gcc/tree-ssanames.c

   1 /* Generic routines for manipulating SSA_NAME expressions
   2    Copyright (C) 2003, 2004, 2005, 2007, 2008, 2009, 2010
   3    Free Software Foundation, Inc.
   4
   5 This file is part of GCC.
   6
   7 GCC is free software; you can redistribute it and/or modify
   8 it under the terms of the GNU General Public License as published by
   9 the Free Software Foundation; either version 3, or (at your option)
  10 any later version.
  11
  12 GCC is distributed in the hope that it will be useful,
  13 but WITHOUT ANY WARRANTY; without even the implied warranty of
  14 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  15 GNU General Public License for more details.
  16
  17 You should have received a copy of the GNU General Public License
  18 along with GCC; see the file COPYING3.  If not see
  19 <http://www.gnu.org/licenses/>.  */
  20
  21 #include "config.h"
  22 #include "system.h"
  23 #include "coretypes.h"
  24 #include "tm.h"
  25 #include "tree.h"
  26 #include "tree-flow.h"
  27 #include "tree-pass.h"
  28
  29 /* Rewriting a function into SSA form can create a huge number of SSA_NAMEs,
  30    many of which may be thrown away shortly after their creation if jumps
  31    were threaded through PHI nodes.
  32
  33    While our garbage collection mechanisms will handle this situation, it
  34    is extremely wasteful to create nodes and throw them away, especially
  35    when the nodes can be reused.
  36
  37    For PR 8361, we can significantly reduce the number of nodes allocated
  38    and thus the total amount of memory allocated by managing SSA_NAMEs a
  39    little.  This additionally helps reduce the amount of work done by the
  40    garbage collector.  Similar results have been seen on a wider variety
  41    of tests (such as the compiler itself).
  42
  43    Right now we maintain our free list on a per-function basis.  It may
  44    or may not make sense to maintain the free list for the duration of
  45    a compilation unit.
  46
  47    External code should rely solely upon HIGHEST_SSA_VERSION and the
  48    externally defined functions.  External code should not know about
  49    the details of the free list management.
  50
  51    External code should also not assume the version number on nodes is
  52    monotonically increasing.  We reuse the version number when we
  53    reuse an SSA_NAME expression.  This helps keep arrays and bitmaps
  54    more compact.
  55
  56    We could also use a zone allocator for these objects since they have
  57    a very well defined lifetime.  If someone wants to experiment with that
  58    this is the place to try it.  */
  59
  60 /* Version numbers with special meanings.  We start allocating new version
  61    numbers after the special ones.  */
  62 #define UNUSED_NAME_VERSION 0
  63
  64 unsigned int ssa_name_nodes_reused;
  65 unsigned int ssa_name_nodes_created;
  66
  67 /* Initialize management of SSA_NAMEs to default SIZE.  If SIZE is
  68    zero use default.  */
  69
  70 void
  71 init_ssanames (struct function *fn, int size)
  72 {
  73   if (size < 50)
  74     size = 50;
  75
  76   SSANAMES (fn) = VEC_alloc (tree, gc, size);
  77
  78   /* Version 0 is special, so reserve the first slot in the table.  Though
  79      currently unused, we may use version 0 in alias analysis as part of
  80      the heuristics used to group aliases when the alias sets are too
  81      large.
  82
  83      We use VEC_quick_push here because we know that SSA_NAMES has at
  84      least 50 elements reserved in it.  */
  85   VEC_quick_push (tree, SSANAMES (fn), NULL_TREE);
  86   FREE_SSANAMES (fn) = NULL;
  87
  88   SYMS_TO_RENAME (fn) = BITMAP_GGC_ALLOC ();
  89 }
  90
  91 /* Finalize management of SSA_NAMEs.  */
  92
  93 void
  94 fini_ssanames (void)
  95 {
  96   VEC_free (tree, gc, SSANAMES (cfun));
  97   VEC_free (tree, gc, FREE_SSANAMES (cfun));
  98 }
  99
 100 /* Dump some simple statistics regarding the re-use of SSA_NAME nodes.  */
 101
 102 void
 103 ssanames_print_statistics (void)
 104 {
 105   fprintf (stderr, "SSA_NAME nodes allocated: %u\n", ssa_name_nodes_created);
 106   fprintf (stderr, "SSA_NAME nodes reused: %u\n", ssa_name_nodes_reused);
 107 }
 108
 109 /* Return an SSA_NAME node for variable VAR defined in statement STMT
 110    in function FN.  STMT may be an empty statement for artificial
 111    references (e.g., default definitions created when a variable is
 112    used without a preceding definition).  */
 113
 114 tree
 115 make_ssa_name_fn (struct function *fn, tree var, gimple stmt)
 116 {
 117   tree t;
 118   use_operand_p imm;
 119
 120   gcc_assert (DECL_P (var));
 121
 122   /* If our free list has an element, then use it.  */
 123   if (!VEC_empty (tree, FREE_SSANAMES (fn)))
 124     {
 125       t = VEC_pop (tree, FREE_SSANAMES (fn));
 126       if (GATHER_STATISTICS)
 127         ssa_name_nodes_reused++;
 128
 129       /* The node was cleared out when we put it on the free list, so
 130          there is no need to do so again here.  */
 131       gcc_assert (ssa_name (SSA_NAME_VERSION (t)) == NULL);
 132       VEC_replace (tree, SSANAMES (fn), SSA_NAME_VERSION (t), t);
 133     }
 134   else
 135     {
 136       t = make_node (SSA_NAME);
 137       SSA_NAME_VERSION (t) = VEC_length (tree, SSANAMES (fn));
 138       VEC_safe_push (tree, gc, SSANAMES (fn), t);
 139       if (GATHER_STATISTICS)
 140         ssa_name_nodes_created++;
 141     }
 142
 143   TREE_TYPE (t) = TREE_TYPE (var);
 144   SSA_NAME_VAR (t) = var;
 145   SSA_NAME_DEF_STMT (t) = stmt;
 146   SSA_NAME_PTR_INFO (t) = NULL;
 147   SSA_NAME_IN_FREE_LIST (t) = 0;
 148   SSA_NAME_IS_DEFAULT_DEF (t) = 0;
 149   imm = &(SSA_NAME_IMM_USE_NODE (t));
 150   imm->use = NULL;
 151   imm->prev = imm;
 152   imm->next = imm;
 153   imm->loc.ssa_name = t;
 154
 155   return t;
 156 }
 157
 158
 159 /* We no longer need the SSA_NAME expression VAR, release it so that
 160    it may be reused.
 161
 162    Note it is assumed that no calls to make_ssa_name will be made
 163    until all uses of the ssa name are released and that the only
 164    use of the SSA_NAME expression is to check its SSA_NAME_VAR.  All
 165    other fields must be assumed clobbered.  */
 166
 167 void
 168 release_ssa_name (tree var)
 169 {
 170   if (!var)
 171     return;
 172
 173   /* Never release the default definition for a symbol.  It's a
 174      special SSA name that should always exist once it's created.  */
 175   if (SSA_NAME_IS_DEFAULT_DEF (var))
 176     return;
 177
 178   /* If VAR has been registered for SSA updating, don't remove it.
 179      After update_ssa has run, the name will be released.  */
 180   if (name_registered_for_update_p (var))
 181     {
 182       release_ssa_name_after_update_ssa (var);
 183       return;
 184     }
 185
 186   /* release_ssa_name can be called multiple times on a single SSA_NAME.
 187      However, it should only end up on our free list one time.   We
 188      keep a status bit in the SSA_NAME node itself to indicate it has
 189      been put on the free list.
 190
 191      Note that once on the freelist you can not reference the SSA_NAME's
 192      defining statement.  */
 193   if (! SSA_NAME_IN_FREE_LIST (var))
 194     {
 195       tree saved_ssa_name_var = SSA_NAME_VAR (var);
 196       int saved_ssa_name_version = SSA_NAME_VERSION (var);
 197       use_operand_p imm = &(SSA_NAME_IMM_USE_NODE (var));
 198
 199       if (MAY_HAVE_DEBUG_STMTS)
 200         insert_debug_temp_for_var_def (NULL, var);
 201
 202 #ifdef ENABLE_CHECKING
 203       verify_imm_links (stderr, var);
 204 #endif
 205       while (imm->next != imm)
 206         delink_imm_use (imm->next);
 207
 208       VEC_replace (tree, SSANAMES (cfun),
 209                    SSA_NAME_VERSION (var), NULL_TREE);
 210       memset (var, 0, tree_size (var));
 211
 212       imm->prev = imm;
 213       imm->next = imm;
 214       imm->loc.ssa_name = var;
 215
 216       /* First put back the right tree node so that the tree checking
 217          macros do not complain.  */
 218       TREE_SET_CODE (var, SSA_NAME);
 219
 220       /* Restore the version number.  */
 221       SSA_NAME_VERSION (var) = saved_ssa_name_version;
 222
 223       /* Hopefully this can go away once we have the new incremental
 224          SSA updating code installed.  */
 225       SSA_NAME_VAR (var) = saved_ssa_name_var;
 226
 227       /* Note this SSA_NAME is now in the first list.  */
 228       SSA_NAME_IN_FREE_LIST (var) = 1;
 229
 230       /* And finally put it on the free list.  */
 231       VEC_safe_push (tree, gc, FREE_SSANAMES (cfun), var);
 232     }
 233 }
 234
 235 /* If the alignment of the pointer described by PI is known, return true and
 236    store the alignment and the deviation from it into *ALIGNP and *MISALIGNP
 237    respectively.  Otherwise return false.  */
 238
 239 bool
 240 get_ptr_info_alignment (struct ptr_info_def *pi, unsigned int *alignp,
 241                         unsigned int *misalignp)
 242 {
 243   if (pi->align)
 244     {
 245       *alignp = pi->align;
 246       *misalignp = pi->misalign;
 247       return true;
 248     }
 249   else
 250     return false;
 251 }
 252
 253 /* State that the pointer described by PI has unknown alignment.  */
 254
 255 void
 256 mark_ptr_info_alignment_unknown (struct ptr_info_def *pi)
 257 {
 258   pi->align = 0;
 259   pi->misalign = 0;
 260 }
 261
 262 /* Store the the power-of-two byte alignment and the deviation from that
 263    alignment of pointer described by PI to ALIOGN and MISALIGN
 264    respectively.  */
 265
 266 void
 267 set_ptr_info_alignment (struct ptr_info_def *pi, unsigned int align,
 268                             unsigned int misalign)
 269 {
 270   gcc_checking_assert (align != 0);
 271   gcc_assert ((align & (align - 1)) == 0);
 272   gcc_assert ((misalign & ~(align - 1)) == 0);
 273
 274   pi->align = align;
 275   pi->misalign = misalign;
 276 }
 277
 278 /* If pointer described by PI has known alignment, increase its known
 279    misalignment by INCREMENT modulo its current alignment.  */
 280
 281 void
 282 adjust_ptr_info_misalignment (struct ptr_info_def *pi,
 283                               unsigned int increment)
 284 {
 285   if (pi->align != 0)
 286     {
 287       pi->misalign += increment;
 288       pi->misalign &= (pi->align - 1);
 289     }
 290 }
 291
 292 /* Return the alias information associated with pointer T.  It creates a
 293    new instance if none existed.  */
 294
 295 struct ptr_info_def *
 296 get_ptr_info (tree t)
 297 {
 298   struct ptr_info_def *pi;
 299
 300   gcc_assert (POINTER_TYPE_P (TREE_TYPE (t)));
 301
 302   pi = SSA_NAME_PTR_INFO (t);
 303   if (pi == NULL)
 304     {
 305       pi = ggc_alloc_cleared_ptr_info_def ();
 306       pt_solution_reset (&pi->pt);
 307       mark_ptr_info_alignment_unknown (pi);
 308       SSA_NAME_PTR_INFO (t) = pi;
 309     }
 310
 311   return pi;
 312 }
 313
 314 /* Creates a duplicate of the ptr_info_def at PTR_INFO for use by
 315    the SSA name NAME.  */
 316
 317 void
 318 duplicate_ssa_name_ptr_info (tree name, struct ptr_info_def *ptr_info)
 319 {
 320   struct ptr_info_def *new_ptr_info;
 321
 322   gcc_assert (POINTER_TYPE_P (TREE_TYPE (name)));
 323   gcc_assert (!SSA_NAME_PTR_INFO (name));
 324
 325   if (!ptr_info)
 326     return;
 327
 328   new_ptr_info = ggc_alloc_ptr_info_def ();
 329   *new_ptr_info = *ptr_info;
 330
 331   SSA_NAME_PTR_INFO (name) = new_ptr_info;
 332 }
 333
 334
 335 /* Creates a duplicate of a ssa name NAME tobe defined by statement STMT.  */
 336
 337 tree
 338 duplicate_ssa_name (tree name, gimple stmt)
 339 {
 340   tree new_name = make_ssa_name (SSA_NAME_VAR (name), stmt);
 341   struct ptr_info_def *old_ptr_info = SSA_NAME_PTR_INFO (name);
 342
 343   if (old_ptr_info)
 344     duplicate_ssa_name_ptr_info (new_name, old_ptr_info);
 345
 346   return new_name;
 347 }
 348
 349
 350 /* Release all the SSA_NAMEs created by STMT.  */
 351
 352 void
 353 release_defs (gimple stmt)
 354 {
 355   tree def;
 356   ssa_op_iter iter;
 357
 358   /* Make sure that we are in SSA.  Otherwise, operand cache may point
 359      to garbage.  */
 360   gcc_assert (gimple_in_ssa_p (cfun));
 361
 362   FOR_EACH_SSA_TREE_OPERAND (def, stmt, iter, SSA_OP_ALL_DEFS)
 363     if (TREE_CODE (def) == SSA_NAME)
 364       release_ssa_name (def);
 365 }
 366
 367
 368 /* Replace the symbol associated with SSA_NAME with SYM.  */
 369
 370 void
 371 replace_ssa_name_symbol (tree ssa_name, tree sym)
 372 {
 373   SSA_NAME_VAR (ssa_name) = sym;
 374   TREE_TYPE (ssa_name) = TREE_TYPE (sym);
 375 }
 376
 377 /* Return SSA names that are unused to GGC memory and compact the SSA
 378    version namespace.  This is used to keep footprint of compiler during
 379    interprocedural optimization.  */
 380 static unsigned int
 381 release_dead_ssa_names (void)
 382 {
 383   tree t;
 384   unsigned i, j;
 385   int n = VEC_length (tree, FREE_SSANAMES (cfun));
 386   referenced_var_iterator rvi;
 387
 388   /* Current defs point to various dead SSA names that in turn point to
 389      eventually dead variables so a bunch of memory is held live.  */
 390   FOR_EACH_REFERENCED_VAR (cfun, t, rvi)
 391     set_current_def (t, NULL);
 392
 393   /* Now release the freelist.  */
 394   VEC_free (tree, gc, FREE_SSANAMES (cfun));
 395   FREE_SSANAMES (cfun) = NULL;
 396
 397   /* And compact the SSA number space.  We make sure to not change the
 398      relative order of SSA versions.  */
 399   for (i = 1, j = 1; i < VEC_length (tree, cfun->gimple_df->ssa_names); ++i)
 400     {
 401       tree name = ssa_name (i);
 402       if (name)
 403         {
 404           if (i != j)
 405             {
 406               SSA_NAME_VERSION (name) = j;
 407               VEC_replace (tree, cfun->gimple_df->ssa_names, j, name);
 408             }
 409           j++;
 410         }
 411     }
 412   VEC_truncate (tree, cfun->gimple_df->ssa_names, j);
 413
 414   statistics_counter_event (cfun, "SSA names released", n);
 415   statistics_counter_event (cfun, "SSA name holes removed", i - j);
 416   if (dump_file)
 417     fprintf (dump_file, "Released %i names, %.2f%%, removed %i holes\n",
 418              n, n * 100.0 / num_ssa_names, i - j);
 419   return 0;
 420 }
 421
 422 struct gimple_opt_pass pass_release_ssa_names =
 423 {
 424  {
 425   GIMPLE_PASS,
 426   "release_ssa",                        /* name */
 427   NULL,                                 /* gate */
 428   release_dead_ssa_names,               /* execute */
 429   NULL,                                 /* sub */
 430   NULL,                                 /* next */
 431   0,                                    /* static_pass_number */
 432   TV_TREE_SSA_OTHER,                    /* tv_id */
 433   PROP_ssa,                             /* properties_required */
 434   0,                                    /* properties_provided */
 435   0,                                    /* properties_destroyed */
 436   0,                                    /* todo_flags_start */
 437   0                                     /* todo_flags_finish */
 438  }
 439 };