From: Changpeng Fang <changpeng.fang@amd.com>
Date: Mon, 17 May 2010 22:33:53 +0000 (+0000)
Subject: Define PREFETCH_MOD_TO_UNROLL_FACTOR_RATIO heuristic.
X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=8532678c05ca513c77a9163e1546f4019a82beef;p=gcc.git

Define PREFETCH_MOD_TO_UNROLL_FACTOR_RATIO heuristic.

2010-05-17  Changpeng Fang  <changpeng.fang@amd.com>

	* tree-ssa-loop-prefetch.c (PREFETCH_MOD_TO_UNROLL_FACTOR_RATIO): New.
	(schedule_prefetches): Do not generate a prefetch if the unroll factor
	is far from what is required by the prefetch.

From-SVN: r159514
---

diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index f1482016985..a0ba83aa970 100644
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,9 @@
+2010-05-17  Changpeng Fang  <changpeng.fang@amd.com>
+
+	* tree-ssa-loop-prefetch.c (PREFETCH_MOD_TO_UNROLL_FACTOR_RATIO): New.
+	(schedule_prefetches): Do not generate a prefetch if the unroll factor
+	is far from what is required by the prefetch.
+
 2010-05-17  Jan Hubicka  <jh@suse.cz>
 
 	* ipa-cp.c (ipcp_update_callgraph): Use ipa_is_param_used.
diff --git a/gcc/tree-ssa-loop-prefetch.c b/gcc/tree-ssa-loop-prefetch.c
index f828ed3f1a1..4889604b6ab 100644
--- a/gcc/tree-ssa-loop-prefetch.c
+++ b/gcc/tree-ssa-loop-prefetch.c
@@ -225,6 +225,17 @@ struct mem_ref_group
 
 #define PREFETCH_ALL		(~(unsigned HOST_WIDE_INT) 0)
 
+/* Do not generate a prefetch if the unroll factor is significantly less
+   than what is required by the prefetch.  This is to avoid redundant
+   prefetches.  For example, if prefetch_mod is 16 and unroll_factor is
+   1, this means prefetching requires unrolling the loop 16 times, but
+   the loop is not going to be unrolled.  In this case (ratio = 16),
+   prefetching is not likely to be beneficial.  */
+
+#ifndef PREFETCH_MOD_TO_UNROLL_FACTOR_RATIO
+#define PREFETCH_MOD_TO_UNROLL_FACTOR_RATIO 8
+#endif
+
 /* The memory reference.  */
 
 struct mem_ref
@@ -921,6 +932,12 @@ schedule_prefetches (struct mem_ref_group *groups, unsigned unroll_factor,
 	if (!should_issue_prefetch_p (ref))
 	  continue;
 
+        /* The loop is far from being sufficiently unrolled for this
+           prefetch.  Do not generate prefetch to avoid many redudant
+           prefetches.  */
+        if (ref->prefetch_mod / unroll_factor > PREFETCH_MOD_TO_UNROLL_FACTOR_RATIO)
+          continue;
+
 	/* If we need to prefetch the reference each PREFETCH_MOD iterations,
 	   and we unroll the loop UNROLL_FACTOR times, we need to insert
 	   ceil (UNROLL_FACTOR / PREFETCH_MOD) instructions in each