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

[Jeff Law <law@redhat.com>]

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