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Reduce the cost in miss rate computation.

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2010-07-02  Changpeng Fang  <changpeng.fang@amd.com>

	* tree-ssa-loop-prefetch.c (compute_miss_rate): Rename to
	is_miss_rate_acceptable. Pull total_positions computation
	out of the loops.  Early return if miss_positions exceeds
	the acceptable threshold.
	* tree-ssa-loop-prefetch.c (prune_ref_by_group_reuse): Call
	is_miss_rate_acceptable after renaming of compute_miss_rate.

From-SVN: r161728
This commit is contained in:
Changpeng Fang 2010-07-02 16:34:38 +00:00 • committed by Sebastian Pop
parent a245c04bcc
commit 14e444c392
2 changed files with 29 additions and 19 deletions
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9
gcc/ChangeLog
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@ -1,3 +1,12 @@
2010-07-02 Changpeng Fang <changpeng.fang@amd.com>
* tree-ssa-loop-prefetch.c (compute_miss_rate): Rename to
is_miss_rate_acceptable. Pull total_positions computation
out of the loops. Early return if miss_positions exceeds
the acceptable threshold.
* tree-ssa-loop-prefetch.c (prune_ref_by_group_reuse): Call
is_miss_rate_acceptable after renaming of compute_miss_rate.
2010-07-02 Changpeng Fang <changpeng.fang@amd.com>
PR middle-end/44576

39
gcc/tree-ssa-loop-prefetch.c
View file

@ -640,27 +640,29 @@ ddown (HOST_WIDE_INT x, unsigned HOST_WIDE_INT by)
/* Given a CACHE_LINE_SIZE and two inductive memory references
with a common STEP greater than CACHE_LINE_SIZE and an address
difference DELTA, compute the probability that they will fall
in different cache lines. DISTINCT_ITERS is the number of
distinct iterations after which the pattern repeats itself.
in different cache lines. Return true if the computed miss rate
is not greater than the ACCEPTABLE_MISS_RATE. DISTINCT_ITERS is the
number of distinct iterations after which the pattern repeats itself.
ALIGN_UNIT is the unit of alignment in bytes. */
static int
compute_miss_rate (unsigned HOST_WIDE_INT cache_line_size,
static bool
is_miss_rate_acceptable (unsigned HOST_WIDE_INT cache_line_size,
HOST_WIDE_INT step, HOST_WIDE_INT delta,
unsigned HOST_WIDE_INT distinct_iters,
int align_unit)
{
unsigned align, iter;
int total_positions, miss_positions, miss_rate;
int total_positions, miss_positions, max_allowed_miss_positions;
int address1, address2, cache_line1, cache_line2;
/* It always misses if delta is greater than or equal to the cache
line size. */
if (delta >= cache_line_size)
return 1000;
if (delta >= (HOST_WIDE_INT) cache_line_size)
return false;
total_positions = 0;
miss_positions = 0;
total_positions = (cache_line_size / align_unit) * distinct_iters;
max_allowed_miss_positions = (ACCEPTABLE_MISS_RATE * total_positions) / 1000;
/* Iterate through all possible alignments of the first
memory reference within its cache line. */
@ -673,12 +675,14 @@ compute_miss_rate (unsigned HOST_WIDE_INT cache_line_size,
address2 = address1 + delta;
cache_line1 = address1 / cache_line_size;
cache_line2 = address2 / cache_line_size;
total_positions += 1;
if (cache_line1 != cache_line2)
miss_positions += 1;
{
miss_positions += 1;
if (miss_positions > max_allowed_miss_positions)
return false;
}
}
miss_rate = 1000 * miss_positions / total_positions;
return miss_rate;
return true;
}
/* Prune the prefetch candidate REF using the reuse with BY.
@ -694,7 +698,6 @@ prune_ref_by_group_reuse (struct mem_ref *ref, struct mem_ref *by,
HOST_WIDE_INT delta = delta_b - delta_r;
HOST_WIDE_INT hit_from;
unsigned HOST_WIDE_INT prefetch_before, prefetch_block;
int miss_rate;
HOST_WIDE_INT reduced_step;
unsigned HOST_WIDE_INT reduced_prefetch_block;
tree ref_type;
@ -793,9 +796,8 @@ prune_ref_by_group_reuse (struct mem_ref *ref, struct mem_ref *by,
delta %= step;
ref_type = TREE_TYPE (ref->mem);
align_unit = TYPE_ALIGN (ref_type) / 8;
miss_rate = compute_miss_rate(prefetch_block, step, delta,
reduced_prefetch_block, align_unit);
if (miss_rate <= ACCEPTABLE_MISS_RATE)
if (is_miss_rate_acceptable (prefetch_block, step, delta,
reduced_prefetch_block, align_unit))
{
/* Do not reduce prefetch_before if we meet beyond cache size. */
if (prefetch_before > L2_CACHE_SIZE_BYTES / PREFETCH_BLOCK)
@ -809,9 +811,8 @@ prune_ref_by_group_reuse (struct mem_ref *ref, struct mem_ref *by,
/* Try also the following iteration. */
prefetch_before++;
delta = step - delta;
miss_rate = compute_miss_rate(prefetch_block, step, delta,
reduced_prefetch_block, align_unit);
if (miss_rate <= ACCEPTABLE_MISS_RATE)
if (is_miss_rate_acceptable (prefetch_block, step, delta,
reduced_prefetch_block, align_unit))
{
if (prefetch_before < ref->prefetch_before)
ref->prefetch_before = prefetch_before;