2 Copyright (C) 2005, 2007, 2008 Free Software Foundation, Inc.
4 This file is part of GCC.
6 GCC is free software; you can redistribute it and/or modify it
7 under the terms of the GNU General Public License as published by the
8 Free Software Foundation; either version 3, or (at your option) any
11 GCC is distributed in the hope that it will be useful, but WITHOUT
12 ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
13 FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
16 You should have received a copy of the GNU General Public License
17 along with GCC; see the file COPYING3. If not see
18 <http://www.gnu.org/licenses/>. */
22 #include "coretypes.h"
27 #include "hard-reg-set.h"
28 #include "basic-block.h"
30 #include "diagnostic.h"
31 #include "tree-flow.h"
32 #include "tree-dump.h"
36 #include "tree-pass.h"
38 #include "insn-config.h"
41 #include "tree-chrec.h"
42 #include "tree-scalar-evolution.h"
45 #include "langhooks.h"
46 #include "tree-inline.h"
47 #include "tree-data-ref.h"
50 /* This pass inserts prefetch instructions to optimize cache usage during
51 accesses to arrays in loops. It processes loops sequentially and:
53 1) Gathers all memory references in the single loop.
54 2) For each of the references it decides when it is profitable to prefetch
55 it. To do it, we evaluate the reuse among the accesses, and determines
56 two values: PREFETCH_BEFORE (meaning that it only makes sense to do
57 prefetching in the first PREFETCH_BEFORE iterations of the loop) and
58 PREFETCH_MOD (meaning that it only makes sense to prefetch in the
59 iterations of the loop that are zero modulo PREFETCH_MOD). For example
60 (assuming cache line size is 64 bytes, char has size 1 byte and there
61 is no hardware sequential prefetch):
64 for (i = 0; i < max; i++)
71 a[187*i + 50] = ...; (5)
74 (0) obviously has PREFETCH_BEFORE 1
75 (1) has PREFETCH_BEFORE 64, since (2) accesses the same memory
76 location 64 iterations before it, and PREFETCH_MOD 64 (since
77 it hits the same cache line otherwise).
78 (2) has PREFETCH_MOD 64
79 (3) has PREFETCH_MOD 4
80 (4) has PREFETCH_MOD 1. We do not set PREFETCH_BEFORE here, since
81 the cache line accessed by (4) is the same with probability only
83 (5) has PREFETCH_MOD 1 as well.
85 Additionally, we use data dependence analysis to determine for each
86 reference the distance till the first reuse; this information is used
87 to determine the temporality of the issued prefetch instruction.
89 3) We determine how much ahead we need to prefetch. The number of
90 iterations needed is time to fetch / time spent in one iteration of
91 the loop. The problem is that we do not know either of these values,
92 so we just make a heuristic guess based on a magic (possibly)
93 target-specific constant and size of the loop.
95 4) Determine which of the references we prefetch. We take into account
96 that there is a maximum number of simultaneous prefetches (provided
97 by machine description). We prefetch as many prefetches as possible
98 while still within this bound (starting with those with lowest
99 prefetch_mod, since they are responsible for most of the cache
102 5) We unroll and peel loops so that we are able to satisfy PREFETCH_MOD
103 and PREFETCH_BEFORE requirements (within some bounds), and to avoid
104 prefetching nonaccessed memory.
105 TODO -- actually implement peeling.
107 6) We actually emit the prefetch instructions. ??? Perhaps emit the
108 prefetch instructions with guards in cases where 5) was not sufficient
109 to satisfy the constraints?
111 The function is_loop_prefetching_profitable() implements a cost model
112 to determine if prefetching is profitable for a given loop. The cost
113 model has two heuristcs:
114 1. A heuristic that determines whether the given loop has enough CPU
115 ops that can be overlapped with cache missing memory ops.
116 If not, the loop won't benefit from prefetching. This is implemented
117 by requirung the ratio between the instruction count and the mem ref
118 count to be above a certain minimum.
119 2. A heuristic that disables prefetching in a loop with an unknown trip
120 count if the prefetching cost is above a certain limit. The relative
121 prefetching cost is estimated by taking the ratio between the
122 prefetch count and the total intruction count (this models the I-cache
124 The limits used in these heuristics are defined as parameters with
125 reasonable default values. Machine-specific default values will be
129 -- write and use more general reuse analysis (that could be also used
130 in other cache aimed loop optimizations)
131 -- make it behave sanely together with the prefetches given by user
132 (now we just ignore them; at the very least we should avoid
133 optimizing loops in that user put his own prefetches)
134 -- we assume cache line size alignment of arrays; this could be
137 /* Magic constants follow. These should be replaced by machine specific
140 /* True if write can be prefetched by a read prefetch. */
142 #ifndef WRITE_CAN_USE_READ_PREFETCH
143 #define WRITE_CAN_USE_READ_PREFETCH 1
146 /* True if read can be prefetched by a write prefetch. */
148 #ifndef READ_CAN_USE_WRITE_PREFETCH
149 #define READ_CAN_USE_WRITE_PREFETCH 0
152 /* The size of the block loaded by a single prefetch. Usually, this is
153 the same as cache line size (at the moment, we only consider one level
154 of cache hierarchy). */
156 #ifndef PREFETCH_BLOCK
157 #define PREFETCH_BLOCK L1_CACHE_LINE_SIZE
160 /* Do we have a forward hardware sequential prefetching? */
162 #ifndef HAVE_FORWARD_PREFETCH
163 #define HAVE_FORWARD_PREFETCH 0
166 /* Do we have a backward hardware sequential prefetching? */
168 #ifndef HAVE_BACKWARD_PREFETCH
169 #define HAVE_BACKWARD_PREFETCH 0
172 /* In some cases we are only able to determine that there is a certain
173 probability that the two accesses hit the same cache line. In this
174 case, we issue the prefetches for both of them if this probability
175 is less then (1000 - ACCEPTABLE_MISS_RATE) per thousand. */
177 #ifndef ACCEPTABLE_MISS_RATE
178 #define ACCEPTABLE_MISS_RATE 50
181 #ifndef HAVE_prefetch
182 #define HAVE_prefetch 0
185 #define L1_CACHE_SIZE_BYTES ((unsigned) (L1_CACHE_SIZE * 1024))
186 #define L2_CACHE_SIZE_BYTES ((unsigned) (L2_CACHE_SIZE * 1024))
188 /* We consider a memory access nontemporal if it is not reused sooner than
189 after L2_CACHE_SIZE_BYTES of memory are accessed. However, we ignore
190 accesses closer than L1_CACHE_SIZE_BYTES / NONTEMPORAL_FRACTION,
191 so that we use nontemporal prefetches e.g. if single memory location
192 is accessed several times in a single iteration of the loop. */
193 #define NONTEMPORAL_FRACTION 16
195 /* In case we have to emit a memory fence instruction after the loop that
196 uses nontemporal stores, this defines the builtin to use. */
198 #ifndef FENCE_FOLLOWING_MOVNT
199 #define FENCE_FOLLOWING_MOVNT NULL_TREE
202 /* It is not profitable to prefetch when the trip count is not at
203 least TRIP_COUNT_TO_AHEAD_RATIO times the prefetch ahead distance.
204 For example, in a loop with a prefetch ahead distance of 10,
205 supposing that TRIP_COUNT_TO_AHEAD_RATIO is equal to 4, it is
206 profitable to prefetch when the trip count is greater or equal to
207 40. In that case, 30 out of the 40 iterations will benefit from
210 #ifndef TRIP_COUNT_TO_AHEAD_RATIO
211 #define TRIP_COUNT_TO_AHEAD_RATIO 4
214 /* The group of references between that reuse may occur. */
218 tree base
; /* Base of the reference. */
219 HOST_WIDE_INT step
; /* Step of the reference. */
220 struct mem_ref
*refs
; /* References in the group. */
221 struct mem_ref_group
*next
; /* Next group of references. */
224 /* Assigned to PREFETCH_BEFORE when all iterations are to be prefetched. */
226 #define PREFETCH_ALL (~(unsigned HOST_WIDE_INT) 0)
228 /* The memory reference. */
232 gimple stmt
; /* Statement in that the reference appears. */
233 tree mem
; /* The reference. */
234 HOST_WIDE_INT delta
; /* Constant offset of the reference. */
235 struct mem_ref_group
*group
; /* The group of references it belongs to. */
236 unsigned HOST_WIDE_INT prefetch_mod
;
237 /* Prefetch only each PREFETCH_MOD-th
239 unsigned HOST_WIDE_INT prefetch_before
;
240 /* Prefetch only first PREFETCH_BEFORE
242 unsigned reuse_distance
; /* The amount of data accessed before the first
243 reuse of this value. */
244 struct mem_ref
*next
; /* The next reference in the group. */
245 unsigned write_p
: 1; /* Is it a write? */
246 unsigned independent_p
: 1; /* True if the reference is independent on
247 all other references inside the loop. */
248 unsigned issue_prefetch_p
: 1; /* Should we really issue the prefetch? */
249 unsigned storent_p
: 1; /* True if we changed the store to a
253 /* Dumps information about reference REF to FILE. */
256 dump_mem_ref (FILE *file
, struct mem_ref
*ref
)
258 fprintf (file
, "Reference %p:\n", (void *) ref
);
260 fprintf (file
, " group %p (base ", (void *) ref
->group
);
261 print_generic_expr (file
, ref
->group
->base
, TDF_SLIM
);
262 fprintf (file
, ", step ");
263 fprintf (file
, HOST_WIDE_INT_PRINT_DEC
, ref
->group
->step
);
264 fprintf (file
, ")\n");
266 fprintf (file
, " delta ");
267 fprintf (file
, HOST_WIDE_INT_PRINT_DEC
, ref
->delta
);
268 fprintf (file
, "\n");
270 fprintf (file
, " %s\n", ref
->write_p
? "write" : "read");
272 fprintf (file
, "\n");
275 /* Finds a group with BASE and STEP in GROUPS, or creates one if it does not
278 static struct mem_ref_group
*
279 find_or_create_group (struct mem_ref_group
**groups
, tree base
,
282 struct mem_ref_group
*group
;
284 for (; *groups
; groups
= &(*groups
)->next
)
286 if ((*groups
)->step
== step
287 && operand_equal_p ((*groups
)->base
, base
, 0))
290 /* Keep the list of groups sorted by decreasing step. */
291 if ((*groups
)->step
< step
)
295 group
= XNEW (struct mem_ref_group
);
299 group
->next
= *groups
;
305 /* Records a memory reference MEM in GROUP with offset DELTA and write status
306 WRITE_P. The reference occurs in statement STMT. */
309 record_ref (struct mem_ref_group
*group
, gimple stmt
, tree mem
,
310 HOST_WIDE_INT delta
, bool write_p
)
312 struct mem_ref
**aref
;
314 /* Do not record the same address twice. */
315 for (aref
= &group
->refs
; *aref
; aref
= &(*aref
)->next
)
317 /* It does not have to be possible for write reference to reuse the read
318 prefetch, or vice versa. */
319 if (!WRITE_CAN_USE_READ_PREFETCH
321 && !(*aref
)->write_p
)
323 if (!READ_CAN_USE_WRITE_PREFETCH
328 if ((*aref
)->delta
== delta
)
332 (*aref
) = XNEW (struct mem_ref
);
333 (*aref
)->stmt
= stmt
;
335 (*aref
)->delta
= delta
;
336 (*aref
)->write_p
= write_p
;
337 (*aref
)->prefetch_before
= PREFETCH_ALL
;
338 (*aref
)->prefetch_mod
= 1;
339 (*aref
)->reuse_distance
= 0;
340 (*aref
)->issue_prefetch_p
= false;
341 (*aref
)->group
= group
;
342 (*aref
)->next
= NULL
;
343 (*aref
)->independent_p
= false;
344 (*aref
)->storent_p
= false;
346 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
347 dump_mem_ref (dump_file
, *aref
);
350 /* Release memory references in GROUPS. */
353 release_mem_refs (struct mem_ref_group
*groups
)
355 struct mem_ref_group
*next_g
;
356 struct mem_ref
*ref
, *next_r
;
358 for (; groups
; groups
= next_g
)
360 next_g
= groups
->next
;
361 for (ref
= groups
->refs
; ref
; ref
= next_r
)
370 /* A structure used to pass arguments to idx_analyze_ref. */
374 struct loop
*loop
; /* Loop of the reference. */
375 gimple stmt
; /* Statement of the reference. */
376 HOST_WIDE_INT
*step
; /* Step of the memory reference. */
377 HOST_WIDE_INT
*delta
; /* Offset of the memory reference. */
380 /* Analyzes a single INDEX of a memory reference to obtain information
381 described at analyze_ref. Callback for for_each_index. */
384 idx_analyze_ref (tree base
, tree
*index
, void *data
)
386 struct ar_data
*ar_data
= (struct ar_data
*) data
;
387 tree ibase
, step
, stepsize
;
388 HOST_WIDE_INT istep
, idelta
= 0, imult
= 1;
391 if (TREE_CODE (base
) == MISALIGNED_INDIRECT_REF
392 || TREE_CODE (base
) == ALIGN_INDIRECT_REF
)
395 if (!simple_iv (ar_data
->loop
, loop_containing_stmt (ar_data
->stmt
),
401 if (!cst_and_fits_in_hwi (step
))
403 istep
= int_cst_value (step
);
405 if (TREE_CODE (ibase
) == POINTER_PLUS_EXPR
406 && cst_and_fits_in_hwi (TREE_OPERAND (ibase
, 1)))
408 idelta
= int_cst_value (TREE_OPERAND (ibase
, 1));
409 ibase
= TREE_OPERAND (ibase
, 0);
411 if (cst_and_fits_in_hwi (ibase
))
413 idelta
+= int_cst_value (ibase
);
414 ibase
= build_int_cst (TREE_TYPE (ibase
), 0);
417 if (TREE_CODE (base
) == ARRAY_REF
)
419 stepsize
= array_ref_element_size (base
);
420 if (!cst_and_fits_in_hwi (stepsize
))
422 imult
= int_cst_value (stepsize
);
428 *ar_data
->step
+= istep
;
429 *ar_data
->delta
+= idelta
;
435 /* Tries to express REF_P in shape &BASE + STEP * iter + DELTA, where DELTA and
436 STEP are integer constants and iter is number of iterations of LOOP. The
437 reference occurs in statement STMT. Strips nonaddressable component
438 references from REF_P. */
441 analyze_ref (struct loop
*loop
, tree
*ref_p
, tree
*base
,
442 HOST_WIDE_INT
*step
, HOST_WIDE_INT
*delta
,
445 struct ar_data ar_data
;
447 HOST_WIDE_INT bit_offset
;
453 /* First strip off the component references. Ignore bitfields. */
454 if (TREE_CODE (ref
) == COMPONENT_REF
455 && DECL_NONADDRESSABLE_P (TREE_OPERAND (ref
, 1)))
456 ref
= TREE_OPERAND (ref
, 0);
460 for (; TREE_CODE (ref
) == COMPONENT_REF
; ref
= TREE_OPERAND (ref
, 0))
462 off
= DECL_FIELD_BIT_OFFSET (TREE_OPERAND (ref
, 1));
463 bit_offset
= TREE_INT_CST_LOW (off
);
464 gcc_assert (bit_offset
% BITS_PER_UNIT
== 0);
466 *delta
+= bit_offset
/ BITS_PER_UNIT
;
469 *base
= unshare_expr (ref
);
473 ar_data
.delta
= delta
;
474 return for_each_index (base
, idx_analyze_ref
, &ar_data
);
477 /* Record a memory reference REF to the list REFS. The reference occurs in
478 LOOP in statement STMT and it is write if WRITE_P. Returns true if the
479 reference was recorded, false otherwise. */
482 gather_memory_references_ref (struct loop
*loop
, struct mem_ref_group
**refs
,
483 tree ref
, bool write_p
, gimple stmt
)
486 HOST_WIDE_INT step
, delta
;
487 struct mem_ref_group
*agrp
;
489 if (get_base_address (ref
) == NULL
)
492 if (!analyze_ref (loop
, &ref
, &base
, &step
, &delta
, stmt
))
495 /* Now we know that REF = &BASE + STEP * iter + DELTA, where DELTA and STEP
496 are integer constants. */
497 agrp
= find_or_create_group (refs
, base
, step
);
498 record_ref (agrp
, stmt
, ref
, delta
, write_p
);
503 /* Record the suitable memory references in LOOP. NO_OTHER_REFS is set to
504 true if there are no other memory references inside the loop. */
506 static struct mem_ref_group
*
507 gather_memory_references (struct loop
*loop
, bool *no_other_refs
, unsigned *ref_count
)
509 basic_block
*body
= get_loop_body_in_dom_order (loop
);
512 gimple_stmt_iterator bsi
;
515 struct mem_ref_group
*refs
= NULL
;
517 *no_other_refs
= true;
520 /* Scan the loop body in order, so that the former references precede the
522 for (i
= 0; i
< loop
->num_nodes
; i
++)
525 if (bb
->loop_father
!= loop
)
528 for (bsi
= gsi_start_bb (bb
); !gsi_end_p (bsi
); gsi_next (&bsi
))
530 stmt
= gsi_stmt (bsi
);
532 if (gimple_code (stmt
) != GIMPLE_ASSIGN
)
534 if (gimple_vuse (stmt
)
535 || (is_gimple_call (stmt
)
536 && !(gimple_call_flags (stmt
) & ECF_CONST
)))
537 *no_other_refs
= false;
541 lhs
= gimple_assign_lhs (stmt
);
542 rhs
= gimple_assign_rhs1 (stmt
);
544 if (REFERENCE_CLASS_P (rhs
))
546 *no_other_refs
&= gather_memory_references_ref (loop
, &refs
,
550 if (REFERENCE_CLASS_P (lhs
))
552 *no_other_refs
&= gather_memory_references_ref (loop
, &refs
,
563 /* Prune the prefetch candidate REF using the self-reuse. */
566 prune_ref_by_self_reuse (struct mem_ref
*ref
)
568 HOST_WIDE_INT step
= ref
->group
->step
;
569 bool backward
= step
< 0;
573 /* Prefetch references to invariant address just once. */
574 ref
->prefetch_before
= 1;
581 if (step
> PREFETCH_BLOCK
)
584 if ((backward
&& HAVE_BACKWARD_PREFETCH
)
585 || (!backward
&& HAVE_FORWARD_PREFETCH
))
587 ref
->prefetch_before
= 1;
591 ref
->prefetch_mod
= PREFETCH_BLOCK
/ step
;
594 /* Divides X by BY, rounding down. */
597 ddown (HOST_WIDE_INT x
, unsigned HOST_WIDE_INT by
)
604 return (x
+ by
- 1) / by
;
607 /* Given a CACHE_LINE_SIZE and two inductive memory references
608 with a common STEP greater than CACHE_LINE_SIZE and an address
609 difference DELTA, compute the probability that they will fall
610 in different cache lines. DISTINCT_ITERS is the number of
611 distinct iterations after which the pattern repeats itself.
612 ALIGN_UNIT is the unit of alignment in bytes. */
615 compute_miss_rate (unsigned HOST_WIDE_INT cache_line_size
,
616 HOST_WIDE_INT step
, HOST_WIDE_INT delta
,
617 unsigned HOST_WIDE_INT distinct_iters
,
620 unsigned align
, iter
;
621 int total_positions
, miss_positions
, miss_rate
;
622 int address1
, address2
, cache_line1
, cache_line2
;
627 /* Iterate through all possible alignments of the first
628 memory reference within its cache line. */
629 for (align
= 0; align
< cache_line_size
; align
+= align_unit
)
631 /* Iterate through all distinct iterations. */
632 for (iter
= 0; iter
< distinct_iters
; iter
++)
634 address1
= align
+ step
* iter
;
635 address2
= address1
+ delta
;
636 cache_line1
= address1
/ cache_line_size
;
637 cache_line2
= address2
/ cache_line_size
;
638 total_positions
+= 1;
639 if (cache_line1
!= cache_line2
)
642 miss_rate
= 1000 * miss_positions
/ total_positions
;
646 /* Prune the prefetch candidate REF using the reuse with BY.
647 If BY_IS_BEFORE is true, BY is before REF in the loop. */
650 prune_ref_by_group_reuse (struct mem_ref
*ref
, struct mem_ref
*by
,
653 HOST_WIDE_INT step
= ref
->group
->step
;
654 bool backward
= step
< 0;
655 HOST_WIDE_INT delta_r
= ref
->delta
, delta_b
= by
->delta
;
656 HOST_WIDE_INT delta
= delta_b
- delta_r
;
657 HOST_WIDE_INT hit_from
;
658 unsigned HOST_WIDE_INT prefetch_before
, prefetch_block
;
660 HOST_WIDE_INT reduced_step
;
661 unsigned HOST_WIDE_INT reduced_prefetch_block
;
667 /* If the references has the same address, only prefetch the
670 ref
->prefetch_before
= 0;
677 /* If the reference addresses are invariant and fall into the
678 same cache line, prefetch just the first one. */
682 if (ddown (ref
->delta
, PREFETCH_BLOCK
)
683 != ddown (by
->delta
, PREFETCH_BLOCK
))
686 ref
->prefetch_before
= 0;
690 /* Only prune the reference that is behind in the array. */
696 /* Transform the data so that we may assume that the accesses
700 delta_r
= PREFETCH_BLOCK
- 1 - delta_r
;
701 delta_b
= PREFETCH_BLOCK
- 1 - delta_b
;
709 /* Check whether the two references are likely to hit the same cache
710 line, and how distant the iterations in that it occurs are from
713 if (step
<= PREFETCH_BLOCK
)
715 /* The accesses are sure to meet. Let us check when. */
716 hit_from
= ddown (delta_b
, PREFETCH_BLOCK
) * PREFETCH_BLOCK
;
717 prefetch_before
= (hit_from
- delta_r
+ step
- 1) / step
;
719 if (prefetch_before
< ref
->prefetch_before
)
720 ref
->prefetch_before
= prefetch_before
;
725 /* A more complicated case with step > prefetch_block. First reduce
726 the ratio between the step and the cache line size to its simplest
727 terms. The resulting denominator will then represent the number of
728 distinct iterations after which each address will go back to its
729 initial location within the cache line. This computation assumes
730 that PREFETCH_BLOCK is a power of two. */
731 prefetch_block
= PREFETCH_BLOCK
;
732 reduced_prefetch_block
= prefetch_block
;
734 while ((reduced_step
& 1) == 0
735 && reduced_prefetch_block
> 1)
738 reduced_prefetch_block
>>= 1;
741 prefetch_before
= delta
/ step
;
743 ref_type
= TREE_TYPE (ref
->mem
);
744 align_unit
= TYPE_ALIGN (ref_type
) / 8;
745 miss_rate
= compute_miss_rate(prefetch_block
, step
, delta
,
746 reduced_prefetch_block
, align_unit
);
747 if (miss_rate
<= ACCEPTABLE_MISS_RATE
)
749 if (prefetch_before
< ref
->prefetch_before
)
750 ref
->prefetch_before
= prefetch_before
;
755 /* Try also the following iteration. */
757 delta
= step
- delta
;
758 miss_rate
= compute_miss_rate(prefetch_block
, step
, delta
,
759 reduced_prefetch_block
, align_unit
);
760 if (miss_rate
<= ACCEPTABLE_MISS_RATE
)
762 if (prefetch_before
< ref
->prefetch_before
)
763 ref
->prefetch_before
= prefetch_before
;
768 /* The ref probably does not reuse by. */
772 /* Prune the prefetch candidate REF using the reuses with other references
776 prune_ref_by_reuse (struct mem_ref
*ref
, struct mem_ref
*refs
)
778 struct mem_ref
*prune_by
;
781 prune_ref_by_self_reuse (ref
);
783 for (prune_by
= refs
; prune_by
; prune_by
= prune_by
->next
)
791 if (!WRITE_CAN_USE_READ_PREFETCH
793 && !prune_by
->write_p
)
795 if (!READ_CAN_USE_WRITE_PREFETCH
797 && prune_by
->write_p
)
800 prune_ref_by_group_reuse (ref
, prune_by
, before
);
804 /* Prune the prefetch candidates in GROUP using the reuse analysis. */
807 prune_group_by_reuse (struct mem_ref_group
*group
)
809 struct mem_ref
*ref_pruned
;
811 for (ref_pruned
= group
->refs
; ref_pruned
; ref_pruned
= ref_pruned
->next
)
813 prune_ref_by_reuse (ref_pruned
, group
->refs
);
815 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
817 fprintf (dump_file
, "Reference %p:", (void *) ref_pruned
);
819 if (ref_pruned
->prefetch_before
== PREFETCH_ALL
820 && ref_pruned
->prefetch_mod
== 1)
821 fprintf (dump_file
, " no restrictions");
822 else if (ref_pruned
->prefetch_before
== 0)
823 fprintf (dump_file
, " do not prefetch");
824 else if (ref_pruned
->prefetch_before
<= ref_pruned
->prefetch_mod
)
825 fprintf (dump_file
, " prefetch once");
828 if (ref_pruned
->prefetch_before
!= PREFETCH_ALL
)
830 fprintf (dump_file
, " prefetch before ");
831 fprintf (dump_file
, HOST_WIDE_INT_PRINT_DEC
,
832 ref_pruned
->prefetch_before
);
834 if (ref_pruned
->prefetch_mod
!= 1)
836 fprintf (dump_file
, " prefetch mod ");
837 fprintf (dump_file
, HOST_WIDE_INT_PRINT_DEC
,
838 ref_pruned
->prefetch_mod
);
841 fprintf (dump_file
, "\n");
846 /* Prune the list of prefetch candidates GROUPS using the reuse analysis. */
849 prune_by_reuse (struct mem_ref_group
*groups
)
851 for (; groups
; groups
= groups
->next
)
852 prune_group_by_reuse (groups
);
855 /* Returns true if we should issue prefetch for REF. */
858 should_issue_prefetch_p (struct mem_ref
*ref
)
860 /* For now do not issue prefetches for only first few of the
862 if (ref
->prefetch_before
!= PREFETCH_ALL
)
865 /* Do not prefetch nontemporal stores. */
872 /* Decide which of the prefetch candidates in GROUPS to prefetch.
873 AHEAD is the number of iterations to prefetch ahead (which corresponds
874 to the number of simultaneous instances of one prefetch running at a
875 time). UNROLL_FACTOR is the factor by that the loop is going to be
876 unrolled. Returns true if there is anything to prefetch. */
879 schedule_prefetches (struct mem_ref_group
*groups
, unsigned unroll_factor
,
882 unsigned remaining_prefetch_slots
, n_prefetches
, prefetch_slots
;
883 unsigned slots_per_prefetch
;
887 /* At most SIMULTANEOUS_PREFETCHES should be running at the same time. */
888 remaining_prefetch_slots
= SIMULTANEOUS_PREFETCHES
;
890 /* The prefetch will run for AHEAD iterations of the original loop, i.e.,
891 AHEAD / UNROLL_FACTOR iterations of the unrolled loop. In each iteration,
892 it will need a prefetch slot. */
893 slots_per_prefetch
= (ahead
+ unroll_factor
/ 2) / unroll_factor
;
894 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
895 fprintf (dump_file
, "Each prefetch instruction takes %u prefetch slots.\n",
898 /* For now we just take memory references one by one and issue
899 prefetches for as many as possible. The groups are sorted
900 starting with the largest step, since the references with
901 large step are more likely to cause many cache misses. */
903 for (; groups
; groups
= groups
->next
)
904 for (ref
= groups
->refs
; ref
; ref
= ref
->next
)
906 if (!should_issue_prefetch_p (ref
))
909 /* If we need to prefetch the reference each PREFETCH_MOD iterations,
910 and we unroll the loop UNROLL_FACTOR times, we need to insert
911 ceil (UNROLL_FACTOR / PREFETCH_MOD) instructions in each
913 n_prefetches
= ((unroll_factor
+ ref
->prefetch_mod
- 1)
914 / ref
->prefetch_mod
);
915 prefetch_slots
= n_prefetches
* slots_per_prefetch
;
917 /* If more than half of the prefetches would be lost anyway, do not
918 issue the prefetch. */
919 if (2 * remaining_prefetch_slots
< prefetch_slots
)
922 ref
->issue_prefetch_p
= true;
924 if (remaining_prefetch_slots
<= prefetch_slots
)
926 remaining_prefetch_slots
-= prefetch_slots
;
933 /* Estimate the number of prefetches in the given GROUPS. */
936 estimate_prefetch_count (struct mem_ref_group
*groups
)
939 int prefetch_count
= 0;
941 for (; groups
; groups
= groups
->next
)
942 for (ref
= groups
->refs
; ref
; ref
= ref
->next
)
943 if (should_issue_prefetch_p (ref
))
946 return prefetch_count
;
949 /* Issue prefetches for the reference REF into loop as decided before.
950 HEAD is the number of iterations to prefetch ahead. UNROLL_FACTOR
951 is the factor by which LOOP was unrolled. */
954 issue_prefetch_ref (struct mem_ref
*ref
, unsigned unroll_factor
, unsigned ahead
)
957 tree addr
, addr_base
, write_p
, local
;
959 gimple_stmt_iterator bsi
;
960 unsigned n_prefetches
, ap
;
961 bool nontemporal
= ref
->reuse_distance
>= L2_CACHE_SIZE_BYTES
;
963 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
964 fprintf (dump_file
, "Issued%s prefetch for %p.\n",
965 nontemporal
? " nontemporal" : "",
968 bsi
= gsi_for_stmt (ref
->stmt
);
970 n_prefetches
= ((unroll_factor
+ ref
->prefetch_mod
- 1)
971 / ref
->prefetch_mod
);
972 addr_base
= build_fold_addr_expr_with_type (ref
->mem
, ptr_type_node
);
973 addr_base
= force_gimple_operand_gsi (&bsi
, unshare_expr (addr_base
),
974 true, NULL
, true, GSI_SAME_STMT
);
975 write_p
= ref
->write_p
? integer_one_node
: integer_zero_node
;
976 local
= build_int_cst (integer_type_node
, nontemporal
? 0 : 3);
978 for (ap
= 0; ap
< n_prefetches
; ap
++)
980 /* Determine the address to prefetch. */
981 delta
= (ahead
+ ap
* ref
->prefetch_mod
) * ref
->group
->step
;
982 addr
= fold_build2 (POINTER_PLUS_EXPR
, ptr_type_node
,
983 addr_base
, size_int (delta
));
984 addr
= force_gimple_operand_gsi (&bsi
, unshare_expr (addr
), true, NULL
,
985 true, GSI_SAME_STMT
);
987 /* Create the prefetch instruction. */
988 prefetch
= gimple_build_call (built_in_decls
[BUILT_IN_PREFETCH
],
989 3, addr
, write_p
, local
);
990 gsi_insert_before (&bsi
, prefetch
, GSI_SAME_STMT
);
994 /* Issue prefetches for the references in GROUPS into loop as decided before.
995 HEAD is the number of iterations to prefetch ahead. UNROLL_FACTOR is the
996 factor by that LOOP was unrolled. */
999 issue_prefetches (struct mem_ref_group
*groups
,
1000 unsigned unroll_factor
, unsigned ahead
)
1002 struct mem_ref
*ref
;
1004 for (; groups
; groups
= groups
->next
)
1005 for (ref
= groups
->refs
; ref
; ref
= ref
->next
)
1006 if (ref
->issue_prefetch_p
)
1007 issue_prefetch_ref (ref
, unroll_factor
, ahead
);
1010 /* Returns true if REF is a memory write for that a nontemporal store insn
1014 nontemporal_store_p (struct mem_ref
*ref
)
1016 enum machine_mode mode
;
1017 enum insn_code code
;
1019 /* REF must be a write that is not reused. We require it to be independent
1020 on all other memory references in the loop, as the nontemporal stores may
1021 be reordered with respect to other memory references. */
1023 || !ref
->independent_p
1024 || ref
->reuse_distance
< L2_CACHE_SIZE_BYTES
)
1027 /* Check that we have the storent instruction for the mode. */
1028 mode
= TYPE_MODE (TREE_TYPE (ref
->mem
));
1029 if (mode
== BLKmode
)
1032 code
= optab_handler (storent_optab
, mode
)->insn_code
;
1033 return code
!= CODE_FOR_nothing
;
1036 /* If REF is a nontemporal store, we mark the corresponding modify statement
1037 and return true. Otherwise, we return false. */
1040 mark_nontemporal_store (struct mem_ref
*ref
)
1042 if (!nontemporal_store_p (ref
))
1045 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
1046 fprintf (dump_file
, "Marked reference %p as a nontemporal store.\n",
1049 gimple_assign_set_nontemporal_move (ref
->stmt
, true);
1050 ref
->storent_p
= true;
1055 /* Issue a memory fence instruction after LOOP. */
1058 emit_mfence_after_loop (struct loop
*loop
)
1060 VEC (edge
, heap
) *exits
= get_loop_exit_edges (loop
);
1063 gimple_stmt_iterator bsi
;
1066 for (i
= 0; VEC_iterate (edge
, exits
, i
, exit
); i
++)
1068 call
= gimple_build_call (FENCE_FOLLOWING_MOVNT
, 0);
1070 if (!single_pred_p (exit
->dest
)
1071 /* If possible, we prefer not to insert the fence on other paths
1073 && !(exit
->flags
& EDGE_ABNORMAL
))
1074 split_loop_exit_edge (exit
);
1075 bsi
= gsi_after_labels (exit
->dest
);
1077 gsi_insert_before (&bsi
, call
, GSI_NEW_STMT
);
1078 mark_virtual_ops_for_renaming (call
);
1081 VEC_free (edge
, heap
, exits
);
1082 update_ssa (TODO_update_ssa_only_virtuals
);
1085 /* Returns true if we can use storent in loop, false otherwise. */
1088 may_use_storent_in_loop_p (struct loop
*loop
)
1092 if (loop
->inner
!= NULL
)
1095 /* If we must issue a mfence insn after using storent, check that there
1096 is a suitable place for it at each of the loop exits. */
1097 if (FENCE_FOLLOWING_MOVNT
!= NULL_TREE
)
1099 VEC (edge
, heap
) *exits
= get_loop_exit_edges (loop
);
1103 for (i
= 0; VEC_iterate (edge
, exits
, i
, exit
); i
++)
1104 if ((exit
->flags
& EDGE_ABNORMAL
)
1105 && exit
->dest
== EXIT_BLOCK_PTR
)
1108 VEC_free (edge
, heap
, exits
);
1114 /* Marks nontemporal stores in LOOP. GROUPS contains the description of memory
1115 references in the loop. */
1118 mark_nontemporal_stores (struct loop
*loop
, struct mem_ref_group
*groups
)
1120 struct mem_ref
*ref
;
1123 if (!may_use_storent_in_loop_p (loop
))
1126 for (; groups
; groups
= groups
->next
)
1127 for (ref
= groups
->refs
; ref
; ref
= ref
->next
)
1128 any
|= mark_nontemporal_store (ref
);
1130 if (any
&& FENCE_FOLLOWING_MOVNT
!= NULL_TREE
)
1131 emit_mfence_after_loop (loop
);
1134 /* Determines whether we can profitably unroll LOOP FACTOR times, and if
1135 this is the case, fill in DESC by the description of number of
1139 should_unroll_loop_p (struct loop
*loop
, struct tree_niter_desc
*desc
,
1142 if (!can_unroll_loop_p (loop
, factor
, desc
))
1145 /* We only consider loops without control flow for unrolling. This is not
1146 a hard restriction -- tree_unroll_loop works with arbitrary loops
1147 as well; but the unrolling/prefetching is usually more profitable for
1148 loops consisting of a single basic block, and we want to limit the
1150 if (loop
->num_nodes
> 2)
1156 /* Determine the coefficient by that unroll LOOP, from the information
1157 contained in the list of memory references REFS. Description of
1158 umber of iterations of LOOP is stored to DESC. NINSNS is the number of
1159 insns of the LOOP. EST_NITER is the estimated number of iterations of
1160 the loop, or -1 if no estimate is available. */
1163 determine_unroll_factor (struct loop
*loop
, struct mem_ref_group
*refs
,
1164 unsigned ninsns
, struct tree_niter_desc
*desc
,
1165 HOST_WIDE_INT est_niter
)
1167 unsigned upper_bound
;
1168 unsigned nfactor
, factor
, mod_constraint
;
1169 struct mem_ref_group
*agp
;
1170 struct mem_ref
*ref
;
1172 /* First check whether the loop is not too large to unroll. We ignore
1173 PARAM_MAX_UNROLL_TIMES, because for small loops, it prevented us
1174 from unrolling them enough to make exactly one cache line covered by each
1175 iteration. Also, the goal of PARAM_MAX_UNROLL_TIMES is to prevent
1176 us from unrolling the loops too many times in cases where we only expect
1177 gains from better scheduling and decreasing loop overhead, which is not
1179 upper_bound
= PARAM_VALUE (PARAM_MAX_UNROLLED_INSNS
) / ninsns
;
1181 /* If we unrolled the loop more times than it iterates, the unrolled version
1182 of the loop would be never entered. */
1183 if (est_niter
>= 0 && est_niter
< (HOST_WIDE_INT
) upper_bound
)
1184 upper_bound
= est_niter
;
1186 if (upper_bound
<= 1)
1189 /* Choose the factor so that we may prefetch each cache just once,
1190 but bound the unrolling by UPPER_BOUND. */
1192 for (agp
= refs
; agp
; agp
= agp
->next
)
1193 for (ref
= agp
->refs
; ref
; ref
= ref
->next
)
1194 if (should_issue_prefetch_p (ref
))
1196 mod_constraint
= ref
->prefetch_mod
;
1197 nfactor
= least_common_multiple (mod_constraint
, factor
);
1198 if (nfactor
<= upper_bound
)
1202 if (!should_unroll_loop_p (loop
, desc
, factor
))
1208 /* Returns the total volume of the memory references REFS, taking into account
1209 reuses in the innermost loop and cache line size. TODO -- we should also
1210 take into account reuses across the iterations of the loops in the loop
1214 volume_of_references (struct mem_ref_group
*refs
)
1216 unsigned volume
= 0;
1217 struct mem_ref_group
*gr
;
1218 struct mem_ref
*ref
;
1220 for (gr
= refs
; gr
; gr
= gr
->next
)
1221 for (ref
= gr
->refs
; ref
; ref
= ref
->next
)
1223 /* Almost always reuses another value? */
1224 if (ref
->prefetch_before
!= PREFETCH_ALL
)
1227 /* If several iterations access the same cache line, use the size of
1228 the line divided by this number. Otherwise, a cache line is
1229 accessed in each iteration. TODO -- in the latter case, we should
1230 take the size of the reference into account, rounding it up on cache
1231 line size multiple. */
1232 volume
+= L1_CACHE_LINE_SIZE
/ ref
->prefetch_mod
;
1237 /* Returns the volume of memory references accessed across VEC iterations of
1238 loops, whose sizes are described in the LOOP_SIZES array. N is the number
1239 of the loops in the nest (length of VEC and LOOP_SIZES vectors). */
1242 volume_of_dist_vector (lambda_vector vec
, unsigned *loop_sizes
, unsigned n
)
1246 for (i
= 0; i
< n
; i
++)
1253 gcc_assert (vec
[i
] > 0);
1255 /* We ignore the parts of the distance vector in subloops, since usually
1256 the numbers of iterations are much smaller. */
1257 return loop_sizes
[i
] * vec
[i
];
1260 /* Add the steps of ACCESS_FN multiplied by STRIDE to the array STRIDE
1261 at the position corresponding to the loop of the step. N is the depth
1262 of the considered loop nest, and, LOOP is its innermost loop. */
1265 add_subscript_strides (tree access_fn
, unsigned stride
,
1266 HOST_WIDE_INT
*strides
, unsigned n
, struct loop
*loop
)
1270 HOST_WIDE_INT astep
;
1271 unsigned min_depth
= loop_depth (loop
) - n
;
1273 while (TREE_CODE (access_fn
) == POLYNOMIAL_CHREC
)
1275 aloop
= get_chrec_loop (access_fn
);
1276 step
= CHREC_RIGHT (access_fn
);
1277 access_fn
= CHREC_LEFT (access_fn
);
1279 if ((unsigned) loop_depth (aloop
) <= min_depth
)
1282 if (host_integerp (step
, 0))
1283 astep
= tree_low_cst (step
, 0);
1285 astep
= L1_CACHE_LINE_SIZE
;
1287 strides
[n
- 1 - loop_depth (loop
) + loop_depth (aloop
)] += astep
* stride
;
1292 /* Returns the volume of memory references accessed between two consecutive
1293 self-reuses of the reference DR. We consider the subscripts of DR in N
1294 loops, and LOOP_SIZES contains the volumes of accesses in each of the
1295 loops. LOOP is the innermost loop of the current loop nest. */
1298 self_reuse_distance (data_reference_p dr
, unsigned *loop_sizes
, unsigned n
,
1301 tree stride
, access_fn
;
1302 HOST_WIDE_INT
*strides
, astride
;
1303 VEC (tree
, heap
) *access_fns
;
1304 tree ref
= DR_REF (dr
);
1305 unsigned i
, ret
= ~0u;
1307 /* In the following example:
1309 for (i = 0; i < N; i++)
1310 for (j = 0; j < N; j++)
1312 the same cache line is accessed each N steps (except if the change from
1313 i to i + 1 crosses the boundary of the cache line). Thus, for self-reuse,
1314 we cannot rely purely on the results of the data dependence analysis.
1316 Instead, we compute the stride of the reference in each loop, and consider
1317 the innermost loop in that the stride is less than cache size. */
1319 strides
= XCNEWVEC (HOST_WIDE_INT
, n
);
1320 access_fns
= DR_ACCESS_FNS (dr
);
1322 for (i
= 0; VEC_iterate (tree
, access_fns
, i
, access_fn
); i
++)
1324 /* Keep track of the reference corresponding to the subscript, so that we
1326 while (handled_component_p (ref
) && TREE_CODE (ref
) != ARRAY_REF
)
1327 ref
= TREE_OPERAND (ref
, 0);
1329 if (TREE_CODE (ref
) == ARRAY_REF
)
1331 stride
= TYPE_SIZE_UNIT (TREE_TYPE (ref
));
1332 if (host_integerp (stride
, 1))
1333 astride
= tree_low_cst (stride
, 1);
1335 astride
= L1_CACHE_LINE_SIZE
;
1337 ref
= TREE_OPERAND (ref
, 0);
1342 add_subscript_strides (access_fn
, astride
, strides
, n
, loop
);
1345 for (i
= n
; i
-- > 0; )
1347 unsigned HOST_WIDE_INT s
;
1349 s
= strides
[i
] < 0 ? -strides
[i
] : strides
[i
];
1351 if (s
< (unsigned) L1_CACHE_LINE_SIZE
1353 > (unsigned) (L1_CACHE_SIZE_BYTES
/ NONTEMPORAL_FRACTION
)))
1355 ret
= loop_sizes
[i
];
1364 /* Determines the distance till the first reuse of each reference in REFS
1365 in the loop nest of LOOP. NO_OTHER_REFS is true if there are no other
1366 memory references in the loop. */
1369 determine_loop_nest_reuse (struct loop
*loop
, struct mem_ref_group
*refs
,
1372 struct loop
*nest
, *aloop
;
1373 VEC (data_reference_p
, heap
) *datarefs
= NULL
;
1374 VEC (ddr_p
, heap
) *dependences
= NULL
;
1375 struct mem_ref_group
*gr
;
1376 struct mem_ref
*ref
, *refb
;
1377 VEC (loop_p
, heap
) *vloops
= NULL
;
1378 unsigned *loop_data_size
;
1380 unsigned volume
, dist
, adist
;
1382 data_reference_p dr
;
1388 /* Find the outermost loop of the loop nest of loop (we require that
1389 there are no sibling loops inside the nest). */
1393 aloop
= loop_outer (nest
);
1395 if (aloop
== current_loops
->tree_root
1396 || aloop
->inner
->next
)
1402 /* For each loop, determine the amount of data accessed in each iteration.
1403 We use this to estimate whether the reference is evicted from the
1404 cache before its reuse. */
1405 find_loop_nest (nest
, &vloops
);
1406 n
= VEC_length (loop_p
, vloops
);
1407 loop_data_size
= XNEWVEC (unsigned, n
);
1408 volume
= volume_of_references (refs
);
1412 loop_data_size
[i
] = volume
;
1413 /* Bound the volume by the L2 cache size, since above this bound,
1414 all dependence distances are equivalent. */
1415 if (volume
> L2_CACHE_SIZE_BYTES
)
1418 aloop
= VEC_index (loop_p
, vloops
, i
);
1419 vol
= estimated_loop_iterations_int (aloop
, false);
1421 vol
= expected_loop_iterations (aloop
);
1425 /* Prepare the references in the form suitable for data dependence
1426 analysis. We ignore unanalyzable data references (the results
1427 are used just as a heuristics to estimate temporality of the
1428 references, hence we do not need to worry about correctness). */
1429 for (gr
= refs
; gr
; gr
= gr
->next
)
1430 for (ref
= gr
->refs
; ref
; ref
= ref
->next
)
1432 dr
= create_data_ref (nest
, ref
->mem
, ref
->stmt
, !ref
->write_p
);
1436 ref
->reuse_distance
= volume
;
1438 VEC_safe_push (data_reference_p
, heap
, datarefs
, dr
);
1441 no_other_refs
= false;
1444 for (i
= 0; VEC_iterate (data_reference_p
, datarefs
, i
, dr
); i
++)
1446 dist
= self_reuse_distance (dr
, loop_data_size
, n
, loop
);
1447 ref
= (struct mem_ref
*) dr
->aux
;
1448 if (ref
->reuse_distance
> dist
)
1449 ref
->reuse_distance
= dist
;
1452 ref
->independent_p
= true;
1455 compute_all_dependences (datarefs
, &dependences
, vloops
, true);
1457 for (i
= 0; VEC_iterate (ddr_p
, dependences
, i
, dep
); i
++)
1459 if (DDR_ARE_DEPENDENT (dep
) == chrec_known
)
1462 ref
= (struct mem_ref
*) DDR_A (dep
)->aux
;
1463 refb
= (struct mem_ref
*) DDR_B (dep
)->aux
;
1465 if (DDR_ARE_DEPENDENT (dep
) == chrec_dont_know
1466 || DDR_NUM_DIST_VECTS (dep
) == 0)
1468 /* If the dependence cannot be analyzed, assume that there might be
1472 ref
->independent_p
= false;
1473 refb
->independent_p
= false;
1477 /* The distance vectors are normalized to be always lexicographically
1478 positive, hence we cannot tell just from them whether DDR_A comes
1479 before DDR_B or vice versa. However, it is not important,
1480 anyway -- if DDR_A is close to DDR_B, then it is either reused in
1481 DDR_B (and it is not nontemporal), or it reuses the value of DDR_B
1482 in cache (and marking it as nontemporal would not affect
1486 for (j
= 0; j
< DDR_NUM_DIST_VECTS (dep
); j
++)
1488 adist
= volume_of_dist_vector (DDR_DIST_VECT (dep
, j
),
1491 /* If this is a dependence in the innermost loop (i.e., the
1492 distances in all superloops are zero) and it is not
1493 the trivial self-dependence with distance zero, record that
1494 the references are not completely independent. */
1495 if (lambda_vector_zerop (DDR_DIST_VECT (dep
, j
), n
- 1)
1497 || DDR_DIST_VECT (dep
, j
)[n
-1] != 0))
1499 ref
->independent_p
= false;
1500 refb
->independent_p
= false;
1503 /* Ignore accesses closer than
1504 L1_CACHE_SIZE_BYTES / NONTEMPORAL_FRACTION,
1505 so that we use nontemporal prefetches e.g. if single memory
1506 location is accessed several times in a single iteration of
1508 if (adist
< L1_CACHE_SIZE_BYTES
/ NONTEMPORAL_FRACTION
)
1516 if (ref
->reuse_distance
> dist
)
1517 ref
->reuse_distance
= dist
;
1518 if (refb
->reuse_distance
> dist
)
1519 refb
->reuse_distance
= dist
;
1522 free_dependence_relations (dependences
);
1523 free_data_refs (datarefs
);
1524 free (loop_data_size
);
1526 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
1528 fprintf (dump_file
, "Reuse distances:\n");
1529 for (gr
= refs
; gr
; gr
= gr
->next
)
1530 for (ref
= gr
->refs
; ref
; ref
= ref
->next
)
1531 fprintf (dump_file
, " ref %p distance %u\n",
1532 (void *) ref
, ref
->reuse_distance
);
1536 /* Do a cost-benefit analysis to determine if prefetching is profitable
1537 for the current loop given the following parameters:
1538 AHEAD: the iteration ahead distance,
1539 EST_NITER: the estimated trip count,
1540 NINSNS: estimated number of instructions in the loop,
1541 PREFETCH_COUNT: an estimate of the number of prefetches
1542 MEM_REF_COUNT: total number of memory references in the loop. */
1545 is_loop_prefetching_profitable (unsigned ahead
, HOST_WIDE_INT est_niter
,
1546 unsigned ninsns
, unsigned prefetch_count
,
1547 unsigned mem_ref_count
, unsigned unroll_factor
)
1549 int insn_to_mem_ratio
, insn_to_prefetch_ratio
;
1551 if (mem_ref_count
== 0)
1554 /* Prefetching improves performance by overlapping cache missing
1555 memory accesses with CPU operations. If the loop does not have
1556 enough CPU operations to overlap with memory operations, prefetching
1557 won't give a significant benefit. One approximate way of checking
1558 this is to require the ratio of instructions to memory references to
1559 be above a certain limit. This approximation works well in practice.
1560 TODO: Implement a more precise computation by estimating the time
1561 for each CPU or memory op in the loop. Time estimates for memory ops
1562 should account for cache misses. */
1563 insn_to_mem_ratio
= ninsns
/ mem_ref_count
;
1565 if (insn_to_mem_ratio
< PREFETCH_MIN_INSN_TO_MEM_RATIO
)
1567 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
1569 "Not prefetching -- instruction to memory reference ratio (%d) too small\n",
1574 /* Profitability of prefetching is highly dependent on the trip count.
1575 For a given AHEAD distance, the first AHEAD iterations do not benefit
1576 from prefetching, and the last AHEAD iterations execute useless
1577 prefetches. So, if the trip count is not large enough relative to AHEAD,
1578 prefetching may cause serious performance degradation. To avoid this
1579 problem when the trip count is not known at compile time, we
1580 conservatively skip loops with high prefetching costs. For now, only
1581 the I-cache cost is considered. The relative I-cache cost is estimated
1582 by taking the ratio between the number of prefetches and the total
1583 number of instructions. Since we are using integer arithmetic, we
1584 compute the reciprocal of this ratio.
1585 (unroll_factor * ninsns) is used to estimate the number of instructions in
1586 the unrolled loop. This implementation is a bit simplistic -- the number
1587 of issued prefetch instructions is also affected by unrolling. So,
1588 prefetch_mod and the unroll factor should be taken into account when
1589 determining prefetch_count. Also, the number of insns of the unrolled
1590 loop will usually be significantly smaller than the number of insns of the
1591 original loop * unroll_factor (at least the induction variable increases
1592 and the exit branches will get eliminated), so it might be better to use
1593 tree_estimate_loop_size + estimated_unrolled_size. */
1596 insn_to_prefetch_ratio
= (unroll_factor
* ninsns
) / prefetch_count
;
1597 return insn_to_prefetch_ratio
>= MIN_INSN_TO_PREFETCH_RATIO
;
1600 if (est_niter
< (HOST_WIDE_INT
) (TRIP_COUNT_TO_AHEAD_RATIO
* ahead
))
1602 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
1604 "Not prefetching -- loop estimated to roll only %d times\n",
1612 /* Issue prefetch instructions for array references in LOOP. Returns
1613 true if the LOOP was unrolled. */
1616 loop_prefetch_arrays (struct loop
*loop
)
1618 struct mem_ref_group
*refs
;
1619 unsigned ahead
, ninsns
, time
, unroll_factor
;
1620 HOST_WIDE_INT est_niter
;
1621 struct tree_niter_desc desc
;
1622 bool unrolled
= false, no_other_refs
;
1623 unsigned prefetch_count
;
1624 unsigned mem_ref_count
;
1626 if (optimize_loop_nest_for_size_p (loop
))
1628 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
1629 fprintf (dump_file
, " ignored (cold area)\n");
1633 /* Step 1: gather the memory references. */
1634 refs
= gather_memory_references (loop
, &no_other_refs
, &mem_ref_count
);
1636 /* Step 2: estimate the reuse effects. */
1637 prune_by_reuse (refs
);
1639 prefetch_count
= estimate_prefetch_count (refs
);
1640 if (prefetch_count
== 0)
1643 determine_loop_nest_reuse (loop
, refs
, no_other_refs
);
1645 /* Step 3: determine the ahead and unroll factor. */
1647 /* FIXME: the time should be weighted by the probabilities of the blocks in
1649 time
= tree_num_loop_insns (loop
, &eni_time_weights
);
1650 ahead
= (PREFETCH_LATENCY
+ time
- 1) / time
;
1651 est_niter
= estimated_loop_iterations_int (loop
, false);
1653 ninsns
= tree_num_loop_insns (loop
, &eni_size_weights
);
1654 unroll_factor
= determine_unroll_factor (loop
, refs
, ninsns
, &desc
,
1656 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
1657 fprintf (dump_file
, "Ahead %d, unroll factor %d, trip count "
1658 HOST_WIDE_INT_PRINT_DEC
"\n"
1659 "insn count %d, mem ref count %d, prefetch count %d\n",
1660 ahead
, unroll_factor
, est_niter
,
1661 ninsns
, mem_ref_count
, prefetch_count
);
1663 if (!is_loop_prefetching_profitable (ahead
, est_niter
, ninsns
, prefetch_count
,
1664 mem_ref_count
, unroll_factor
))
1667 mark_nontemporal_stores (loop
, refs
);
1669 /* Step 4: what to prefetch? */
1670 if (!schedule_prefetches (refs
, unroll_factor
, ahead
))
1673 /* Step 5: unroll the loop. TODO -- peeling of first and last few
1674 iterations so that we do not issue superfluous prefetches. */
1675 if (unroll_factor
!= 1)
1677 tree_unroll_loop (loop
, unroll_factor
,
1678 single_dom_exit (loop
), &desc
);
1682 /* Step 6: issue the prefetches. */
1683 issue_prefetches (refs
, unroll_factor
, ahead
);
1686 release_mem_refs (refs
);
1690 /* Issue prefetch instructions for array references in loops. */
1693 tree_ssa_prefetch_arrays (void)
1697 bool unrolled
= false;
1701 /* It is possible to ask compiler for say -mtune=i486 -march=pentium4.
1702 -mtune=i486 causes us having PREFETCH_BLOCK 0, since this is part
1703 of processor costs and i486 does not have prefetch, but
1704 -march=pentium4 causes HAVE_prefetch to be true. Ugh. */
1705 || PREFETCH_BLOCK
== 0)
1708 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
1710 fprintf (dump_file
, "Prefetching parameters:\n");
1711 fprintf (dump_file
, " simultaneous prefetches: %d\n",
1712 SIMULTANEOUS_PREFETCHES
);
1713 fprintf (dump_file
, " prefetch latency: %d\n", PREFETCH_LATENCY
);
1714 fprintf (dump_file
, " prefetch block size: %d\n", PREFETCH_BLOCK
);
1715 fprintf (dump_file
, " L1 cache size: %d lines, %d kB\n",
1716 L1_CACHE_SIZE_BYTES
/ L1_CACHE_LINE_SIZE
, L1_CACHE_SIZE
);
1717 fprintf (dump_file
, " L1 cache line size: %d\n", L1_CACHE_LINE_SIZE
);
1718 fprintf (dump_file
, " L2 cache size: %d kB\n", L2_CACHE_SIZE
);
1719 fprintf (dump_file
, " min insn-to-prefetch ratio: %d \n",
1720 MIN_INSN_TO_PREFETCH_RATIO
);
1721 fprintf (dump_file
, " min insn-to-mem ratio: %d \n",
1722 PREFETCH_MIN_INSN_TO_MEM_RATIO
);
1723 fprintf (dump_file
, "\n");
1726 initialize_original_copy_tables ();
1728 if (!built_in_decls
[BUILT_IN_PREFETCH
])
1730 tree type
= build_function_type (void_type_node
,
1731 tree_cons (NULL_TREE
,
1732 const_ptr_type_node
,
1734 tree decl
= add_builtin_function ("__builtin_prefetch", type
,
1735 BUILT_IN_PREFETCH
, BUILT_IN_NORMAL
,
1737 DECL_IS_NOVOPS (decl
) = true;
1738 built_in_decls
[BUILT_IN_PREFETCH
] = decl
;
1741 /* We assume that size of cache line is a power of two, so verify this
1743 gcc_assert ((PREFETCH_BLOCK
& (PREFETCH_BLOCK
- 1)) == 0);
1745 FOR_EACH_LOOP (li
, loop
, LI_FROM_INNERMOST
)
1747 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
1748 fprintf (dump_file
, "Processing loop %d:\n", loop
->num
);
1750 unrolled
|= loop_prefetch_arrays (loop
);
1752 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
1753 fprintf (dump_file
, "\n\n");
1759 todo_flags
|= TODO_cleanup_cfg
;
1762 free_original_copy_tables ();