Introduce prefetch-dynamic-strides option.

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.

2018-05-07  Luis Machado  <luis.machado@linaro.org>

	gcc/
	* config/aarch64/aarch64-protos.h (cpu_prefetch_tune)
	<prefetch_dynamic_strides>: New const bool 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 false.
	(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.

From-SVN: r259996

diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index 535204d3b677e0fb74a19c148154669e6e8787c1..64a385e851a3b026ea2dde65c168a6bff2199f55 100644 (file)
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,23 @@
+2018-05-07  Luis Machado  <luis.machado@linaro.org>
+
+       gcc/
+       * config/aarch64/aarch64-protos.h (cpu_prefetch_tune)
+       <prefetch_dynamic_strides>: New const bool 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 false.
+       (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.
+
 2018-05-07  Luis Machado  <luis.machado@linaro.org>
 
        gcc/

diff --git a/gcc/config/aarch64/aarch64-protos.h b/gcc/config/aarch64/aarch64-protos.h
index 5d3b9d7a06e3f68a20e90d6f9dfa560ea21cb380..157308ddb79373e2cb18baaff0acc261e13211a2 100644 (file)
--- 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 bool 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 7d0ba35b7be15950e40aefbe3213e7aa54ecf036..3d3a9763824d11d96938c29acfd03de2523da77f 100644 (file)
--- 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  */
+  true,                        /* 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  */
+  true,                        /* 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  */
+  false,               /* 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  */
+  true,                        /* 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  */
+  true,                        /* 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  */
+  true,                        /* prefetch_dynamic_strides */
   -1,                  /* minimum_stride */
   -1                   /* default_opt_level  */
 };
@@ -10602,6 +10608,11 @@ 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)
+    maybe_set_param_value (PARAM_PREFETCH_DYNAMIC_STRIDES,
+                          0,
+                          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 7c90abcef05a906d551651d654a980370d324d7a..7e57e9650f65b38b4dc78f78cc9732c2f4cf7d0e 100644 (file)
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -10733,6 +10733,16 @@ 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 2166deb6a687a96f480df764bcaefa183814da0a..7408843c8cc28c2e8341684d615ddab1e190f3d0 100644 (file)
--- 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 96012db31ff7f038a6b6689c71ce8a79bf0dad8f..8aa960a904ee7f7ce239aa1323ab25e6043ae7ba 100644 (file)
--- 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 53104026e3c3d57f5e3f6010357c971d8185fb18..af89d7070e3d1044e72361679692961e353e551e 100644 (file)
--- 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