Re: [PATCH 1/5] Add register filter operand to define_register_constraint

[Jeff Law <jeffreyalaw@gmail.com>]

On 11/12/23 07:52, Richard Sandiford wrote:
> The main way of enforcing registers to be aligned is through
> HARD_REGNO_MODE_OK.  But this is a global property that applies
> to all operands.  A given (regno, mode) pair is either globally
> valid or globally invalid.
> 
> This patch instead adds a way of specifying that individual operands
> must be aligned.  More generally, it allows constraints to specify
> a C++ condition that the operand's REGNO must satisfy.  The condition
> must be invariant for a given set of target options, so that it can
> be precomputed and cached as a HARD_REG_SET.
> 
> This information will be used in very compile-time-sensitive
> parts of the compiler.  A lot of the complication is in allowing
> the information to be stored and tested without much memory cost,
> and without impacting targets that don't use the feature.
> 
> Specifically:
> 
> - Constraints are encouraged to test the absolute REGNO rather than
>    an offset from the start of the containing class.  For example,
>    all constraints for even registers should use the same condition,
>    such as "regno % 2 == 0".  This requires the classes to start at
>    even register boundaries, but that's already an implicit
>    requirement due to things like the ira-costs.cc code that begins:
> 
>        /* Some targets allow pseudos to be allocated to unaligned sequences
> 	 of hard registers.  However, selecting an unaligned sequence can
> 	 unnecessarily restrict later allocations.  So increase the cost of
> 	 unaligned hard regs to encourage the use of aligned hard regs.  */
> 
> - Each unique condition is given a "filter identifier".
> 
> - The total number of filters is given by NUM_REGISTER_FILTERS,
>    defined automatically in insn-config.h.  Structures can therefore use
>    a bitfield of NUM_REGISTER_FILTERS to represent a mask of filters.
> 
> - There is a new target global, target_constraints, that caches the
>    HARD_REG_SET for each filter.
> 
> - There is a function for looking up the HARD_REG_SET filter for a given
>    constraint and one for looking up the filter id.  Both simply return
>    a constant on targets that don't use the feature.
> 
> - There are functions for testing a register against a specific filter,
>    or against a mask of filters.
> 
> This patch just adds the information.  Later ones make use of it.
> 
> gcc/
> 	* rtl.def (DEFINE_REGISTER_CONSTRAINT): Add an optional filter
> 	operand.
> 	* doc/md.texi (define_register_constraint): Document it.
> 	* doc/tm.texi.in: Reference it in discussion about aligned registers.
> 	* doc/tm.texi: Regenerate.
> 	* gensupport.h (register_filters, get_register_filter_id): Declare.
> 	* gensupport.cc (register_filter_map, register_filters): New variables.
> 	(get_register_filter_id): New function.
> 	(process_define_register_constraint): Likewise.
> 	(process_rtx): Pass define_register_constraints to
> 	process_define_register_constraint.
> 	* genconfig.cc (main): Emit a definition of NUM_REGISTER_FILTERS.
> 	* genpreds.cc (constraint_data): Add a filter field.
> 	(add_constraint): Update accordingly.
> 	(process_define_register_constraint): Pass the filter operand.
> 	(write_init_reg_class_start_regs): New function.
> 	(write_get_register_filter): Likewise.
> 	(write_get_register_filter_id): Likewise.
> 	(write_tm_preds_h): Write a definition of target_constraints,
> 	plus helpers to test its contents.  Write the get_register_filter*
> 	functions.
> 	(write_insn_preds_c): Write init_reg_class_start_regs.
> 	* reginfo.cc (init_reg_class_start_regs): Declare.
> 	(init_reg_sets): Call it.
> 	* target-globals.h (this_target_constraints): Declare.
> 	(target_globals): Add a constraints field.
> 	(restore_target_globals): Update accordingly.
> 	* target-globals.cc: Include tm_p.h.
> 	(default_target_globals): Initialize the constraints field.
> 	(save_target_globals): Handle the constraints field.
> 	(target_globals::~target_globals): Likewise.
OK.  Mostly focused on the concept -- if we need to iterate on the 
implementation after your using it we can certainly do that.

Jeff