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
a diagnostic info when the insn-to-mem ratio is too small.
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;
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;
}
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);