Robin Dapp <[email protected]> writes:
>> It looks like this is an existing (potential) problem,
>> but default_noce_conversion_profitable_p uses seq_cost, which in turn
>> uses insn_cost. And insn_cost has an optional target hook behind it,
>> which allows for costing based on insn attributes etc. For a true
>> apples-with-apples comparison we should use insn_cost here too.
>
> Good point, fixed that.
>
>> I think the detail that COSTS_N_INSNS (2) is the default is useful here.
>> (In other words, I'd forgotten by the time I'd poked around other bits of
>> ifcvt and was about to ask why we didn't cost the condition “properly”.)
>> So how about something like:
>>
>> The original costs already include a base cost of COSTS_N_INSNS (2):
>> one instruction for the compare (which we will be needing either way)
>> and one instruction for the branch.
>
> Yes, this is much clearer. I went with that wording in the attached v2.
>
> Regards
> Robin
>
> From 54508fa00fbee082fa62f4e1a8167f477938e781 Mon Sep 17 00:00:00 2001
> From: Robin Dapp <[email protected]>
> Date: Wed, 27 Nov 2019 13:46:17 +0100
> Subject: [PATCH v2 3/7] ifcvt: Improve costs handling for
> noce_convert_multiple.
>
> When noce_convert_multiple is called the original costs are not yet
> initialized. Therefore, up to now, costs were only ever unfairly
> compared against COSTS_N_INSNS (2). This would lead to
> default_noce_conversion_profitable_p () rejecting all but the most
> contrived of sequences.
>
> This patch temporarily initializes the original costs by counting
> adding costs for all sets inside the then_bb.
OK, thanks.
Richard
> ---
> gcc/ifcvt.c | 31 +++++++++++++++++++++++++++----
> 1 file changed, 27 insertions(+), 4 deletions(-)
>
> diff --git a/gcc/ifcvt.c b/gcc/ifcvt.c
> index f9d4478ec18..91b54dbea9c 100644
> --- a/gcc/ifcvt.c
> +++ b/gcc/ifcvt.c
> @@ -3404,14 +3404,17 @@ noce_convert_multiple_sets (struct noce_if_info
> *if_info)
> (SET (REG) (REG)) insns suitable for conversion to a series
> of conditional moves. Also check that we have more than one set
> (other routines can handle a single set better than we would), and
> - fewer than PARAM_MAX_RTL_IF_CONVERSION_INSNS sets. */
> + fewer than PARAM_MAX_RTL_IF_CONVERSION_INSNS sets. While going
> + through the insns store the sum of their potential costs in COST. */
>
> static bool
> -bb_ok_for_noce_convert_multiple_sets (basic_block test_bb)
> +bb_ok_for_noce_convert_multiple_sets (basic_block test_bb, unsigned *cost)
> {
> rtx_insn *insn;
> unsigned count = 0;
> unsigned param = param_max_rtl_if_conversion_insns;
> + bool speed_p = optimize_bb_for_speed_p (test_bb);
> + unsigned potential_cost = 0;
>
> FOR_BB_INSNS (test_bb, insn)
> {
> @@ -3447,9 +3450,13 @@ bb_ok_for_noce_convert_multiple_sets (basic_block
> test_bb)
> if (!can_conditionally_move_p (GET_MODE (dest)))
> return false;
>
> + potential_cost += insn_cost (insn, speed_p);
> +
> count++;
> }
>
> + *cost += potential_cost;
> +
> /* If we would only put out one conditional move, the other strategies
> this pass tries are better optimized and will be more appropriate.
> Some targets want to strictly limit the number of conditional moves
> @@ -3497,11 +3504,24 @@ noce_process_if_block (struct noce_if_info *if_info)
> to calculate a value for x.
> ??? For future expansion, further expand the "multiple X" rules. */
>
> - /* First look for multiple SETS. */
> + /* First look for multiple SETS. The original costs already include
> + a base cost of COSTS_N_INSNS (2): one instruction for the compare
> + (which we will be needing either way) and one instruction for the
> + branch. When comparing costs we want to use the branch instruction
> + cost and the sets vs. the cmovs generated here. Therefore subtract
> + the costs of the compare before checking.
> + ??? Actually, instead of the branch instruction costs we might want
> + to use COSTS_N_INSNS (BRANCH_COST ()) as in other places. */
> +
> + unsigned potential_cost = if_info->original_cost - COSTS_N_INSNS (1);
> + unsigned old_cost = if_info->original_cost;
> if (!else_bb
> && HAVE_conditional_move
> - && bb_ok_for_noce_convert_multiple_sets (then_bb))
> + && bb_ok_for_noce_convert_multiple_sets (then_bb, &potential_cost))
> {
> + /* Temporarily set the original costs to what we estimated so
> + we can determine if the transformation is worth it. */
> + if_info->original_cost = potential_cost;
> if (noce_convert_multiple_sets (if_info))
> {
> if (dump_file && if_info->transform_name)
> @@ -3509,6 +3529,9 @@ noce_process_if_block (struct noce_if_info *if_info)
> if_info->transform_name);
> return TRUE;
> }
> +
> + /* Restore the original costs. */
> + if_info->original_cost = old_cost;
> }
>
> bool speed_p = optimize_bb_for_speed_p (test_bb);
