1 /* Generic routines for manipulating PHIs
2 Copyright (C) 2003, 2005, 2007 Free Software Foundation, Inc.
4 This file is part of GCC.
6 GCC is free software; you can redistribute it and/or modify
7 it under the terms of the GNU General Public License as published by
8 the Free Software Foundation; either version 3, or (at your option)
11 GCC is distributed in the hope that it will be useful,
12 but WITHOUT ANY WARRANTY; without even the implied warranty of
13 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
14 GNU General Public License for more details.
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"
28 #include "basic-block.h"
29 #include "tree-flow.h"
33 /* Rewriting a function into SSA form can create a huge number of PHIs
34 many of which may be thrown away shortly after their creation if jumps
35 were threaded through PHI nodes.
37 While our garbage collection mechanisms will handle this situation, it
38 is extremely wasteful to create nodes and throw them away, especially
39 when the nodes can be reused.
41 For PR 8361, we can significantly reduce the number of nodes allocated
42 and thus the total amount of memory allocated by managing PHIs a
43 little. This additionally helps reduce the amount of work done by the
44 garbage collector. Similar results have been seen on a wider variety
45 of tests (such as the compiler itself).
47 Right now we maintain our free list on a per-function basis. It may
48 or may not make sense to maintain the free list for the duration of
51 We could also use a zone allocator for these objects since they have
52 a very well defined lifetime. If someone wants to experiment with that
53 this is the place to try it.
55 PHI nodes have different sizes, so we can't have a single list of all
56 the PHI nodes as it would be too expensive to walk down that list to
57 find a PHI of a suitable size.
59 Instead we have an array of lists of free PHI nodes. The array is
60 indexed by the number of PHI alternatives that PHI node can hold.
61 Except for the last array member, which holds all remaining PHI
64 So to find a free PHI node, we compute its index into the free PHI
65 node array and see if there are any elements with an exact match.
66 If so, then we are done. Otherwise, we test the next larger size
67 up and continue until we are in the last array element.
69 We do not actually walk members of the last array element. While it
70 might allow us to pick up a few reusable PHI nodes, it could potentially
71 be very expensive if the program has released a bunch of large PHI nodes,
72 but keeps asking for even larger PHI nodes. Experiments have shown that
73 walking the elements of the last array entry would result in finding less
74 than .1% additional reusable PHI nodes.
76 Note that we can never have less than two PHI argument slots. Thus,
77 the -2 on all the calculations below. */
79 #define NUM_BUCKETS 10
80 static GTY ((deletable (""))) VEC(gimple,gc) *free_phinodes[NUM_BUCKETS - 2];
81 static unsigned long free_phinode_count;
83 static int ideal_phi_node_len (int);
85 #ifdef GATHER_STATISTICS
86 unsigned int phi_nodes_reused;
87 unsigned int phi_nodes_created;
90 /* Initialize management of PHIs. */
97 for (i = 0; i < NUM_BUCKETS - 2; i++)
98 free_phinodes[i] = NULL;
99 free_phinode_count = 0;
102 /* Finalize management of PHIs. */
109 for (i = 0; i < NUM_BUCKETS - 2; i++)
110 free_phinodes[i] = NULL;
111 free_phinode_count = 0;
114 /* Dump some simple statistics regarding the re-use of PHI nodes. */
116 #ifdef GATHER_STATISTICS
118 phinodes_print_statistics (void)
120 fprintf (stderr, "PHI nodes allocated: %u\n", phi_nodes_created);
121 fprintf (stderr, "PHI nodes reused: %u\n", phi_nodes_reused);
125 /* Allocate a PHI node with at least LEN arguments. If the free list
126 happens to contain a PHI node with LEN arguments or more, return
130 allocate_phi_node (size_t len)
133 size_t bucket = NUM_BUCKETS - 2;
134 size_t size = sizeof (struct gimple_statement_phi)
135 + (len - 1) * sizeof (struct phi_arg_d);
137 if (free_phinode_count)
138 for (bucket = len - 2; bucket < NUM_BUCKETS - 2; bucket++)
139 if (free_phinodes[bucket])
142 /* If our free list has an element, then use it. */
143 if (bucket < NUM_BUCKETS - 2
144 && gimple_phi_capacity (VEC_index (gimple, free_phinodes[bucket], 0))
147 free_phinode_count--;
148 phi = VEC_pop (gimple, free_phinodes[bucket]);
149 if (VEC_empty (gimple, free_phinodes[bucket]))
150 VEC_free (gimple, gc, free_phinodes[bucket]);
151 #ifdef GATHER_STATISTICS
157 phi = (gimple) ggc_alloc (size);
158 #ifdef GATHER_STATISTICS
161 enum gimple_alloc_kind kind = gimple_alloc_kind (GIMPLE_PHI);
162 gimple_alloc_counts[(int) kind]++;
163 gimple_alloc_sizes[(int) kind] += size;
171 /* Given LEN, the original number of requested PHI arguments, return
172 a new, "ideal" length for the PHI node. The "ideal" length rounds
173 the total size of the PHI node up to the next power of two bytes.
175 Rounding up will not result in wasting any memory since the size request
176 will be rounded up by the GC system anyway. [ Note this is not entirely
177 true since the original length might have fit on one of the special
178 GC pages. ] By rounding up, we may avoid the need to reallocate the
179 PHI node later if we increase the number of arguments for the PHI. */
182 ideal_phi_node_len (int len)
184 size_t size, new_size;
187 /* We do not support allocations of less than two PHI argument slots. */
191 /* Compute the number of bytes of the original request. */
192 size = sizeof (struct gimple_statement_phi)
193 + (len - 1) * sizeof (struct phi_arg_d);
195 /* Round it up to the next power of two. */
196 log2 = ceil_log2 (size);
197 new_size = 1 << log2;
199 /* Now compute and return the number of PHI argument slots given an
200 ideal size allocation. */
201 new_len = len + (new_size - size) / sizeof (struct phi_arg_d);
206 /* Return a PHI node with LEN argument slots for variable VAR. */
209 make_phi_node (tree var, int len)
214 capacity = ideal_phi_node_len (len);
216 phi = allocate_phi_node (capacity);
218 /* We need to clear the entire PHI node, including the argument
219 portion, because we represent a "missing PHI argument" by placing
220 NULL_TREE in PHI_ARG_DEF. */
221 memset (phi, 0, (sizeof (struct gimple_statement_phi)
222 - sizeof (struct phi_arg_d)
223 + sizeof (struct phi_arg_d) * len));
224 phi->gsbase.code = GIMPLE_PHI;
225 phi->gimple_phi.nargs = len;
226 phi->gimple_phi.capacity = capacity;
227 if (TREE_CODE (var) == SSA_NAME)
228 gimple_phi_set_result (phi, var);
230 gimple_phi_set_result (phi, make_ssa_name (var, phi));
232 for (i = 0; i < capacity; i++)
235 imm = gimple_phi_arg_imm_use_ptr (phi, i);
236 imm->use = gimple_phi_arg_def_ptr (phi, i);
245 /* We no longer need PHI, release it so that it may be reused. */
248 release_phi_node (gimple phi)
251 size_t len = gimple_phi_capacity (phi);
254 for (x = 0; x < gimple_phi_num_args (phi); x++)
257 imm = gimple_phi_arg_imm_use_ptr (phi, x);
258 delink_imm_use (imm);
261 bucket = len > NUM_BUCKETS - 1 ? NUM_BUCKETS - 1 : len;
263 VEC_safe_push (gimple, gc, free_phinodes[bucket], phi);
264 free_phinode_count++;
268 /* Resize an existing PHI node. The only way is up. Return the
269 possibly relocated phi. */
272 resize_phi_node (gimple *phi, size_t len)
277 gcc_assert (len > gimple_phi_capacity (*phi));
279 /* The garbage collector will not look at the PHI node beyond the
280 first PHI_NUM_ARGS elements. Therefore, all we have to copy is a
281 portion of the PHI node currently in use. */
282 old_size = sizeof (struct gimple_statement_phi)
283 + (gimple_phi_num_args (*phi) - 1) * sizeof (struct phi_arg_d);
285 new_phi = allocate_phi_node (len);
287 memcpy (new_phi, *phi, old_size);
289 for (i = 0; i < gimple_phi_num_args (new_phi); i++)
291 use_operand_p imm, old_imm;
292 imm = gimple_phi_arg_imm_use_ptr (new_phi, i);
293 old_imm = gimple_phi_arg_imm_use_ptr (*phi, i);
294 imm->use = gimple_phi_arg_def_ptr (new_phi, i);
295 relink_imm_use_stmt (imm, old_imm, new_phi);
298 new_phi->gimple_phi.capacity = len;
300 for (i = gimple_phi_num_args (new_phi); i < len; i++)
303 imm = gimple_phi_arg_imm_use_ptr (new_phi, i);
304 imm->use = gimple_phi_arg_def_ptr (new_phi, i);
307 imm->loc.stmt = new_phi;
313 /* Reserve PHI arguments for a new edge to basic block BB. */
316 reserve_phi_args_for_new_edge (basic_block bb)
318 size_t len = EDGE_COUNT (bb->preds);
319 size_t cap = ideal_phi_node_len (len + 4);
320 gimple_stmt_iterator gsi;
322 for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); gsi_next (&gsi))
324 gimple *loc = gsi_stmt_ptr (&gsi);
326 if (len > gimple_phi_capacity (*loc))
328 gimple old_phi = *loc;
330 resize_phi_node (loc, cap);
332 /* The result of the PHI is defined by this PHI node. */
333 SSA_NAME_DEF_STMT (gimple_phi_result (*loc)) = *loc;
335 release_phi_node (old_phi);
338 /* We represent a "missing PHI argument" by placing NULL_TREE in
339 the corresponding slot. If PHI arguments were added
340 immediately after an edge is created, this zeroing would not
341 be necessary, but unfortunately this is not the case. For
342 example, the loop optimizer duplicates several basic blocks,
343 redirects edges, and then fixes up PHI arguments later in
345 SET_PHI_ARG_DEF (*loc, len - 1, NULL_TREE);
347 (*loc)->gimple_phi.nargs++;
352 /* Create a new PHI node for variable VAR at basic block BB. */
355 create_phi_node (tree var, basic_block bb)
357 gimple_stmt_iterator gsi;
358 gimple phi = make_phi_node (var, EDGE_COUNT (bb->preds));
360 /* Add the new PHI node to the list of PHI nodes for block BB. */
361 if (phi_nodes (bb) == NULL)
362 set_phi_nodes (bb, gimple_seq_alloc ());
364 gsi = gsi_last (phi_nodes (bb));
365 gsi_insert_after (&gsi, phi, GSI_NEW_STMT);
367 /* Associate BB to the PHI node. */
368 gimple_set_bb (phi, bb);
374 /* Add a new argument to PHI node PHI. DEF is the incoming reaching
375 definition and E is the edge through which DEF reaches PHI. The new
376 argument is added at the end of the argument list.
377 If PHI has reached its maximum capacity, add a few slots. In this case,
378 PHI points to the reallocated phi node when we return. */
381 add_phi_arg (gimple phi, tree def, edge e)
383 basic_block bb = e->dest;
385 gcc_assert (bb == gimple_bb (phi));
387 /* We resize PHI nodes upon edge creation. We should always have
388 enough room at this point. */
389 gcc_assert (gimple_phi_num_args (phi) <= gimple_phi_capacity (phi));
391 /* We resize PHI nodes upon edge creation. We should always have
392 enough room at this point. */
393 gcc_assert (e->dest_idx < gimple_phi_num_args (phi));
395 /* Copy propagation needs to know what object occur in abnormal
396 PHI nodes. This is a convenient place to record such information. */
397 if (e->flags & EDGE_ABNORMAL)
399 SSA_NAME_OCCURS_IN_ABNORMAL_PHI (def) = 1;
400 SSA_NAME_OCCURS_IN_ABNORMAL_PHI (PHI_RESULT (phi)) = 1;
403 SET_PHI_ARG_DEF (phi, e->dest_idx, def);
407 /* Remove the Ith argument from PHI's argument list. This routine
408 implements removal by swapping the last alternative with the
409 alternative we want to delete and then shrinking the vector, which
410 is consistent with how we remove an edge from the edge vector. */
413 remove_phi_arg_num (gimple phi, int i)
415 int num_elem = gimple_phi_num_args (phi);
417 gcc_assert (i < num_elem);
419 /* Delink the item which is being removed. */
420 delink_imm_use (gimple_phi_arg_imm_use_ptr (phi, i));
422 /* If it is not the last element, move the last element
423 to the element we want to delete, resetting all the links. */
424 if (i != num_elem - 1)
426 use_operand_p old_p, new_p;
427 old_p = gimple_phi_arg_imm_use_ptr (phi, num_elem - 1);
428 new_p = gimple_phi_arg_imm_use_ptr (phi, i);
429 /* Set use on new node, and link into last element's place. */
430 *(new_p->use) = *(old_p->use);
431 relink_imm_use (new_p, old_p);
434 /* Shrink the vector and return. Note that we do not have to clear
435 PHI_ARG_DEF because the garbage collector will not look at those
436 elements beyond the first PHI_NUM_ARGS elements of the array. */
437 phi->gimple_phi.nargs--;
441 /* Remove all PHI arguments associated with edge E. */
444 remove_phi_args (edge e)
446 gimple_stmt_iterator gsi;
448 for (gsi = gsi_start_phis (e->dest); !gsi_end_p (gsi); gsi_next (&gsi))
449 remove_phi_arg_num (gsi_stmt (gsi), e->dest_idx);
453 /* Remove the PHI node pointed-to by iterator GSI from basic block BB. After
454 removal, iterator GSI is updated to point to the next PHI node in the
455 sequence. If RELEASE_LHS_P is true, the LHS of this PHI node is released
456 into the free pool of SSA names. */
459 remove_phi_node (gimple_stmt_iterator *gsi, bool release_lhs_p)
461 gimple phi = gsi_stmt (*gsi);
462 gsi_remove (gsi, false);
464 /* If we are deleting the PHI node, then we should release the
465 SSA_NAME node so that it can be reused. */
466 release_phi_node (phi);
468 release_ssa_name (gimple_phi_result (phi));
471 #include "gt-tree-phinodes.h"