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Re: [PATCH] Restrict LOOP_ALIGN to loop headers only.


On 7/9/19 11:56 AM, Jan Hubicka wrote:
>> Hi.
>>
>> I'm suggesting to restrict LOOP_ALIGN to only loop headers. That are the
>> basic blocks for which it makes the biggest sense. I quite some binary
>> size reductions on SPEC2006 and SPEC2017. Speed numbers are also slightly
>> positive.
>>
>> Patch can bootstrap on x86_64-linux-gnu and survives regression tests.
>>
>> Ready to be installed?
> The original idea of distinction between jump alignment and loop
> alignment was that they have two basic meanings:
>  1) jump alignment is there to avoid jumping just to the end of decode
>  window (if the window is aligned) so CPU will get stuck after reaching
>  the jump and also to possibly reduce code cache polution by populating
>  by code that is not executed
>  2) loop alignment aims to fit loop in as few cache windows as possible
> 
> Now if you have loop laid in a way that header of loop is not first
> basic block, 2) IMO still apply.  I.e.
> 
> 		jump loop
> :loopback
> 		loop body
> :loop
> 		if cond jump to loopback
> 
> So dropping loop alignment for those does not seem to make much sense
> from high level.  We may want to have differnt alignment for loops
> starting by header and loops starting in the middle,

That's quite complicated condition, I would not introduce a new alignment.

> but I still liked
> more your patch which did bundles for loops.

The patch caused regression for quite some benchmarks and has it's own
problems (need of a recent GAS, not doing a bundle for bundles that
can't fit in a single bundle window). For that reasons, I decided
to not work on it any longer.

Martin

> 
> modern x86 chips are not very good testing targets on it.  I guess
> generic changes to alignment needs to be tested on other chips too.
> 
> Honza
>> Thanks,
>> Martin
>>
>> gcc/ChangeLog:
>>
>> 2019-07-09  Martin Liska  <mliska@suse.cz>
>>
>> 	* final.c (compute_alignments): Apply the LOOP_ALIGN only
>> 	to basic blocks that all loop headers.
>> ---
>>  gcc/final.c | 1 +
>>  1 file changed, 1 insertion(+)
>>
>>
> 
>> diff --git a/gcc/final.c b/gcc/final.c
>> index fefc4874b24..ce2678da988 100644
>> --- a/gcc/final.c
>> +++ b/gcc/final.c
>> @@ -739,6 +739,7 @@ compute_alignments (void)
>>        if (has_fallthru
>>  	  && !(single_succ_p (bb)
>>  	       && single_succ (bb) == EXIT_BLOCK_PTR_FOR_FN (cfun))
>> +	  && bb->loop_father->header == bb
>>  	  && optimize_bb_for_speed_p (bb)
>>  	  && branch_count + fallthru_count > count_threshold
>>  	  && (branch_count
>>
> 
> 
> 


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