On Thu, 13 Nov 2025, Tamar Christina wrote:
> This adds a new target hook to gimple-sel to allow targets to
> do target specific massaging of the gimple IL to prepare for
> expand.
>
> Tejas will be sending up part 2 of this soon to help convert
> SVE packed boolean VEC_COND_EXPR into something that we can
> handle more efficiently if expanded in a different order.
>
> We also want to use this to e.g. for Adv. SIMD prefer avoiding
> != vector compare expressions because the ISA doesn't have
> this instruction and so we expand to == + ~ but changing the
> expression from a MIN to MAX only for VECTOR_BOOLEAN_TYPE_P
> and flipping the operands we can expand more efficient.
>
> These are the kind of things we want to use the hook for,
> not generic changes that apply to all target.
>
> Bootstrapped Regtested on aarch64-none-linux-gnu and no issues.
>
> Ok for master pending submission of Tejas' patch?
I'm refraining from bike-shedding on the name, but ... "preferred"?
> Thanks,
> Tamar
>
> gcc/ChangeLog:
>
> * target.def (preferred_instruction_selection): New.
> * doc/tm.texi.in: Document it.
> * doc/tm.texi: Regenerate
> * gimple-isel.cc (pass_gimple_isel::execute): Use it.
> * targhooks.cc (default_preferred_instruction_selection): New.
> * targhooks.h (default_preferred_instruction_selection): New.
>
> ---
> diff --git a/gcc/doc/tm.texi b/gcc/doc/tm.texi
> index
> fd208f53844a157721dd8a0282f283da64cf5d93..35c1e2061f3bfed037a50e01c8ff50c199801a1b
> 100644
> --- a/gcc/doc/tm.texi
> +++ b/gcc/doc/tm.texi
> @@ -6628,6 +6628,16 @@ like @code{cond_add@var{m}}. The default
> implementation returns a zero
> constant of type @var{type}.
> @end deftypefn
>
> +@deftypefn {Target Hook} bool TARGET_PREFERRED_INSTRUCTION_SELECTION
> (function *@var{fun}, gimple_stmt_iterator *@var{gsi})
> +This hook allows a target to give a preferred instruction selection for an
> +instruction or sequence of instructions before expand to allow expansion to
> +generate more efficient code.
> +
> +@var{fun} is the current function being considered and @var{gsi} is the
> +iterator pointing to the current instruction being optimized. The default
> +implementation does not do any rewriting and returns false.
> +@end deftypefn
> +
The return value is not documented. As with your patch it would
indicate whether the CFG was changed, but I think as you name it
"preferred" the intent is to not do any further instruction selection
when the target did something?
The current pass stmt iteration is a bit messy in this regard, in
particular also what allowed modifications of 'gsi' are concerned
which looks like some handlers might remove the current stmt
but we advance it anyway (causing to possibly skip a stmt).
Apart from this documentation issue the change looks OK to me.
Richard.
> @deftypefn {Target Hook} tree TARGET_GOACC_ADJUST_PRIVATE_DECL (location_t
> @var{loc}, tree @var{var}, int @var{level})
> This hook, if defined, is used by accelerator target back-ends to adjust
> OpenACC variable declarations that should be made private to the given
> diff --git a/gcc/doc/tm.texi.in b/gcc/doc/tm.texi.in
> index
> 14315dd508051037b7936c89638c05f07b6d3d6f..825599407379dadb2b19e53f38a8af58ec3fca5b
> 100644
> --- a/gcc/doc/tm.texi.in
> +++ b/gcc/doc/tm.texi.in
> @@ -4352,6 +4352,8 @@ address; but often a machine-dependent strategy can
> generate better code.
>
> @hook TARGET_PREFERRED_ELSE_VALUE
>
> +@hook TARGET_PREFERRED_INSTRUCTION_SELECTION
> +
> @hook TARGET_GOACC_ADJUST_PRIVATE_DECL
>
> @hook TARGET_GOACC_EXPAND_VAR_DECL
> diff --git a/gcc/gimple-isel.cc b/gcc/gimple-isel.cc
> index
> e745608904e3db77a88d861bc6512afe67a82480..ac1a4c56a66b608d963f93a2ce80549016fd0f1d
> 100644
> --- a/gcc/gimple-isel.cc
> +++ b/gcc/gimple-isel.cc
> @@ -1357,6 +1357,9 @@ pass_gimple_isel::execute (struct function *fun)
> {
> for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi))
> {
> + /* Give the target first try at replacing the instruction. */
> + cfg_changed |= targetm.preferred_instruction_selection (fun, &gsi);
> +
> /* Pre-expand VEC_COND_EXPRs to .VCOND* internal function
> calls mapping to supported optabs. */
> gimple *g = gimple_expand_vec_cond_expr (&gsi);
> diff --git a/gcc/target.def b/gcc/target.def
> index
> f288329ffcab81e2773c2066e60a470611f29e23..cdf95b8a8c871da18b60dec984be5bc2e8cea5a2
> 100644
> --- a/gcc/target.def
> +++ b/gcc/target.def
> @@ -2137,6 +2137,19 @@ constant of type @var{type}.",
> (unsigned ifn, tree type, unsigned nops, tree *ops),
> default_preferred_else_value)
>
> +DEFHOOK
> +(preferred_instruction_selection,
> + "This hook allows a target to give a preferred instruction selection for
> an\n\
> +instruction or sequence of instructions before expand to allow expansion
> to\n\
> +generate more efficient code.\n\
> +\n\
> +@var{fun} is the current function being considered and @var{gsi} is the\n\
> +iterator pointing to the current instruction being optimized. The default\n\
> +implementation does not do any rewriting and returns false.",
> + bool,
> + (function *fun, gimple_stmt_iterator *gsi),
> + default_preferred_instruction_selection)
> +
> DEFHOOK
> (record_offload_symbol,
> "Used when offloaded functions are seen in the compilation unit and no
> named\n\
> diff --git a/gcc/targhooks.cc b/gcc/targhooks.cc
> index
> 1873d572ba3fb59c4713e10b5c5b14afb029b953..c906a8055cc565f7ad86fba878645aa6e1b61337
> 100644
> --- a/gcc/targhooks.cc
> +++ b/gcc/targhooks.cc
> @@ -2764,6 +2764,14 @@ default_preferred_else_value (unsigned, tree type,
> unsigned, tree *)
> return build_zero_cst (type);
> }
>
> +/* The default implementation of TARGET_PREFERRED_INSTRUCTION_SELECTION. */
> +
> +bool
> +default_preferred_instruction_selection (function *, gimple_stmt_iterator *)
> +{
> + return false;
> +}
> +
> /* Default implementation of TARGET_HAVE_SPECULATION_SAFE_VALUE. */
> bool
> default_have_speculation_safe_value (bool active ATTRIBUTE_UNUSED)
> diff --git a/gcc/targhooks.h b/gcc/targhooks.h
> index
> 92e7a4cb10f109978301d8c10f69b4c5a3952f56..4bfcbdd0c3012fdd67201aa5da638eb8177e945e
> 100644
> --- a/gcc/targhooks.h
> +++ b/gcc/targhooks.h
> @@ -305,7 +305,8 @@ extern machine_mode default_mode_for_floating_type (enum
> tree_index);
> extern HOST_WIDE_INT default_stack_clash_protection_alloca_probe_range
> (void);
> extern void default_select_early_remat_modes (sbitmap);
> extern tree default_preferred_else_value (unsigned, tree, unsigned, tree *);
> -
> +extern bool default_preferred_instruction_selection (function *,
> + gimple_stmt_iterator *);
> extern bool default_have_speculation_safe_value (bool);
> extern bool speculation_safe_value_not_needed (bool);
> extern rtx default_speculation_safe_value (machine_mode, rtx, rtx, rtx);
>
>
>
--
Richard Biener <[email protected]>
SUSE Software Solutions Germany GmbH,
Frankenstrasse 146, 90461 Nuernberg, Germany;
GF: Jochen Jaser, Andrew McDonald, Werner Knoblich; (HRB 36809, AG Nuernberg)
