prefetch instructions with guards in cases where 5) was not sufficient
to satisfy the constraints?
- The function is_loop_prefetching_profitable() implements a cost model
- to determine if prefetching is profitable for a given loop. The cost
- model has two heuristcs:
- 1. A heuristic that determines whether the given loop has enough CPU
- ops that can be overlapped with cache missing memory ops.
- If not, the loop won't benefit from prefetching. This is implemented
- by requirung the ratio between the instruction count and the mem ref
- count to be above a certain minimum.
- 2. A heuristic that disables prefetching in a loop with an unknown trip
- count if the prefetching cost is above a certain limit. The relative
- prefetching cost is estimated by taking the ratio between the
- prefetch count and the total intruction count (this models the I-cache
- cost).
+ A cost model is implemented to determine whether or not prefetching is
+ profitable for a given loop. The cost model has three heuristics:
+
+ 1. Function trip_count_to_ahead_ratio_too_small_p implements a
+ heuristic that determines whether or not the loop has too few
+ iterations (compared to ahead). Prefetching is not likely to be
+ beneficial if the trip count to ahead ratio is below a certain
+ minimum.
+
+ 2. Function mem_ref_count_reasonable_p implements a heuristic that
+ determines whether the given loop has enough CPU ops that can be
+ overlapped with cache missing memory ops. If not, the loop
+ won't benefit from prefetching. In the implementation,
+ prefetching is not considered beneficial if the ratio between
+ the instruction count and the mem ref count is below a certain
+ minimum.
+
+ 3. Function insn_to_prefetch_ratio_too_small_p implements a
+ heuristic that disables prefetching in a loop if the prefetching
+ cost is above a certain limit. The relative prefetching cost is
+ estimated by taking the ratio between the prefetch count and the
+ total intruction count (this models the I-cache cost).
+
The limits used in these heuristics are defined as parameters with
reasonable default values. Machine-specific default values will be
added later.
#define PREFETCH_MOD_TO_UNROLL_FACTOR_RATIO 4
#endif
+/* Some of the prefetch computations have quadratic complexity. We want to
+ avoid huge compile times and, therefore, want to limit the amount of
+ memory references per loop where we consider prefetching. */
+
+#ifndef PREFETCH_MAX_MEM_REFS_PER_LOOP
+#define PREFETCH_MAX_MEM_REFS_PER_LOOP 200
+#endif
+
/* The memory reference. */
struct mem_ref
}
}
-/* Do a cost-benefit analysis to determine if prefetching is profitable
- for the current loop given the following parameters:
+/* Determine whether or not the trip count to ahead ratio is too small based
+ on prefitablility consideration.
AHEAD: the iteration ahead distance,
- EST_NITER: the estimated trip count,
+ EST_NITER: the estimated trip count. */
+
+static bool
+trip_count_to_ahead_ratio_too_small_p (unsigned ahead, HOST_WIDE_INT est_niter)
+{
+ /* Assume trip count to ahead ratio is big enough if the trip count could not
+ be estimated at compile time. */
+ if (est_niter < 0)
+ return false;
+
+ if (est_niter < (HOST_WIDE_INT) (TRIP_COUNT_TO_AHEAD_RATIO * ahead))
+ {
+ if (dump_file && (dump_flags & TDF_DETAILS))
+ fprintf (dump_file,
+ "Not prefetching -- loop estimated to roll only %d times\n",
+ (int) est_niter);
+ return true;
+ }
+
+ return false;
+}
+
+/* Determine whether or not the number of memory references in the loop is
+ reasonable based on the profitablity and compilation time considerations.
NINSNS: estimated number of instructions in the loop,
- PREFETCH_COUNT: an estimate of the number of prefetches
MEM_REF_COUNT: total number of memory references in the loop. */
static bool
-is_loop_prefetching_profitable (unsigned ahead, HOST_WIDE_INT est_niter,
- unsigned ninsns, unsigned prefetch_count,
- unsigned mem_ref_count, unsigned unroll_factor)
+mem_ref_count_reasonable_p (unsigned ninsns, unsigned mem_ref_count)
{
- int insn_to_mem_ratio, insn_to_prefetch_ratio;
+ int insn_to_mem_ratio;
if (mem_ref_count == 0)
return false;
+ /* Miss rate computation (is_miss_rate_acceptable) and dependence analysis
+ (compute_all_dependences) have high costs based on quadratic complexity.
+ To avoid huge compilation time, we give up prefetching if mem_ref_count
+ is too large. */
+ if (mem_ref_count > PREFETCH_MAX_MEM_REFS_PER_LOOP)
+ return false;
+
/* Prefetching improves performance by overlapping cache missing
memory accesses with CPU operations. If the loop does not have
enough CPU operations to overlap with memory operations, prefetching
return false;
}
+ return true;
+}
+
+/* Determine whether or not the instruction to prefetch ratio in the loop is
+ too small based on the profitablity consideration.
+ NINSNS: estimated number of instructions in the loop,
+ PREFETCH_COUNT: an estimate of the number of prefetches,
+ UNROLL_FACTOR: the factor to unroll the loop if prefetching. */
+
+static bool
+insn_to_prefetch_ratio_too_small_p (unsigned ninsns, unsigned prefetch_count,
+ unsigned unroll_factor)
+{
+ int insn_to_prefetch_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.
fprintf (dump_file,
"Not prefetching -- instruction to prefetch ratio (%d) too small\n",
insn_to_prefetch_ratio);
- return false;
+ return true;
}
- /* 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))
- fprintf (dump_file,
- "Not prefetching -- loop estimated to roll only %d times\n",
- (int) est_niter);
- return false;
- }
- return true;
+ return false;
}
return false;
}
+ /* FIXME: the time should be weighted by the probabilities of the blocks in
+ the loop body. */
+ time = tree_num_loop_insns (loop, &eni_time_weights);
+ if (time == 0)
+ return false;
+
+ ahead = (PREFETCH_LATENCY + time - 1) / time;
+ est_niter = estimated_loop_iterations_int (loop, false);
+
+ /* Prefetching is not likely to be profitable if the trip count to ahead
+ ratio is too small. */
+ if (trip_count_to_ahead_ratio_too_small_p (ahead, est_niter))
+ return false;
+
+ ninsns = tree_num_loop_insns (loop, &eni_size_weights);
+
/* Step 1: gather the memory references. */
refs = gather_memory_references (loop, &no_other_refs, &mem_ref_count);
+ /* Give up prefetching if the number of memory references in the
+ loop is not reasonable based on profitablity and compilation time
+ considerations. */
+ if (!mem_ref_count_reasonable_p (ninsns, mem_ref_count))
+ goto fail;
+
/* Step 2: estimate the reuse effects. */
prune_by_reuse (refs);
determine_loop_nest_reuse (loop, refs, no_other_refs);
- /* Step 3: determine the ahead and unroll factor. */
-
- /* FIXME: the time should be weighted by the probabilities of the blocks in
- the loop body. */
- time = tree_num_loop_insns (loop, &eni_time_weights);
- ahead = (PREFETCH_LATENCY + time - 1) / time;
- est_niter = estimated_loop_iterations_int (loop, false);
-
- ninsns = tree_num_loop_insns (loop, &eni_size_weights);
+ /* Step 3: determine unroll factor. */
unroll_factor = determine_unroll_factor (loop, refs, ninsns, &desc,
est_niter);
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, unroll_factor))
+ /* Prefetching is not likely to be profitable if the instruction to prefetch
+ ratio is too small. */
+ if (insn_to_prefetch_ratio_too_small_p (ninsns, prefetch_count,
+ unroll_factor))
goto fail;
mark_nontemporal_stores (loop, refs);
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