Re: [04/77] Add FOR_EACH iterators for modes

[Jeff Law <law at redhat dot com>]

On 07/13/2017 02:39 AM, Richard Sandiford wrote:
> The new iterators are:
> 
> - FOR_EACH_MODE_IN_CLASS: iterate over all the modes in a mode class.
> 
> - FOR_EACH_MODE_FROM: iterate over all the modes in a class,
>   starting at a given mode.
> 
> - FOR_EACH_WIDER_MODE: iterate over all the modes in a class,
>   starting at the next widest mode after a given mode.
> 
> - FOR_EACH_2XWIDER_MODE: same, but considering only modes that
>   are two times wider than the previous mode.
> 
> - FOR_EACH_MODE_UNTIL: iterate over all the modes in a class until
>   a given mode is reached.
> 
> - FOR_EACH_MODE: iterate over all the modes in a class between
>   two given modes, inclusive of the first but not the second.
> 
> These help with the stronger type checking added by later patches,
> since every new mode will be in the same class as the previous one.
> 
> 2017-07-13  Richard Sandiford  <richard.sandiford@linaro.org>
>             Alan Hayward  <alan.hayward@arm.com>
>             David Sherwood  <david.sherwood@arm.com>
> 
> gcc/
> 	* machmode.h (mode_traits): New structure.
> 	(get_narrowest_mode): New function.
> 	(mode_iterator::start): Likewise.
> 	(mode_iterator::iterate_p): Likewise.
> 	(mode_iterator::get_wider): Likewise.
> 	(mode_iterator::get_known_wider): Likewise.
> 	(mode_iterator::get_2xwider): Likewise.
> 	(FOR_EACH_MODE_IN_CLASS): New mode iterator.
> 	(FOR_EACH_MODE): Likewise.
> 	(FOR_EACH_MODE_FROM): Likewise.
> 	(FOR_EACH_MODE_UNTIL): Likewise.
> 	(FOR_EACH_WIDER_MODE): Likewise.
> 	(FOR_EACH_2XWIDER_MODE): Likewise.
> 	* builtins.c (expand_builtin_strlen): Use new mode iterators.
> 	* combine.c (simplify_comparison): Likewise
> 	* config/i386/i386.c (type_natural_mode): Likewise.
> 	* cse.c (cse_insn): Likewise.
> 	* dse.c (find_shift_sequence): Likewise.
> 	* emit-rtl.c (init_derived_machine_modes): Likewise.
> 	(init_emit_once): Likewise.
> 	* explow.c (hard_function_value): Likewise.
> 	* expmed.c (init_expmed_one_mode): Likewise.
> 	(extract_fixed_bit_field_1): Likewise.
> 	(extract_bit_field_1): Likewise.
> 	(expand_divmod): Likewise.
> 	(emit_store_flag_1): Likewise.
> 	* expr.c (init_expr_target): Likewise.
> 	(convert_move): Likewise.
> 	(alignment_for_piecewise_move): Likewise.
> 	(widest_int_mode_for_size): Likewise.
> 	(emit_block_move_via_movmem): Likewise.
> 	(copy_blkmode_to_reg): Likewise.
> 	(set_storage_via_setmem): Likewise.
> 	(compress_float_constant): Likewise.
> 	* omp-low.c (omp_clause_aligned_alignment): Likewise.
> 	* optabs-query.c (get_best_extraction_insn): Likewise.
> 	* optabs.c (expand_binop): Likewise.
> 	(expand_twoval_unop): Likewise.
> 	(expand_twoval_binop): Likewise.
> 	(widen_leading): Likewise.
> 	(widen_bswap): Likewise.
> 	(expand_parity): Likewise.
> 	(expand_unop): Likewise.
> 	(prepare_cmp_insn): Likewise.
> 	(prepare_float_lib_cmp): Likewise.
> 	(expand_float): Likewise.
> 	(expand_fix): Likewise.
> 	(expand_sfix_optab): Likewise.
> 	* postreload.c (move2add_use_add2_insn): Likewise.
> 	* reg-stack.c (reg_to_stack): Likewise.
> 	* reginfo.c (choose_hard_reg_mode): Likewise.
> 	* rtlanal.c (init_num_sign_bit_copies_in_rep): Likewise.
> 	* stmt.c (emit_case_decision_tree): Likewise.
> 	* stor-layout.c (mode_for_size): Likewise.
> 	(smallest_mode_for_size): Likewise.
> 	(mode_for_vector): Likewise.
> 	(finish_bitfield_representative): Likewise.
> 	* tree-ssa-math-opts.c (target_supports_divmod_p): Likewise.
> 	* tree-vect-generic.c (type_for_widest_vector_mode): Likewise.
> 	* tree-vect-stmts.c (vectorizable_conversion): Likewise.
> 	* var-tracking.c (prepare_call_arguments): Likewise.
> 
> gcc/ada/
> 	* gcc-interface/misc.c (fp_prec_to_size): Use new mode iterators.
> 	(fp_size_to_prec): Likewise.
> 
> gcc/c-family/
> 	* c-common.c (c_common_fixed_point_type_for_size): Use new mode
> 	iterators.
> 	* c-cppbuiltin.c (c_cpp_builtins): Likewise.
So in some cases I see that we have loops like this:

for (; mode != VOIDmode; mode = GET_WIDER_MODE (mode))

Which iterates from the current mode through all the remaining modes,
with each iteration using a wider mode than the previous iteration.
GET_WIDER_MODE is always going to give us something in the same class or
VOIDmode.

This is getting translated into:

FOR_EACH_MODE_FROM (mode, mode)

The two loops have different steps.  The original steps to a wider mode
each iteration.  The new one may step to a different mode of the same
width IIUC.

Am I missing something here?

I realize you won't want to use FOR_EACH_WIDER_MODE since that's got a
different starting point.  But we might want to have an iterator that
starts at the current mode, but steps to the net wider mode within the
class.  That would match the existing loops in several places and would
avoid having to review each one of those use cases to ensure that the
difference in steps isn't important.

Thoughts?  Am I missing something?

jeff