Re: [PATCH 1/2] Introduce prefetch-minimum stride option

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

On 05/01/2018 03:30 PM, Jeff Law wrote:
> On 01/22/2018 06:46 AM, Luis Machado wrote:
>> This patch adds a new option to control the minimum stride, for a memory
>> reference, after which the loop prefetch pass may issue software prefetch
>> hints for. There are two motivations:
>>
>> * Make the pass less aggressive, only issuing prefetch hints for bigger strides
>> that are more likely to benefit from prefetching. I've noticed a case in cpu2017
>> where we were issuing thousands of hints, for example.
>>
>> * For processors that have a hardware prefetcher, like Falkor, it allows the
>> loop prefetch pass to defer prefetching of smaller (less than the threshold)
>> strides to the hardware prefetcher instead. This prevents conflicts between
>> the software prefetcher and the hardware prefetcher.
>>
>> I've noticed considerable reduction in the number of prefetch hints and
>> slightly positive performance numbers. This aligns GCC and LLVM in terms of
>> prefetch behavior for Falkor.
>>
>> The default settings should guarantee no changes for existing targets. Those
>> are free to tweak the settings as necessary.
>>
>> No regressions in the testsuite and bootstrapped ok on aarch64-linux.
>>
>> Ok?
>>
>> 2018-01-22  Luis Machado  <luis.machado@linaro.org>
>>
>> 	Introduce option to limit software prefetching to known constant
>> 	strides above a specific threshold with the goal of preventing
>> 	conflicts with a hardware prefetcher.
>>
>> 	gcc/
>> 	* config/aarch64/aarch64-protos.h (cpu_prefetch_tune)
>> 	<minimum_stride>: New const int field.
>> 	* config/aarch64/aarch64.c (generic_prefetch_tune): Update to include
>> 	minimum_stride field.
>> 	(exynosm1_prefetch_tune): Likewise.
>> 	(thunderxt88_prefetch_tune): Likewise.
>> 	(thunderx_prefetch_tune): Likewise.
>> 	(thunderx2t99_prefetch_tune): Likewise.
>> 	(qdf24xx_prefetch_tune): Likewise. Set minimum_stride to 2048.
>> 	(aarch64_override_options_internal): Update to set
>> 	PARAM_PREFETCH_MINIMUM_STRIDE.
>> 	* doc/invoke.texi (prefetch-minimum-stride): Document new option.
>> 	* params.def (PARAM_PREFETCH_MINIMUM_STRIDE): New.
>> 	* params.h (PARAM_PREFETCH_MINIMUM_STRIDE): Define.
>> 	* tree-ssa-loop-prefetch.c (should_issue_prefetch_p): Return false if
>> 	stride is constant and is below the minimum stride threshold.
> OK for the trunk.
> jeff
> 

Thanks. Committed as revision 259995 now.