[Patch 01/02] Introduce prefetch-minimum stride option

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.

gcc/ChangeLog:

2018-05-23  Luis Machado  <luis.machado@linaro.org>

	* 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 defaulting to -1.
	(exynosm1_prefetch_tune): Likewise.
	(thunderxt88_prefetch_tune): Likewise.
	(thunderx_prefetch_tune): Likewise.
	(thunderx2t99_prefetch_tune): Likewise.
	(qdf24xx_prefetch_tune) <minimum_stride>: Set to 2048.
	<default_opt_level>: Set to 3.
	(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.

From-SVN: r260617

1 files changed, 15 insertions, 0 deletions

diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 30d2a31..d1e50a0 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -10734,6 +10734,21 @@ The size of L1 cache, in kilobytes.
 @item l2-cache-size
 The size of L2 cache, in kilobytes.
 
+@item prefetch-minimum-stride
+Minimum constant stride, in bytes, to start using prefetch hints for.  If
+the stride is less than this threshold, prefetch hints will not be issued.
+
+This setting is useful for processors that have hardware prefetchers, in
+which case there may be conflicts between the hardware prefetchers and
+the software prefetchers.  If the hardware prefetchers have a maximum
+stride they can handle, it should be used here to improve the use of
+software prefetchers.
+
+A value of -1, the default, means we don't have a threshold and therefore
+prefetch hints can be issued for any constant stride.
+
+This setting is only useful for strides that are known and constant.
+
 @item loop-interchange-max-num-stmts
 The maximum number of stmts in a loop to be interchanged.