Adjust the threshold value of PREFETCH_MOD_TO_UNROLL_FACTOR_RATIO

2010-06-09  Changpeng Fang  <changpeng.fang@amd.com>

	* tree-ssa-loop-prefetch.c (PREFETCH_MOD_TO_UNROLL_FACTOR_RATIO) :
	Change the PREFETCH_MOD_TO_UNROLL_FACTOR_RATIO threshold value from
	8 to 4.  Minor change of the related comments.

From-SVN: r160514

gcc/ChangeLog

+2010-06-09  Changpeng Fang  <changpeng.fang@amd.com>
+
+        * tree-ssa-loop-prefetch.c (PREFETCH_MOD_TO_UNROLL_FACTOR_RATIO) :
+        Change the PREFETCH_MOD_TO_UNROLL_FACTOR_RATIO threshold value from
+        8 to 4.  Minor change of the related comments.
+
 2010-06-09  Sebastian Pop  <sebastian.pop@amd.com>
 
 	* tree-scalar-evolution.c (instantiate_scev_name): Do not fail

gcc/tree-ssa-loop-prefetch.c

@@ -228,13 +228,13 @@ struct mem_ref_group
 
 /* 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),
+   prefetches.  For example, when prefetch_mod is 16 and unroll_factor is
+   2, prefetching requires unrolling the loop 16 times, but
+   the loop is actually unrolled twice.  In this case (ratio = 8),
    prefetching is not likely to be beneficial.  */
 
 #ifndef PREFETCH_MOD_TO_UNROLL_FACTOR_RATIO
-#define PREFETCH_MOD_TO_UNROLL_FACTOR_RATIO 8
+#define PREFETCH_MOD_TO_UNROLL_FACTOR_RATIO 4
 #endif
 
 /* The memory reference.  */