gcc/tree-ssanames.c

   1 /* Generic routines for manipulating SSA_NAME expressions
   2    Copyright (C) 2003, 2004, 2005, 2007, 2008, 2009
   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 #ifdef GATHER_STATISTICS
  65 unsigned int ssa_name_nodes_reused;
  66 unsigned int ssa_name_nodes_created;
  67 #endif
  68
  69 /* Initialize management of SSA_NAMEs to default SIZE.  If SIZE is
  70    zero use default.  */
  71
  72 void
  73 init_ssanames (struct function *fn, int size)
  74 {
  75   if (size < 50)
  76     size = 50;
  77
  78   SSANAMES (fn) = VEC_alloc (tree, gc, size);
  79
  80   /* Version 0 is special, so reserve the first slot in the table.  Though
  81      currently unused, we may use version 0 in alias analysis as part of
  82      the heuristics used to group aliases when the alias sets are too
  83      large.
  84
  85      We use VEC_quick_push here because we know that SSA_NAMES has at
  86      least 50 elements reserved in it.  */
  87   VEC_quick_push (tree, SSANAMES (fn), NULL_TREE);
  88   FREE_SSANAMES (fn) = NULL;
  89
  90   SYMS_TO_RENAME (fn) = BITMAP_GGC_ALLOC ();
  91 }
  92
  93 /* Finalize management of SSA_NAMEs.  */
  94
  95 void
  96 fini_ssanames (void)
  97 {
  98   VEC_free (tree, gc, SSANAMES (cfun));
  99   FREE_SSANAMES (cfun) = NULL;
 100 }
 101
 102 /* Dump some simple statistics regarding the re-use of SSA_NAME nodes.  */
 103
 104 #ifdef GATHER_STATISTICS
 105 void
 106 ssanames_print_statistics (void)
 107 {
 108   fprintf (stderr, "SSA_NAME nodes allocated: %u\n", ssa_name_nodes_created);
 109   fprintf (stderr, "SSA_NAME nodes reused: %u\n", ssa_name_nodes_reused);
 110 }
 111 #endif
 112
 113 /* Return an SSA_NAME node for variable VAR defined in statement STMT
 114    in function FN.  STMT may be an empty statement for artificial
 115    references (e.g., default definitions created when a variable is
 116    used without a preceding definition).  */
 117
 118 tree
 119 make_ssa_name_fn (struct function *fn, tree var, gimple stmt)
 120 {
 121   tree t;
 122   use_operand_p imm;
 123
 124   gcc_assert (DECL_P (var));
 125
 126   /* If our free list has an element, then use it.  */
 127   if (FREE_SSANAMES (fn))
 128     {
 129       t = FREE_SSANAMES (fn);
 130       FREE_SSANAMES (fn) = TREE_CHAIN (FREE_SSANAMES (fn));
 131 #ifdef GATHER_STATISTICS
 132       ssa_name_nodes_reused++;
 133 #endif
 134
 135       /* The node was cleared out when we put it on the free list, so
 136          there is no need to do so again here.  */
 137       gcc_assert (ssa_name (SSA_NAME_VERSION (t)) == NULL);
 138       VEC_replace (tree, SSANAMES (fn), SSA_NAME_VERSION (t), t);
 139     }
 140   else
 141     {
 142       t = make_node (SSA_NAME);
 143       SSA_NAME_VERSION (t) = VEC_length (tree, SSANAMES (fn));
 144       VEC_safe_push (tree, gc, SSANAMES (fn), t);
 145 #ifdef GATHER_STATISTICS
 146       ssa_name_nodes_created++;
 147 #endif
 148     }
 149
 150   TREE_TYPE (t) = TREE_TYPE (var);
 151   SSA_NAME_VAR (t) = var;
 152   SSA_NAME_DEF_STMT (t) = stmt;
 153   SSA_NAME_PTR_INFO (t) = NULL;
 154   SSA_NAME_IN_FREE_LIST (t) = 0;
 155   SSA_NAME_IS_DEFAULT_DEF (t) = 0;
 156   imm = &(SSA_NAME_IMM_USE_NODE (t));
 157   imm->use = NULL;
 158   imm->prev = imm;
 159   imm->next = imm;
 160   imm->loc.ssa_name = t;
 161
 162   return t;
 163 }
 164
 165
 166 /* We no longer need the SSA_NAME expression VAR, release it so that
 167    it may be reused.
 168
 169    Note it is assumed that no calls to make_ssa_name will be made
 170    until all uses of the ssa name are released and that the only
 171    use of the SSA_NAME expression is to check its SSA_NAME_VAR.  All
 172    other fields must be assumed clobbered.  */
 173
 174 void
 175 release_ssa_name (tree var)
 176 {
 177   if (!var)
 178     return;
 179
 180   /* Never release the default definition for a symbol.  It's a
 181      special SSA name that should always exist once it's created.  */
 182   if (SSA_NAME_IS_DEFAULT_DEF (var))
 183     return;
 184
 185   /* If VAR has been registered for SSA updating, don't remove it.
 186      After update_ssa has run, the name will be released.  */
 187   if (name_registered_for_update_p (var))
 188     {
 189       release_ssa_name_after_update_ssa (var);
 190       return;
 191     }
 192
 193   /* release_ssa_name can be called multiple times on a single SSA_NAME.
 194      However, it should only end up on our free list one time.   We
 195      keep a status bit in the SSA_NAME node itself to indicate it has
 196      been put on the free list.
 197
 198      Note that once on the freelist you can not reference the SSA_NAME's
 199      defining statement.  */
 200   if (! SSA_NAME_IN_FREE_LIST (var))
 201     {
 202       tree saved_ssa_name_var = SSA_NAME_VAR (var);
 203       int saved_ssa_name_version = SSA_NAME_VERSION (var);
 204       use_operand_p imm = &(SSA_NAME_IMM_USE_NODE (var));
 205
 206       if (MAY_HAVE_DEBUG_STMTS)
 207         insert_debug_temp_for_var_def (NULL, var);
 208
 209 #ifdef ENABLE_CHECKING
 210       verify_imm_links (stderr, var);
 211 #endif
 212       while (imm->next != imm)
 213         delink_imm_use (imm->next);
 214
 215       VEC_replace (tree, SSANAMES (cfun),
 216                    SSA_NAME_VERSION (var), NULL_TREE);
 217       memset (var, 0, tree_size (var));
 218
 219       imm->prev = imm;
 220       imm->next = imm;
 221       imm->loc.ssa_name = var;
 222
 223       /* First put back the right tree node so that the tree checking
 224          macros do not complain.  */
 225       TREE_SET_CODE (var, SSA_NAME);
 226
 227       /* Restore the version number.  */
 228       SSA_NAME_VERSION (var) = saved_ssa_name_version;
 229
 230       /* Hopefully this can go away once we have the new incremental
 231          SSA updating code installed.  */
 232       SSA_NAME_VAR (var) = saved_ssa_name_var;
 233
 234       /* Note this SSA_NAME is now in the first list.  */
 235       SSA_NAME_IN_FREE_LIST (var) = 1;
 236
 237       /* And finally link it into the free list.  */
 238       TREE_CHAIN (var) = FREE_SSANAMES (cfun);
 239       FREE_SSANAMES (cfun) = var;
 240     }
 241 }
 242
 243
 244 /* Return the alias information associated with pointer T.  It creates a
 245    new instance if none existed.  */
 246
 247 struct ptr_info_def *
 248 get_ptr_info (tree t)
 249 {
 250   struct ptr_info_def *pi;
 251
 252   gcc_assert (POINTER_TYPE_P (TREE_TYPE (t)));
 253
 254   pi = SSA_NAME_PTR_INFO (t);
 255   if (pi == NULL)
 256     {
 257       pi = ggc_alloc_cleared_ptr_info_def ();
 258       pt_solution_reset (&pi->pt);
 259       pi->align = 1;
 260       pi->misalign = 0;
 261       SSA_NAME_PTR_INFO (t) = pi;
 262     }
 263
 264   return pi;
 265 }
 266
 267 /* Creates a duplicate of the ptr_info_def at PTR_INFO for use by
 268    the SSA name NAME.  */
 269
 270 void
 271 duplicate_ssa_name_ptr_info (tree name, struct ptr_info_def *ptr_info)
 272 {
 273   struct ptr_info_def *new_ptr_info;
 274
 275   gcc_assert (POINTER_TYPE_P (TREE_TYPE (name)));
 276   gcc_assert (!SSA_NAME_PTR_INFO (name));
 277
 278   if (!ptr_info)
 279     return;
 280
 281   new_ptr_info = ggc_alloc_ptr_info_def ();
 282   *new_ptr_info = *ptr_info;
 283
 284   SSA_NAME_PTR_INFO (name) = new_ptr_info;
 285 }
 286
 287
 288 /* Creates a duplicate of a ssa name NAME tobe defined by statement STMT.  */
 289
 290 tree
 291 duplicate_ssa_name (tree name, gimple stmt)
 292 {
 293   tree new_name = make_ssa_name (SSA_NAME_VAR (name), stmt);
 294   struct ptr_info_def *old_ptr_info = SSA_NAME_PTR_INFO (name);
 295
 296   if (old_ptr_info)
 297     duplicate_ssa_name_ptr_info (new_name, old_ptr_info);
 298
 299   return new_name;
 300 }
 301
 302
 303 /* Release all the SSA_NAMEs created by STMT.  */
 304
 305 void
 306 release_defs (gimple stmt)
 307 {
 308   tree def;
 309   ssa_op_iter iter;
 310
 311   /* Make sure that we are in SSA.  Otherwise, operand cache may point
 312      to garbage.  */
 313   gcc_assert (gimple_in_ssa_p (cfun));
 314
 315   FOR_EACH_SSA_TREE_OPERAND (def, stmt, iter, SSA_OP_ALL_DEFS)
 316     if (TREE_CODE (def) == SSA_NAME)
 317       release_ssa_name (def);
 318 }
 319
 320
 321 /* Replace the symbol associated with SSA_NAME with SYM.  */
 322
 323 void
 324 replace_ssa_name_symbol (tree ssa_name, tree sym)
 325 {
 326   SSA_NAME_VAR (ssa_name) = sym;
 327   TREE_TYPE (ssa_name) = TREE_TYPE (sym);
 328 }
 329
 330 /* Return SSA names that are unused to GGC memory.  This is used to keep
 331    footprint of compiler during interprocedural optimization.
 332    As a side effect the SSA_NAME_VERSION number reuse is reduced
 333    so this function should not be used too often.  */
 334 static unsigned int
 335 release_dead_ssa_names (void)
 336 {
 337   tree t, next;
 338   int n = 0;
 339   referenced_var_iterator rvi;
 340
 341   /* Current defs point to various dead SSA names that in turn point to
 342      eventually dead variables so a bunch of memory is held live.  */
 343   FOR_EACH_REFERENCED_VAR (t, rvi)
 344     set_current_def (t, NULL);
 345   /* Now release the freelist.  */
 346   for (t = FREE_SSANAMES (cfun); t; t = next)
 347     {
 348       next = TREE_CHAIN (t);
 349       /* Dangling pointers might make GGC to still see dead SSA names, so it is
 350          important to unlink the list and avoid GGC from seeing all subsequent
 351          SSA names.  In longer run we want to have all dangling pointers here
 352          removed (since they usually go through dead statements that consume
 353          considerable amounts of memory).  */
 354       TREE_CHAIN (t) = NULL_TREE;
 355       n++;
 356     }
 357   FREE_SSANAMES (cfun) = NULL;
 358
 359   statistics_counter_event (cfun, "SSA names released", n);
 360   if (dump_file)
 361     fprintf (dump_file, "Released %i names, %.2f%%\n",
 362              n, n * 100.0 / num_ssa_names);
 363   return 0;
 364 }
 365
 366 struct gimple_opt_pass pass_release_ssa_names =
 367 {
 368  {
 369   GIMPLE_PASS,
 370   "release_ssa",                        /* name */
 371   NULL,                                 /* gate */
 372   release_dead_ssa_names,               /* execute */
 373   NULL,                                 /* sub */
 374   NULL,                                 /* next */
 375   0,                                    /* static_pass_number */
 376   TV_NONE,                              /* tv_id */
 377   PROP_ssa,                             /* properties_required */
 378   0,                                    /* properties_provided */
 379   0,                                    /* properties_destroyed */
 380   0,                                    /* todo_flags_start */
 381   TODO_dump_func                        /* todo_flags_finish */
 382  }
 383 };