diff options
Diffstat (limited to 'gcc/tree-ssa-loop-prefetch.c')
| -rw-r--r-- | gcc/tree-ssa-loop-prefetch.c | 29 |
1 files changed, 15 insertions, 14 deletions
diff --git a/gcc/tree-ssa-loop-prefetch.c b/gcc/tree-ssa-loop-prefetch.c index 4889604..4d85f54 100644 --- a/gcc/tree-ssa-loop-prefetch.c +++ b/gcc/tree-ssa-loop-prefetch.c @@ -1603,17 +1603,9 @@ is_loop_prefetching_profitable (unsigned ahead, HOST_WIDE_INT est_niter, return false; } - /* Profitability of prefetching is highly dependent on the trip count. - For a given AHEAD distance, the first AHEAD iterations do not benefit - from prefetching, and the last AHEAD iterations execute useless - prefetches. So, if the trip count is not large enough relative to AHEAD, - prefetching may cause serious performance degradation. To avoid this - problem when the trip count is not known at compile time, we - conservatively skip loops with high prefetching costs. For now, only - the I-cache cost is considered. The relative I-cache cost is estimated - 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. + /* Prefetching most likely causes performance degradation when the instruction + to prefetch ratio is too small. Too many prefetch instructions in a loop + may reduce the I-cache performance. (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, @@ -1623,12 +1615,21 @@ is_loop_prefetching_profitable (unsigned ahead, HOST_WIDE_INT est_niter, 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 = (unroll_factor * ninsns) / prefetch_count; + if (insn_to_prefetch_ratio < MIN_INSN_TO_PREFETCH_RATIO) { - insn_to_prefetch_ratio = (unroll_factor * ninsns) / prefetch_count; - return insn_to_prefetch_ratio >= MIN_INSN_TO_PREFETCH_RATIO; + if (dump_file && (dump_flags & TDF_DETAILS)) + fprintf (dump_file, + "Not prefetching -- instruction to prefetch ratio (%d) too small\n", + insn_to_prefetch_ratio); + return false; } + /* Could not do further estimation if the trip count is unknown. Just assume + prefetching is profitable. Too aggressive??? */ + if (est_niter < 0) + return true; + if (est_niter < (HOST_WIDE_INT) (TRIP_COUNT_TO_AHEAD_RATIO * ahead)) { if (dump_file && (dump_flags & TDF_DETAILS)) |