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

[Luis Machado <luis dot machado at linaro dot org>]

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?

2018-01-22  Luis Machado  <luis.machado@linaro.org>

	Introduce option to control whether the software prefetch pass should
	issue prefetch hints for non-constant strides.

	gcc/
	* config/aarch64/aarch64-protos.h (cpu_prefetch_tune)
	<prefetch_dynamic_strides>: New const unsigned int field.
	* config/aarch64/aarch64.c (generic_prefetch_tune): Update to include
	prefetch_dynamic_strides.
	(exynosm1_prefetch_tune): Likewise.
	(thunderxt88_prefetch_tune): Likewise.
	(thunderx_prefetch_tune): Likewise.
	(thunderx2t99_prefetch_tune): Likewise.
	(qdf24xx_prefetch_tune): Likewise. Set prefetch_dynamic_strides to 0.
	(aarch64_override_options_internal): Update to set
	PARAM_PREFETCH_DYNAMIC_STRIDES.
	* doc/invoke.texi (prefetch-dynamic-strides): Document new option.
	* params.def (PARAM_PREFETCH_DYNAMIC_STRIDES): New.
	* params.h (PARAM_PREFETCH_DYNAMIC_STRIDES): Define.
	* tree-ssa-loop-prefetch.c (should_issue_prefetch_p): Account for
	prefetch-dynamic-strides setting.
---
 gcc/config/aarch64/aarch64-protos.h |  3 +++
 gcc/config/aarch64/aarch64.c        | 11 +++++++++++
 gcc/doc/invoke.texi                 | 10 ++++++++++
 gcc/params.def                      |  9 +++++++++
 gcc/params.h                        |  2 ++
 gcc/tree-ssa-loop-prefetch.c        | 10 ++++++++++
 6 files changed, 45 insertions(+)

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;
   /* The minimum constant stride beyond which we should use prefetch
      hints for.  */
   const int minimum_stride;
diff --git a/gcc/config/aarch64/aarch64.c b/gcc/config/aarch64/aarch64.c
index 0ed9f14..713b230 100644
--- a/gcc/config/aarch64/aarch64.c
+++ b/gcc/config/aarch64/aarch64.c
@@ -547,6 +547,7 @@ static const cpu_prefetch_tune generic_prefetch_tune =
   -1,			/* l1_cache_size  */
   -1,			/* l1_cache_line_size  */
   -1,			/* l2_cache_size  */
+  1,			/* prefetch_dynamic_strides */
   -1,			/* minimum_stride */
   -1			/* default_opt_level  */
 };
@@ -557,6 +558,7 @@ static const cpu_prefetch_tune exynosm1_prefetch_tune =
   -1,			/* l1_cache_size  */
   64,			/* l1_cache_line_size  */
   -1,			/* l2_cache_size  */
+  1,			/* prefetch_dynamic_strides */
   -1,			/* minimum_stride */
   -1			/* default_opt_level  */
 };
@@ -567,6 +569,7 @@ static const cpu_prefetch_tune qdf24xx_prefetch_tune =
   32,			/* l1_cache_size  */
   64,			/* l1_cache_line_size  */
   1024,			/* l2_cache_size  */
+  0,			/* prefetch_dynamic_strides */
   2048,			/* minimum_stride */
   3			/* default_opt_level  */
 };
@@ -577,6 +580,7 @@ static const cpu_prefetch_tune thunderxt88_prefetch_tune =
   32,			/* l1_cache_size  */
   128,			/* l1_cache_line_size  */
   16*1024,		/* l2_cache_size  */
+  1,			/* prefetch_dynamic_strides */
   -1,			/* minimum_stride */
   3			/* default_opt_level  */
 };
@@ -587,6 +591,7 @@ static const cpu_prefetch_tune thunderx_prefetch_tune =
   32,			/* l1_cache_size  */
   128,			/* l1_cache_line_size  */
   -1,			/* l2_cache_size  */
+  1,			/* prefetch_dynamic_strides */
   -1,			/* minimum_stride */
   -1			/* default_opt_level  */
 };
@@ -597,6 +602,7 @@ static const cpu_prefetch_tune thunderx2t99_prefetch_tune =
   32,			/* l1_cache_size  */
   64,			/* l1_cache_line_size  */
   256,			/* l2_cache_size  */
+  1,			/* prefetch_dynamic_strides */
   -1,			/* minimum_stride */
   -1			/* default_opt_level  */
 };
@@ -10467,6 +10473,11 @@ aarch64_override_options_internal (struct gcc_options *opts)
 			   aarch64_tune_params.prefetch->l2_cache_size,
 			   opts->x_param_values,
 			   global_options_set.x_param_values);
+  if (aarch64_tune_params.prefetch->prefetch_dynamic_strides == 0)
+    maybe_set_param_value (PARAM_PREFETCH_DYNAMIC_STRIDES,
+			   aarch64_tune_params.prefetch->prefetch_dynamic_strides,
+			   opts->x_param_values,
+			   global_options_set.x_param_values);
   if (aarch64_tune_params.prefetch->minimum_stride >= 0)
     maybe_set_param_value (PARAM_PREFETCH_MINIMUM_STRIDE,
 			   aarch64_tune_params.prefetch->minimum_stride,
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 1cb1ef5..541c24c 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -10567,6 +10567,16 @@ The size of L1 cache, in kilobytes.
 @item l2-cache-size
 The size of L2 cache, in kilobytes.
 
+@item prefetch-dynamic-strides
+Whether the loop array prefetch pass should issue software prefetch hints
+for strides that are non-constant.  In some cases this may be
+beneficial, though the fact the stride is non-constant may make it
+hard to predict when there is clear benefit to issuing these hints.
+
+Set to 1, the default, if the prefetch hints should be issued for non-constant
+strides.  Set to 0 if prefetch hints should be issued only for strides that
+are known to be constant and below @option{prefetch-minimum-stride}.
+
 @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.
diff --git a/gcc/params.def b/gcc/params.def
index bf2d12c..c564178 100644
--- a/gcc/params.def
+++ b/gcc/params.def
@@ -790,6 +790,15 @@ DEFPARAM (PARAM_L2_CACHE_SIZE,
 	  "The size of L2 cache.",
 	  512, 0, 0)
 
+/* Whether software prefetch hints should be issued for non-constant
+   strides.  */
+
+DEFPARAM (PARAM_PREFETCH_DYNAMIC_STRIDES,
+	  "prefetch-dynamic-strides",
+	  "Whether software prefetch hints should be issued for non-constant "
+	  "strides.",
+	  1, 0, 1)
+
 /* The minimum constant stride beyond which we should use prefetch hints
    for.  */
 
diff --git a/gcc/params.h b/gcc/params.h
index 96012db..8aa960a 100644
--- a/gcc/params.h
+++ b/gcc/params.h
@@ -196,6 +196,8 @@ extern void init_param_values (int *params);
   PARAM_VALUE (PARAM_L1_CACHE_LINE_SIZE)
 #define L2_CACHE_SIZE \
   PARAM_VALUE (PARAM_L2_CACHE_SIZE)
+#define PREFETCH_DYNAMIC_STRIDES \
+  PARAM_VALUE (PARAM_PREFETCH_DYNAMIC_STRIDES)
 #define PREFETCH_MINIMUM_STRIDE \
   PARAM_VALUE (PARAM_PREFETCH_MINIMUM_STRIDE)
 #define USE_CANONICAL_TYPES \
diff --git a/gcc/tree-ssa-loop-prefetch.c b/gcc/tree-ssa-loop-prefetch.c
index 112ccac..de2acc8 100644
--- a/gcc/tree-ssa-loop-prefetch.c
+++ b/gcc/tree-ssa-loop-prefetch.c
@@ -992,6 +992,16 @@ prune_by_reuse (struct mem_ref_group *groups)
 static bool
 should_issue_prefetch_p (struct mem_ref *ref)
 {
+  /* Do we want to issue prefetches for non-constant strides?  */
+  if (!cst_and_fits_in_hwi (ref->group->step) && PREFETCH_DYNAMIC_STRIDES == 0)
+    {
+      if (dump_file && (dump_flags & TDF_DETAILS))
+	fprintf (dump_file,
+		 "Skipping non-constant step for reference %u:%u\n",
+		 ref->group->uid, ref->uid);
+      return false;
+    }
+
   /* Some processors may have a hardware prefetcher that may conflict with
      prefetch hints for a range of strides.  Make sure we don't issue
      prefetches for such cases if the stride is within this particular
-- 
2.7.4