diff options
Diffstat (limited to 'gcc/tree-ssa-loop-prefetch.c')
-rw-r--r-- | gcc/tree-ssa-loop-prefetch.c | 227 |
1 files changed, 151 insertions, 76 deletions
diff --git a/gcc/tree-ssa-loop-prefetch.c b/gcc/tree-ssa-loop-prefetch.c index 65474898ad9..4840704be19 100644 --- a/gcc/tree-ssa-loop-prefetch.c +++ b/gcc/tree-ssa-loop-prefetch.c @@ -36,6 +36,7 @@ along with GCC; see the file COPYING3. If not see #include "hashtab.h" #include "tree-chrec.h" #include "tree-scalar-evolution.h" +#include "diagnostic-core.h" #include "toplev.h" #include "params.h" #include "langhooks.h" @@ -109,19 +110,29 @@ along with GCC; see the file COPYING3. If not see 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. @@ -237,6 +248,14 @@ struct mem_ref_group #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 @@ -404,8 +423,7 @@ idx_analyze_ref (tree base, tree *index, void *data) HOST_WIDE_INT idelta = 0, imult = 1; affine_iv iv; - if (TREE_CODE (base) == MISALIGNED_INDIRECT_REF - || TREE_CODE (base) == ALIGN_INDIRECT_REF) + if (TREE_CODE (base) == MISALIGNED_INDIRECT_REF) return false; if (!simple_iv (ar_data->loop, loop_containing_stmt (ar_data->stmt), @@ -468,10 +486,18 @@ analyze_ref (struct loop *loop, tree *ref_p, tree *base, *step = NULL_TREE; *delta = 0; - /* First strip off the component references. Ignore bitfields. */ - if (TREE_CODE (ref) == COMPONENT_REF - && DECL_NONADDRESSABLE_P (TREE_OPERAND (ref, 1))) - ref = TREE_OPERAND (ref, 0); + /* First strip off the component references. Ignore bitfields. + Also strip off the real and imagine parts of a complex, so that + they can have the same base. */ + if (TREE_CODE (ref) == REALPART_EXPR + || TREE_CODE (ref) == IMAGPART_EXPR + || (TREE_CODE (ref) == COMPONENT_REF + && DECL_NONADDRESSABLE_P (TREE_OPERAND (ref, 1)))) + { + if (TREE_CODE (ref) == IMAGPART_EXPR) + *delta += int_size_in_bytes (TREE_TYPE (ref)); + ref = TREE_OPERAND (ref, 0); + } *ref_p = ref; @@ -640,22 +666,29 @@ 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; - total_positions = 0; + /* It always misses if delta is greater than or equal to the cache + line size. */ + if (delta >= (HOST_WIDE_INT) cache_line_size) + return false; + 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. */ @@ -668,12 +701,14 @@ 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. @@ -689,7 +724,6 @@ 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; @@ -788,9 +822,8 @@ 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) @@ -804,9 +837,8 @@ 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; @@ -1059,7 +1091,7 @@ issue_prefetch_ref (struct mem_ref *ref, unsigned unroll_factor, unsigned ahead) addr_base = force_gimple_operand_gsi (&bsi, unshare_expr (addr_base), true, NULL, true, GSI_SAME_STMT); write_p = ref->write_p ? integer_one_node : integer_zero_node; - local = build_int_cst (integer_type_node, nontemporal ? 0 : 3); + local = nontemporal ? integer_zero_node : integer_three_node; for (ap = 0; ap < n_prefetches; ap++) { @@ -1130,7 +1162,7 @@ nontemporal_store_p (struct mem_ref *ref) if (mode == BLKmode) return false; - code = optab_handler (storent_optab, mode)->insn_code; + code = optab_handler (storent_optab, mode); return code != CODE_FOR_nothing; } @@ -1634,24 +1666,51 @@ determine_loop_nest_reuse (struct loop *loop, struct mem_ref_group *refs, } } -/* 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 @@ -1672,6 +1731,21 @@ is_loop_prefetching_profitable (unsigned ahead, HOST_WIDE_INT est_niter, 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. @@ -1691,23 +1765,10 @@ is_loop_prefetching_profitable (unsigned ahead, HOST_WIDE_INT est_niter, 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; } @@ -1732,9 +1793,31 @@ loop_prefetch_arrays (struct loop *loop) 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); @@ -1743,15 +1826,7 @@ loop_prefetch_arrays (struct loop *loop) 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); @@ -1767,8 +1842,10 @@ 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, 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); @@ -1834,10 +1911,8 @@ tree_ssa_prefetch_arrays (void) if (!built_in_decls[BUILT_IN_PREFETCH]) { - tree type = build_function_type (void_type_node, - tree_cons (NULL_TREE, - const_ptr_type_node, - NULL_TREE)); + tree type = build_function_type_list (void_type_node, + const_ptr_type_node, NULL_TREE); tree decl = add_builtin_function ("__builtin_prefetch", type, BUILT_IN_PREFETCH, BUILT_IN_NORMAL, NULL, NULL_TREE); |