Re: [PATCH 2/2] Introduce prefetch-dynamic-strides option.

[Luis Machado <luis.machado@linaro.org>]

Hi,

On 01/23/2018 07:40 AM, Kyrill Tkachov wrote:
> Hi Luis,
> 
> On 22/01/18 13:46, Luis Machado wrote:
>> The following patch adds an option to control software prefetching of 
>> memory
>> references with non-constant/unknown strides.
>>
>> Currently we prefetch these references if the pass thinks there is 
>> benefit to
>> doing so. But, since this is all based on heuristics, it's not always 
>> the case
>> that we end up with better performance.
>>
>> For Falkor there is also the problem of conflicts with the hardware 
>> prefetcher,
>> so we need to be more conservative in terms of what we issue software 
>> prefetch
>> hints for.
>>
>> This also aligns GCC with what LLVM does for Falkor.
>>
>> Similarly to the previous patch, the defaults guarantee no change in 
>> behavior
>> for other targets and architectures.
>>
>> I've regression-tested and bootstrapped it on aarch64-linux. No 
>> problems found.
>>
>> Ok?
>>
> 
> This also looks like a sensible approach to me with a caveat inline.
> The same general comments as for patch [1/2] apply.
>> diff --git a/gcc/config/aarch64/aarch64-protos.h 
>> b/gcc/config/aarch64/aarch64-protos.h
>> index 8736bd9..22bd9ae 100644
>> --- a/gcc/config/aarch64/aarch64-protos.h
>> +++ b/gcc/config/aarch64/aarch64-protos.h
>> @@ -230,6 +230,9 @@ struct cpu_prefetch_tune
>>    const int l1_cache_size;
>>    const int l1_cache_line_size;
>>    const int l2_cache_size;
>> +  /* Whether software prefetch hints should be issued for non-constant
>> +     strides.  */
>> +  const unsigned int prefetch_dynamic_strides;
> 
> I understand that the midend PARAMs are defined as integers, but I think
> the backend tuning option here is better represented as a boolean as it 
> really
> is just a yes/no decision.
>
I started off with a boolean to be honest. Then i noticed the midend 
only used integers, which i restricted to the range of 0..1.

I'll change this locally to use booleans again.

Thanks!
Luis