PING^2 [PATCH v2] rs6000: Add load density heuristic

Mon Jun 28 07:01:12 GMT 2021


Gentle ping this:


on 2021/6/9 上午10:26, Kewen.Lin via Gcc-patches wrote:
> Hi,
> Gentle ping this:
> BR,
> Kewen
> on 2021/5/26 上午10:59, Kewen.Lin via Gcc-patches wrote:
>> Hi,
>> This is the updated version of patch to deal with the bwaves_r
>> degradation due to vector construction fed by strided loads.
>> As Richi's comments [1], this follows the similar idea to over
>> price the vector construction fed by VMAT_ELEMENTWISE or
>> VMAT_STRIDED_SLP.  Instead of adding the extra cost on vector
>> construction costing immediately, it firstly records how many
>> loads and vectorized statements in the given loop, later in
>> rs6000_density_test (called by finish_cost) it computes the
>> load density ratio against all vectorized stmts, and check
>> with the corresponding thresholds DENSITY_LOAD_NUM_THRESHOLD
>> and DENSITY_LOAD_PCT_THRESHOLD, do the actual extra pricing
>> if both thresholds are exceeded.
>> Note that this new load density heuristic check is based on
>> some fields in target cost which are updated as needed when
>> scanning each add_stmt_cost entry, it's independent of the
>> current function rs6000_density_test which requires to scan
>> non_vect stmts.  Since it's checking the load stmts count
>> vs. all vectorized stmts, it's kind of density, so I put
>> it in function rs6000_density_test.  With the same reason to
>> keep it independent, I didn't put it as an else arm of the
>> current existing density threshold check hunk or before this
>> hunk.
>> In the investigation of -1.04% degradation from 526.blender_r
>> on Power8, I noticed that the extra penalized cost 320 on one
>> single vector construction with type V16QI is much exaggerated,
>> which makes the final body cost unreliable, so this patch adds
>> one maximum bound for the extra penalized cost for each vector
>> construction statement.
>> Bootstrapped/regtested on powerpc64le-linux-gnu P9.
>> Full SPEC2017 performance evaluation on Power8/Power9 with
>> option combinations:
>>   * -O2 -ftree-vectorize {,-fvect-cost-model=very-cheap} {,-ffast-math}
>>   * {-O3, -Ofast} {,-funroll-loops}
>> bwaves_r degradations on P8/P9 have been fixed, nothing else
>> remarkable was observed.
>> Is it ok for trunk?
>> [1]
>> BR,
>> Kewen
>> -----
>> gcc/ChangeLog:
>> 	* config/rs6000/rs6000.c (struct rs6000_cost_data): New members
>> 	nstmts, nloads and extra_ctor_cost.
>> 	(rs6000_density_test): Add load density related heuristics and the
>> 	checks, do extra costing on vector construction statements if need.
>> 	(rs6000_init_cost): Init new members.
>> 	(rs6000_update_target_cost_per_stmt): New function.
>> 	(rs6000_add_stmt_cost): Factor vect_nonmem hunk out to function
>> 	rs6000_update_target_cost_per_stmt and call it.

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