> Sent: 17 September 2026 13:48
> To: [email protected]
> Cc: [email protected]; Tamar Christina <[email protected]>;
> [email protected]; [email protected];
> [email protected]; [email protected]; [email protected];
> Alfie Richards <[email protected]>
> Subject: [PATCH 2/2] aarch64: Add option to specify particular mode in --
> param=aarch64_validate_autovec
>
> gcc/ChangeLog:
>
> * config/aarch64/aarch64-opts.h (enum
> aarch64_autovec_preference_enum):
> Add AARCH64_AUTOVEC_SPECIFIC_MODE value.
> * config/aarch64/aarch64-protos.h
> (aarch64_validate_autovec_preference):
> New function.
> * config/aarch64/aarch64.cc (aarch64_autovec_preference):
> New static variable.
> (aarch64_autovec_preference_mode): Likewise.
> (aarch64_override_options_internal): Update
> aarch64_autovec_preference logic
> (aarch64_override_options): Handle aarch64_autovec_preference.
> (aarch64_autovectorize_vector_modes): Add logic for selecting one
> particular mode with aarch64_autovec_preference.
> * config/aarch64/aarch64.opt: Update aarch64_autovec_preference.
> * doc/params.texi: Update documentation.
>
> -- >8 --
>
> I thought about this some more and thought that Kyrylo's suggestion on IRC
> was actually easier and better than I initially thought.
>
> I only change --param=aarch64-autovec-preference as this is a pretty
> hacky and non-user friendly option. But it is useful for testing
> and debugging.
>
> Similar to riscv's --param=riscv-autovec-mode.
While I like the idea, I don't like the re-using of the parameter and the
dynamic parsing of modes.
The first part is because it changes the parameter to free form mode,
and so the parameter loses the very useful spelling and autocorrect.
I'd prefer we leave the parameter alone and make a new one for this which is
just a machine_mode enum. There are only a small fixed number of modes
(as listed in aarch64_autovectorize_vector_modes) that are useable as input
into the vectorizer.
So I suggest something like
-param=aarch64-force-vector-mode=
Target Undocumented Joined Var(aarch64_force_slp_mode)
Enum(aarch64_autovec_mode) Init(VOIDmode) Param
Enum
Name(aarch64_autovec_mode) Type(machine_mode) UnknownError(unknown mode %qs)
EnumValue
Enum(aarch64_autovec_mode) String(VNx16QI) Value(VNx16QImode)
EnumValue
Enum(aarch64_autovec_mode) String(VNx8QI) Value(VNx8QImode)
EnumValue
Enum(aarch64_autovec_mode) String(VNx4QI) Value(VNx4QImode)
...
And then the only change you need aside from the .opt file is
if (aarch64_force_slp_mode != VOIDmode)
{
modes->safe_push (aarch64_force_slp_mode);
return 0;
}
At the start of aarch64_autovectorize_vector_modes. If the vectorizer
can't use it it'll just reject vectorization.
Also please add "Undocumented" tag to the param. End users should not use this
and I
want to reserve the right to change it at any given time. And I'm not married
to the name.
Happy to use aarch64-autovec-mode too.
Thanks,
Tamar
>
> Regression tested and bootstrapped on aarch64-linux-gnu and
> x86_64-linux-gnu.
>
> Thoughts?
> Okay for master?
>
> Thanks,
> Alfie
> ---
> gcc/config/aarch64/aarch64-opts.h | 3 +-
> gcc/config/aarch64/aarch64-protos.h | 3 ++
> gcc/config/aarch64/aarch64.cc | 58 +++++++++++++++++++++++++++--
> gcc/config/aarch64/aarch64.opt | 44 +++++++++++-----------
> gcc/doc/params.texi | 8 ++--
> 5 files changed, 87 insertions(+), 29 deletions(-)
>
> diff --git a/gcc/config/aarch64/aarch64-opts.h
> b/gcc/config/aarch64/aarch64-opts.h
> index 423285d6b0f..2b3870a3c36 100644
> --- a/gcc/config/aarch64/aarch64-opts.h
> +++ b/gcc/config/aarch64/aarch64-opts.h
> @@ -121,7 +121,8 @@ enum aarch64_autovec_preference_enum {
> AARCH64_AUTOVEC_ASIMD_ONLY,
> AARCH64_AUTOVEC_SVE_ONLY,
> AARCH64_AUTOVEC_PREFER_ASIMD,
> - AARCH64_AUTOVEC_PREFER_SVE
> + AARCH64_AUTOVEC_PREFER_SVE,
> + AARCH64_AUTOVEC_SPECIFIC_MODE
> };
>
> /* An enum specifying how to handle load and store pairs using
> diff --git a/gcc/config/aarch64/aarch64-protos.h
> b/gcc/config/aarch64/aarch64-protos.h
> index e8ae3d42794..22fd1c68b05 100644
> --- a/gcc/config/aarch64/aarch64-protos.h
> +++ b/gcc/config/aarch64/aarch64-protos.h
> @@ -1257,6 +1257,9 @@ bool aarch64_validate_march (const char *,
> aarch64_arch *,
> bool aarch64_validate_mcpu (const char *, aarch64_cpu *,
> aarch64_feature_flags *);
> bool aarch64_validate_mtune (const char *, aarch64_cpu *);
> +bool aarch64_validate_autovec_preference (const char *,
> + aarch64_autovec_preference_enum
> *,
> + opt_machine_mode *);
> std::string aarch64_get_extension_string_for_isa_flags
> (aarch64_feature_flags,
>
> aarch64_feature_flags);
> std::string aarch64_get_arch_string_for_assembler (aarch64_arch,
> diff --git a/gcc/config/aarch64/aarch64.cc b/gcc/config/aarch64/aarch64.cc
> index f9d906f449f..d8bd0cc74f1 100644
> --- a/gcc/config/aarch64/aarch64.cc
> +++ b/gcc/config/aarch64/aarch64.cc
> @@ -602,6 +602,9 @@ aarch64_lookup_sysreg_map (const char *regname)
>
> /* The current tuning set. */
> struct tune_params aarch64_tune_params = generic_tunings;
> +aarch64_autovec_preference_enum aarch64_autovec_preference
> + = AARCH64_AUTOVEC_DEFAULT;
> +machine_mode aarch64_autovec_preference_mode = VOIDmode;
>
> /* If NAME is the name of an arm:: attribute that describes shared state,
> return its associated AARCH64_STATE_* flags, otherwise return 0. */
> @@ -20252,10 +20255,9 @@ aarch64_override_options_internal (struct
> gcc_options *opts,
> /* Synchronize the -mautovec-preference and aarch64_autovec_preference
> using
> whichever one is not default. If both are set then prefer the param
> flag
> over the parameters. */
> - if (opts->x_autovec_preference != AARCH64_AUTOVEC_DEFAULT)
> - SET_OPTION_IF_UNSET (opts, opts_set,
> - aarch64_autovec_preference,
> - opts->x_autovec_preference);
> + if (opts->x_autovec_preference != AARCH64_AUTOVEC_DEFAULT
> + && aarch64_autovec_preference == AARCH64_AUTOVEC_DEFAULT)
> + aarch64_autovec_preference = opts->x_autovec_preference;
>
> aarch64_override_options_after_change_1 (opts);
> }
> @@ -20427,6 +20429,30 @@ static const struct aarch_branch_protect_type
> aarch64_branch_protect_types[] =
> { NULL, false, NULL, NULL, 0 }
> };
>
> +/* Validate a command-line param autovec preference option. */
> +
> +bool
> +aarch64_validate_autovec_preference (
> + const char *str,
> + aarch64_autovec_preference_enum *preference,
> + machine_mode *preferred_mode)
> +{
> + if (opt_enum_arg_to_value (OPT_mautovec_preference_, str, (int *)
> preference,
> + CL_TARGET))
> + return true;
> +
> + machine_mode mode;
> + if (parse_machine_mode (str, &mode) && VECTOR_MODE_P (mode))
> + {
> + *preference = AARCH64_AUTOVEC_SPECIFIC_MODE;
> + *preferred_mode = mode;
> + return true;
> + }
> +
> + error ("invalid mode for param=aarch64-autovec-preference %<%s%>", str);
> + return false;
> +}
> +
> /* Implement TARGET_OPTION_OVERRIDE. This is called once in the
> beginning
> and is used to parse the -m{cpu,tune,arch} strings and setup the initial
> tuning structs. In particular it must set selected_tune and
> @@ -20465,6 +20491,11 @@ aarch64_override_options (void)
> if (aarch64_tune_string)
> aarch64_validate_mtune (aarch64_tune_string, &tune);
>
> + if (aarch64_autovec_preference_string)
> + aarch64_validate_autovec_preference
> (aarch64_autovec_preference_string,
> + &aarch64_autovec_preference,
> +
> &aarch64_autovec_preference_mode);
> +
> #ifdef SUBTARGET_OVERRIDE_OPTIONS
> SUBTARGET_OVERRIDE_OPTIONS;
> #endif
> @@ -24083,6 +24114,7 @@ aarch64_autovectorize_vector_modes
> (vector_modes *modes, bool)
> for this case. */
> V2SImode
> };
> + static bool selected_mode_warning_emitted = false;
>
> /* Try using N-byte SVE modes only after trying N-byte Advanced SIMD
> mode.
> This is because:
> @@ -24117,6 +24149,24 @@ aarch64_autovectorize_vector_modes
> (vector_modes *modes, bool)
> while (sve_i < ARRAY_SIZE (sve_modes))
> modes->safe_push (sve_modes[sve_i++]);
>
> + if (aarch64_autovec_preference == AARCH64_AUTOVEC_SPECIFIC_MODE)
> + {
> + if (modes->contains (aarch64_autovec_preference_mode))
> + {
> + modes->truncate (0);
> + modes->safe_push (aarch64_autovec_preference_mode);
> + return 0;
> + }
> + else if (!selected_mode_warning_emitted)
> + {
> + selected_mode_warning_emitted = true;
> + warning (0,
> + "selected mode %<%s%> not an available vector mode, "
> + "using default cost model behaviour",
> + aarch64_autovec_preference_string);
> + }
> + }
> +
> unsigned int flags = 0;
> if (aarch64_vect_compare_costs)
> flags |= VECT_COMPARE_COSTS;
> diff --git a/gcc/config/aarch64/aarch64.opt
> b/gcc/config/aarch64/aarch64.opt
> index deccc6c8887..63342c83070 100644
> --- a/gcc/config/aarch64/aarch64.opt
> +++ b/gcc/config/aarch64/aarch64.opt
> @@ -315,6 +315,24 @@ Force an ISA selection strategy for auto-
> vectorization. For best performance it
> is highly recommended to use -mcpu or -mtune instead. This parameter
> should
> only be used for code exploration.
>
> +Enum
> +Name(aarch64_autovec_preference) Type(enum
> aarch64_autovec_preference_enum) UnknownError(unknown autovec
> preference %qs)
> +
> +EnumValue
> +Enum(aarch64_autovec_preference) String(default)
> Value(AARCH64_AUTOVEC_DEFAULT)
> +
> +EnumValue
> +Enum(aarch64_autovec_preference) String(asimd-only)
> Value(AARCH64_AUTOVEC_ASIMD_ONLY)
> +
> +EnumValue
> +Enum(aarch64_autovec_preference) String(sve-only)
> Value(AARCH64_AUTOVEC_SVE_ONLY)
> +
> +EnumValue
> +Enum(aarch64_autovec_preference) String(prefer-asimd)
> Value(AARCH64_AUTOVEC_PREFER_ASIMD)
> +
> +EnumValue
> +Enum(aarch64_autovec_preference) String(prefer-sve)
> Value(AARCH64_AUTOVEC_PREFER_SVE)
> +
> mmax-vectorization
> Target Var(flag_aarch64_max_vectorization) Save
> Override the scalar cost model such that vectorization is always profitable.
> @@ -384,27 +402,11 @@ Target Joined UInteger
> Var(aarch64_double_recp_precision) Init(2) IntegerRange(1
> The number of Newton iterations for calculating the reciprocal for double
> type. The precision of division is proportional to this param when division
> approximation is enabled. The default value is 2.
>
> -param=aarch64-autovec-preference=
> -Target Joined Var(aarch64_autovec_preference)
> Enum(aarch64_autovec_preference) Init(AARCH64_AUTOVEC_DEFAULT)
> Param
> -An old alias for -mautovec-preference. If both -mautovec-preference and
> ---param=aarch64-autovec-preference are passed, the --param value will be
> used.
> -
> -Enum
> -Name(aarch64_autovec_preference) Type(enum
> aarch64_autovec_preference_enum) UnknownError(unknown autovec
> preference %qs)
> -
> -EnumValue
> -Enum(aarch64_autovec_preference) String(default)
> Value(AARCH64_AUTOVEC_DEFAULT)
> -
> -EnumValue
> -Enum(aarch64_autovec_preference) String(asimd-only)
> Value(AARCH64_AUTOVEC_ASIMD_ONLY)
> -
> -EnumValue
> -Enum(aarch64_autovec_preference) String(sve-only)
> Value(AARCH64_AUTOVEC_SVE_ONLY)
> -
> -EnumValue
> -Enum(aarch64_autovec_preference) String(prefer-asimd)
> Value(AARCH64_AUTOVEC_PREFER_ASIMD)
> -
> -EnumValue
> -Enum(aarch64_autovec_preference) String(prefer-sve)
> Value(AARCH64_AUTOVEC_PREFER_SVE)
> +Target RejectNegative Joined Var(aarch64_autovec_preference_string) Param
> +Force an ISA selection strategy for auto-vectorization, similarly to
> +-mautovec-preference, or select a specific mode to be preferred.
> +If both -mautovec-preference and --param=aarch64-autovec-preference are
> passed,
> +the --param value will be used.
>
> -param=aarch64-loop-vect-issue-rate-niters=
> Target Joined UInteger Var(aarch64_loop_vect_issue_rate_niters) Init(6)
> IntegerRange(0, 65536) Param
> diff --git a/gcc/doc/params.texi b/gcc/doc/params.texi
> index e69be2d2923..d06589a3584 100644
> --- a/gcc/doc/params.texi
> +++ b/gcc/doc/params.texi
> @@ -2023,9 +2023,11 @@ approximation is enabled. The default value is 2.
>
> @paindex aarch64-autovec-preference
> @item aarch64-autovec-preference
> -An old alias for @option{-mautovec-preference}. If both
> -@option{-mautovec-preference} and @option{--param=aarch64-autovec-
> preference}
> -are passed, the @option{--param} value is used.
> +Force an ISA selection strategy for auto-vectorization, similarly to
> +@option{-mautovec-preference}, or select a specific machine mode to be
> +used for vectorization. If both @option{-mautovec-preference} and
> +@option{--param=aarch64-autovec-preference} are passed, the @option{--
> param}
> +value is used.
>
> @paindex aarch64-ldp-policy
> @item aarch64-ldp-policy
> --
> 2.43.0
