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Account for loop unrolling in the insn-to-prefetch ratio heuristic. 

2010-05-07  Changpeng Fang  <changpeng.fang@amd.com>

	* tree-ssa-loop-prefetch.c (is_loop_prefetching_profitable):
	Account for loop unrolling in the insn-to-prefetch ratio heuristic.
	(loop_prefetch_arrays): Pass to is_loop_prefetching_profitable
	the unroll_factor.

From-SVN: r159162
This commit is contained in:
Changpeng Fang 2010-05-07 16:15:52 +00:00 committed by Sebastian Pop
parent 55e5a2ebc0
commit ccacf0e193
2 changed files with 20 additions and 8 deletions
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7
gcc/ChangeLog
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@ -1,3 +1,10 @@
2010-05-07 Changpeng Fang <changpeng.fang@amd.com>
* tree-ssa-loop-prefetch.c (is_loop_prefetching_profitable):
Account for loop unrolling in the insn-to-prefetch ratio heuristic.
(loop_prefetch_arrays): Pass to is_loop_prefetching_profitable
the unroll_factor.
2010-05-07 Changpeng Fang <changpeng.fang@amd.com>
* tree-ssa-loop-prefetch.c (is_loop_prefetching_profitable): Dump

21
gcc/tree-ssa-loop-prefetch.c
View File

@ -1532,7 +1532,7 @@ determine_loop_nest_reuse (struct loop *loop, struct mem_ref_group *refs,
static bool
is_loop_prefetching_profitable (unsigned ahead, HOST_WIDE_INT est_niter,
unsigned ninsns, unsigned prefetch_count,
unsigned mem_ref_count)
unsigned mem_ref_count, unsigned unroll_factor)
{
int insn_to_mem_ratio, insn_to_prefetch_ratio;
@ -1570,13 +1570,18 @@ is_loop_prefetching_profitable (unsigned ahead, HOST_WIDE_INT est_niter,
by taking the ratio between the number of prefetches and the total
number of instructions. Since we are using integer arithmetic, we
compute the reciprocal of this ratio.
TODO: Account for loop unrolling, which may reduce the costs of
shorter stride prefetches. Note that not accounting for loop
unrolling over-estimates the cost and hence gives more conservative
results. */
(unroll_factor * ninsns) is used to estimate the number of instructions in
the unrolled loop. This implementation is a bit simplistic -- the number
of issued prefetch instructions is also affected by unrolling. So,
prefetch_mod and the unroll factor should be taken into account when
determining prefetch_count. Also, the number of insns of the unrolled
loop will usually be significantly smaller than the number of insns of the
original loop * unroll_factor (at least the induction variable increases
and the exit branches will get eliminated), so it might be better to use
tree_estimate_loop_size + estimated_unrolled_size. */
if (est_niter < 0)
{
insn_to_prefetch_ratio = ninsns / prefetch_count;
insn_to_prefetch_ratio = (unroll_factor * ninsns) / prefetch_count;
return insn_to_prefetch_ratio >= MIN_INSN_TO_PREFETCH_RATIO;
}
@ -1643,8 +1648,8 @@ loop_prefetch_arrays (struct loop *loop)
ahead, unroll_factor, est_niter,
ninsns, mem_ref_count, prefetch_count);
if (!is_loop_prefetching_profitable (ahead, est_niter, ninsns,
prefetch_count, mem_ref_count))
if (!is_loop_prefetching_profitable (ahead, est_niter, ninsns, prefetch_count,
mem_ref_count, unroll_factor))
goto fail;
mark_nontemporal_stores (loop, refs);