#include "params.h"
#include "langhooks.h"
#include "tree-inline.h"
+#include "tree-data-ref.h"
/* This pass inserts prefetch instructions to optimize cache usage during
accesses to arrays in loops. It processes loops sequentially and:
7/32.
(5) has PREFETCH_MOD 1 as well.
+ Additionally, we use data dependence analysis to determine for each
+ reference the distance till the first reuse; this information is used
+ to determine the temporality of the issued prefetch instruction.
+
3) We determine how much ahead we need to prefetch. The number of
iterations needed is time to fetch / time spent in one iteration of
the loop. The problem is that we do not know either of these values,
#define HAVE_prefetch 0
#endif
+#define L1_CACHE_SIZE_BYTES ((unsigned) (L1_CACHE_SIZE * L1_CACHE_LINE_SIZE))
+/* TODO: Add parameter to specify L2 cache size. */
+#define L2_CACHE_SIZE_BYTES (8 * L1_CACHE_SIZE_BYTES)
+
+/* We consider a memory access nontemporal if it is not reused sooner than
+ after L2_CACHE_SIZE_BYTES of memory are accessed. However, we ignore
+ accesses closer than L1_CACHE_SIZE_BYTES / NONTEMPORAL_FRACTION,
+ so that we use nontemporal prefetches e.g. if single memory location
+ is accessed several times in a single iteration of the loop. */
+#define NONTEMPORAL_FRACTION 16
+
/* The group of references between that reuse may occur. */
struct mem_ref_group
unsigned HOST_WIDE_INT prefetch_before;
/* Prefetch only first PREFETCH_BEFORE
iterations. */
+ unsigned reuse_distance; /* The amount of data accessed before the first
+ reuse of this value. */
bool issue_prefetch_p; /* Should we really issue the prefetch? */
struct mem_ref *next; /* The next reference in the group. */
};
break;
}
- group = xcalloc (1, sizeof (struct mem_ref_group));
+ group = XNEW (struct mem_ref_group);
group->base = base;
group->step = step;
group->refs = NULL;
return;
}
- (*aref) = xcalloc (1, sizeof (struct mem_ref));
+ (*aref) = XNEW (struct mem_ref);
(*aref)->stmt = stmt;
(*aref)->mem = mem;
(*aref)->delta = delta;
(*aref)->write_p = write_p;
(*aref)->prefetch_before = PREFETCH_ALL;
(*aref)->prefetch_mod = 1;
+ (*aref)->reuse_distance = 0;
(*aref)->issue_prefetch_p = false;
(*aref)->group = group;
(*aref)->next = NULL;
issue_prefetch_ref (struct mem_ref *ref, unsigned unroll_factor, unsigned ahead)
{
HOST_WIDE_INT delta;
- tree addr, addr_base, prefetch, write_p;
+ tree addr, addr_base, prefetch, write_p, local;
block_stmt_iterator bsi;
unsigned n_prefetches, ap;
+ bool nontemporal = ref->reuse_distance >= L2_CACHE_SIZE_BYTES;
if (dump_file && (dump_flags & TDF_DETAILS))
- fprintf (dump_file, "Issued prefetch for %p.\n", (void *) ref);
+ fprintf (dump_file, "Issued%s prefetch for %p.\n",
+ nontemporal ? " nontemporal" : "",
+ (void *) ref);
bsi = bsi_for_stmt (ref->stmt);
addr_base = build_fold_addr_expr_with_type (ref->mem, ptr_type_node);
addr_base = force_gimple_operand_bsi (&bsi, unshare_expr (addr_base), true, NULL);
write_p = ref->write_p ? integer_one_node : integer_zero_node;
+ local = build_int_cst (integer_type_node, nontemporal ? 0 : 3);
for (ap = 0; ap < n_prefetches; ap++)
{
/* Create the prefetch instruction. */
prefetch = build_call_expr (built_in_decls[BUILT_IN_PREFETCH],
- 2, addr, write_p);
+ 3, addr, write_p, local);
bsi_insert_before (&bsi, prefetch, BSI_SAME_STMT);
}
}
return factor;
}
+/* Returns the total volume of the memory references REFS, taking into account
+ reuses in the innermost loop and cache line size. TODO -- we should also
+ take into account reuses across the iterations of the loops in the loop
+ nest. */
+
+static unsigned
+volume_of_references (struct mem_ref_group *refs)
+{
+ unsigned volume = 0;
+ struct mem_ref_group *gr;
+ struct mem_ref *ref;
+
+ for (gr = refs; gr; gr = gr->next)
+ for (ref = gr->refs; ref; ref = ref->next)
+ {
+ /* Almost always reuses another value? */
+ if (ref->prefetch_before != PREFETCH_ALL)
+ continue;
+
+ /* If several iterations access the same cache line, use the size of
+ the line divided by this number. Otherwise, a cache line is
+ accessed in each iteration. TODO -- in the latter case, we should
+ take the size of the reference into account, rounding it up on cache
+ line size multiple. */
+ volume += L1_CACHE_LINE_SIZE / ref->prefetch_mod;
+ }
+ return volume;
+}
+
+/* Returns the volume of memory references accessed across VEC iterations of
+ loops, whose sizes are described in the LOOP_SIZES array. N is the number
+ of the loops in the nest (length of VEC and LOOP_SIZES vectors). */
+
+static unsigned
+volume_of_dist_vector (lambda_vector vec, unsigned *loop_sizes, unsigned n)
+{
+ unsigned i;
+
+ for (i = 0; i < n; i++)
+ if (vec[i] != 0)
+ break;
+
+ if (i == n)
+ return 0;
+
+ gcc_assert (vec[i] > 0);
+
+ /* We ignore the parts of the distance vector in subloops, since usually
+ the numbers of iterations are much smaller. */
+ return loop_sizes[i] * vec[i];
+}
+
+/* Add the steps of ACCESS_FN multiplied by STRIDE to the array STRIDE
+ at the position corresponding to the loop of the step. N is the depth
+ of the considered loop nest, and, LOOP is its innermost loop. */
+
+static void
+add_subscript_strides (tree access_fn, unsigned stride,
+ HOST_WIDE_INT *strides, unsigned n, struct loop *loop)
+{
+ struct loop *aloop;
+ tree step;
+ HOST_WIDE_INT astep;
+ unsigned min_depth = loop_depth (loop) - n;
+
+ while (TREE_CODE (access_fn) == POLYNOMIAL_CHREC)
+ {
+ aloop = get_chrec_loop (access_fn);
+ step = CHREC_RIGHT (access_fn);
+ access_fn = CHREC_LEFT (access_fn);
+
+ if ((unsigned) loop_depth (aloop) <= min_depth)
+ continue;
+
+ if (host_integerp (step, 0))
+ astep = tree_low_cst (step, 0);
+ else
+ astep = L1_CACHE_LINE_SIZE;
+
+ strides[n - 1 - loop_depth (loop) + loop_depth (aloop)] += astep * stride;
+
+ }
+}
+
+/* Returns the volume of memory references accessed between two consecutive
+ self-reuses of the reference DR. We consider the subscripts of DR in N
+ loops, and LOOP_SIZES contains the volumes of accesses in each of the
+ loops. LOOP is the innermost loop of the current loop nest. */
+
+static unsigned
+self_reuse_distance (data_reference_p dr, unsigned *loop_sizes, unsigned n,
+ struct loop *loop)
+{
+ tree stride, access_fn;
+ HOST_WIDE_INT *strides, astride;
+ VEC (tree, heap) *access_fns;
+ tree ref = DR_REF (dr);
+ unsigned i, ret = ~0u;
+
+ /* In the following example:
+
+ for (i = 0; i < N; i++)
+ for (j = 0; j < N; j++)
+ use (a[j][i]);
+ the same cache line is accessed each N steps (except if the change from
+ i to i + 1 crosses the boundary of the cache line). Thus, for self-reuse,
+ we cannot rely purely on the results of the data dependence analysis.
+
+ Instead, we compute the stride of the reference in each loop, and consider
+ the innermost loop in that the stride is less than cache size. */
+
+ strides = XCNEWVEC (HOST_WIDE_INT, n);
+ access_fns = DR_ACCESS_FNS (dr);
+
+ for (i = 0; VEC_iterate (tree, access_fns, i, access_fn); i++)
+ {
+ /* Keep track of the reference corresponding to the subscript, so that we
+ know its stride. */
+ while (handled_component_p (ref) && TREE_CODE (ref) != ARRAY_REF)
+ ref = TREE_OPERAND (ref, 0);
+
+ if (TREE_CODE (ref) == ARRAY_REF)
+ {
+ stride = TYPE_SIZE_UNIT (TREE_TYPE (ref));
+ if (host_integerp (stride, 1))
+ astride = tree_low_cst (stride, 1);
+ else
+ astride = L1_CACHE_LINE_SIZE;
+
+ ref = TREE_OPERAND (ref, 0);
+ }
+ else
+ astride = 1;
+
+ add_subscript_strides (access_fn, astride, strides, n, loop);
+ }
+
+ for (i = n; i-- > 0; )
+ {
+ unsigned HOST_WIDE_INT s;
+
+ s = strides[i] < 0 ? -strides[i] : strides[i];
+
+ if (s < (unsigned) L1_CACHE_LINE_SIZE
+ && (loop_sizes[i]
+ > (unsigned) (L1_CACHE_SIZE_BYTES / NONTEMPORAL_FRACTION)))
+ {
+ ret = loop_sizes[i];
+ break;
+ }
+ }
+
+ free (strides);
+ return ret;
+}
+
+/* Determines the distance till the first reuse of each reference in REFS
+ in the loop nest of LOOP. */
+
+static void
+determine_loop_nest_reuse (struct loop *loop, struct mem_ref_group *refs)
+{
+ struct loop *nest, *aloop;
+ VEC (data_reference_p, heap) *datarefs = NULL;
+ VEC (ddr_p, heap) *dependences = NULL;
+ struct mem_ref_group *gr;
+ struct mem_ref *ref;
+ VEC (loop_p, heap) *vloops = NULL;
+ unsigned *loop_data_size;
+ unsigned i, j, n;
+ unsigned volume, dist, adist;
+ HOST_WIDE_INT vol;
+ data_reference_p dr;
+ ddr_p dep;
+
+ if (loop->inner)
+ return;
+
+ /* Find the outermost loop of the loop nest of loop (we require that
+ there are no sibling loops inside the nest). */
+ nest = loop;
+ while (1)
+ {
+ aloop = loop_outer (nest);
+
+ if (aloop == current_loops->tree_root
+ || aloop->inner->next)
+ break;
+
+ nest = aloop;
+ }
+
+ /* For each loop, determine the amount of data accessed in each iteration.
+ We use this to estimate whether the reference is evicted from the
+ cache before its reuse. */
+ find_loop_nest (nest, &vloops);
+ n = VEC_length (loop_p, vloops);
+ loop_data_size = XNEWVEC (unsigned, n);
+ volume = volume_of_references (refs);
+ i = n;
+ while (i-- != 0)
+ {
+ loop_data_size[i] = volume;
+ /* Bound the volume by the L2 cache size, since above this bound,
+ all dependence distances are equivalent. */
+ if (volume > L2_CACHE_SIZE_BYTES)
+ continue;
+
+ aloop = VEC_index (loop_p, vloops, i);
+ vol = estimated_loop_iterations_int (aloop, false);
+ if (vol < 0)
+ vol = expected_loop_iterations (aloop);
+ volume *= vol;
+ }
+
+ /* Prepare the references in the form suitable for data dependence
+ analysis. We ignore unanalysable data references (the results
+ are used just as a heuristics to estimate temporality of the
+ references, hence we do not need to worry about correctness). */
+ for (gr = refs; gr; gr = gr->next)
+ for (ref = gr->refs; ref; ref = ref->next)
+ {
+ dr = create_data_ref (nest, ref->mem, ref->stmt, !ref->write_p);
+
+ if (dr)
+ {
+ ref->reuse_distance = volume;
+ dr->aux = ref;
+ VEC_safe_push (data_reference_p, heap, datarefs, dr);
+ }
+ }
+
+ for (i = 0; VEC_iterate (data_reference_p, datarefs, i, dr); i++)
+ {
+ dist = self_reuse_distance (dr, loop_data_size, n, loop);
+ ref = dr->aux;
+ if (ref->reuse_distance > dist)
+ ref->reuse_distance = dist;
+ }
+
+ compute_all_dependences (datarefs, &dependences, vloops, true);
+
+ for (i = 0; VEC_iterate (ddr_p, dependences, i, dep); i++)
+ {
+ if (DDR_ARE_DEPENDENT (dep) == chrec_known)
+ continue;
+
+ if (DDR_ARE_DEPENDENT (dep) == chrec_dont_know
+ || DDR_NUM_DIST_VECTS (dep) == 0)
+ {
+ /* If the dependence cannot be analysed, assume that there might be
+ a reuse. */
+ dist = 0;
+ }
+ else
+ {
+ /* The distance vectors are normalised to be always lexicographically
+ positive, hence we cannot tell just from them whether DDR_A comes
+ before DDR_B or vice versa. However, it is not important,
+ anyway -- if DDR_A is close to DDR_B, then it is either reused in
+ DDR_B (and it is not nontemporal), or it reuses the value of DDR_B
+ in cache (and marking it as nontemporal would not affect
+ anything). */
+
+ dist = volume;
+ for (j = 0; j < DDR_NUM_DIST_VECTS (dep); j++)
+ {
+ adist = volume_of_dist_vector (DDR_DIST_VECT (dep, j),
+ loop_data_size, n);
+
+ /* Ignore accesses closer than
+ L1_CACHE_SIZE_BYTES / NONTEMPORAL_FRACTION,
+ so that we use nontemporal prefetches e.g. if single memory
+ location is accessed several times in a single iteration of
+ the loop. */
+ if (adist < L1_CACHE_SIZE_BYTES / NONTEMPORAL_FRACTION)
+ continue;
+
+ if (adist < dist)
+ dist = adist;
+ }
+ }
+
+ ref = DDR_A (dep)->aux;
+ if (ref->reuse_distance > dist)
+ ref->reuse_distance = dist;
+ ref = DDR_B (dep)->aux;
+ if (ref->reuse_distance > dist)
+ ref->reuse_distance = dist;
+ }
+
+ free_dependence_relations (dependences);
+ free_data_refs (datarefs);
+ free (loop_data_size);
+
+ if (dump_file && (dump_flags & TDF_DETAILS))
+ {
+ fprintf (dump_file, "Reuse distances:\n");
+ for (gr = refs; gr; gr = gr->next)
+ for (ref = gr->refs; ref; ref = ref->next)
+ fprintf (dump_file, " ref %p distance %u\n",
+ (void *) ref, ref->reuse_distance);
+ }
+}
+
/* Issue prefetch instructions for array references in LOOP. Returns
true if the LOOP was unrolled. */
if (!anything_to_prefetch_p (refs))
goto fail;
+ determine_loop_nest_reuse (loop, refs);
+
/* Step 3: determine the ahead and unroll factor. */
/* FIXME: the time should be weighted by the probabilities of the blocks in
fprintf (dump_file, " simultaneous prefetches: %d\n",
SIMULTANEOUS_PREFETCHES);
fprintf (dump_file, " prefetch latency: %d\n", PREFETCH_LATENCY);
- fprintf (dump_file, " L1 cache size: %d (%d bytes)\n",
- L1_CACHE_SIZE, L1_CACHE_SIZE * L1_CACHE_LINE_SIZE);
- fprintf (dump_file, " L1 cache line size: %d\n", L1_CACHE_LINE_SIZE);
fprintf (dump_file, " prefetch block size: %d\n", PREFETCH_BLOCK);
+ fprintf (dump_file, " L1 cache size: %d lines, %d bytes\n",
+ L1_CACHE_SIZE, L1_CACHE_SIZE_BYTES);
+ fprintf (dump_file, " L1 cache line size: %d\n", L1_CACHE_LINE_SIZE);
+ fprintf (dump_file, " L2 cache size: %d bytes\n", L2_CACHE_SIZE_BYTES);
fprintf (dump_file, "\n");
}
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