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 #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   VEC_free (tree, gc, FREE_SSANAMES (cfun));
 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 (!VEC_empty (tree, FREE_SSANAMES (fn)))
 128     {
 129       t = VEC_pop (tree, FREE_SSANAMES (fn));
 130 #ifdef GATHER_STATISTICS
 131       ssa_name_nodes_reused++;
 132 #endif
 133
 134       /* The node was cleared out when we put it on the free list, so
 135          there is no need to do so again here.  */
 136       gcc_assert (ssa_name (SSA_NAME_VERSION (t)) == NULL);
 137       VEC_replace (tree, SSANAMES (fn), SSA_NAME_VERSION (t), t);
 138     }
 139   else
 140     {
 141       t = make_node (SSA_NAME);
 142       SSA_NAME_VERSION (t) = VEC_length (tree, SSANAMES (fn));
 143       VEC_safe_push (tree, gc, SSANAMES (fn), t);
 144 #ifdef GATHER_STATISTICS
 145       ssa_name_nodes_created++;
 146 #endif
 147     }
 148
 149   TREE_TYPE (t) = TREE_TYPE (var);
 150   SSA_NAME_VAR (t) = var;
 151   SSA_NAME_DEF_STMT (t) = stmt;
 152   SSA_NAME_PTR_INFO (t) = NULL;
 153   SSA_NAME_IN_FREE_LIST (t) = 0;
 154   SSA_NAME_IS_DEFAULT_DEF (t) = 0;
 155   imm = &(SSA_NAME_IMM_USE_NODE (t));
 156   imm->use = NULL;
 157   imm->prev = imm;
 158   imm->next = imm;
 159   imm->loc.ssa_name = t;
 160
 161   return t;
 162 }
 163
 164
 165 /* We no longer need the SSA_NAME expression VAR, release it so that
 166    it may be reused.
 167
 168    Note it is assumed that no calls to make_ssa_name will be made
 169    until all uses of the ssa name are released and that the only
 170    use of the SSA_NAME expression is to check its SSA_NAME_VAR.  All
 171    other fields must be assumed clobbered.  */
 172
 173 void
 174 release_ssa_name (tree var)
 175 {
 176   if (!var)
 177     return;
 178
 179   /* Never release the default definition for a symbol.  It's a
 180      special SSA name that should always exist once it's created.  */
 181   if (SSA_NAME_IS_DEFAULT_DEF (var))
 182     return;
 183
 184   /* If VAR has been registered for SSA updating, don't remove it.
 185      After update_ssa has run, the name will be released.  */
 186   if (name_registered_for_update_p (var))
 187     {
 188       release_ssa_name_after_update_ssa (var);
 189       return;
 190     }
 191
 192   /* release_ssa_name can be called multiple times on a single SSA_NAME.
 193      However, it should only end up on our free list one time.   We
 194      keep a status bit in the SSA_NAME node itself to indicate it has
 195      been put on the free list.
 196
 197      Note that once on the freelist you can not reference the SSA_NAME's
 198      defining statement.  */
 199   if (! SSA_NAME_IN_FREE_LIST (var))
 200     {
 201       tree saved_ssa_name_var = SSA_NAME_VAR (var);
 202       int saved_ssa_name_version = SSA_NAME_VERSION (var);
 203       use_operand_p imm = &(SSA_NAME_IMM_USE_NODE (var));
 204
 205       if (MAY_HAVE_DEBUG_STMTS)
 206         insert_debug_temp_for_var_def (NULL, var);
 207
 208 #ifdef ENABLE_CHECKING
 209       verify_imm_links (stderr, var);
 210 #endif
 211       while (imm->next != imm)
 212         delink_imm_use (imm->next);
 213
 214       VEC_replace (tree, SSANAMES (cfun),
 215                    SSA_NAME_VERSION (var), NULL_TREE);
 216       memset (var, 0, tree_size (var));
 217
 218       imm->prev = imm;
 219       imm->next = imm;
 220       imm->loc.ssa_name = var;
 221
 222       /* First put back the right tree node so that the tree checking
 223          macros do not complain.  */
 224       TREE_SET_CODE (var, SSA_NAME);
 225
 226       /* Restore the version number.  */
 227       SSA_NAME_VERSION (var) = saved_ssa_name_version;
 228
 229       /* Hopefully this can go away once we have the new incremental
 230          SSA updating code installed.  */
 231       SSA_NAME_VAR (var) = saved_ssa_name_var;
 232
 233       /* Note this SSA_NAME is now in the first list.  */
 234       SSA_NAME_IN_FREE_LIST (var) = 1;
 235
 236       /* And finally put it on the free list.  */
 237       VEC_safe_push (tree, gc, FREE_SSANAMES (cfun), var);
 238     }
 239 }
 240
 241
 242 /* Return the alias information associated with pointer T.  It creates a
 243    new instance if none existed.  */
 244
 245 struct ptr_info_def *
 246 get_ptr_info (tree t)
 247 {
 248   struct ptr_info_def *pi;
 249
 250   gcc_assert (POINTER_TYPE_P (TREE_TYPE (t)));
 251
 252   pi = SSA_NAME_PTR_INFO (t);
 253   if (pi == NULL)
 254     {
 255       pi = ggc_alloc_cleared_ptr_info_def ();
 256       pt_solution_reset (&pi->pt);
 257       pi->align = 1;
 258       pi->misalign = 0;
 259       SSA_NAME_PTR_INFO (t) = pi;
 260     }
 261
 262   return pi;
 263 }
 264
 265 /* Creates a duplicate of the ptr_info_def at PTR_INFO for use by
 266    the SSA name NAME.  */
 267
 268 void
 269 duplicate_ssa_name_ptr_info (tree name, struct ptr_info_def *ptr_info)
 270 {
 271   struct ptr_info_def *new_ptr_info;
 272
 273   gcc_assert (POINTER_TYPE_P (TREE_TYPE (name)));
 274   gcc_assert (!SSA_NAME_PTR_INFO (name));
 275
 276   if (!ptr_info)
 277     return;
 278
 279   new_ptr_info = ggc_alloc_ptr_info_def ();
 280   *new_ptr_info = *ptr_info;
 281
 282   SSA_NAME_PTR_INFO (name) = new_ptr_info;
 283 }
 284
 285
 286 /* Creates a duplicate of a ssa name NAME tobe defined by statement STMT.  */
 287
 288 tree
 289 duplicate_ssa_name (tree name, gimple stmt)
 290 {
 291   tree new_name = make_ssa_name (SSA_NAME_VAR (name), stmt);
 292   struct ptr_info_def *old_ptr_info = SSA_NAME_PTR_INFO (name);
 293
 294   if (old_ptr_info)
 295     duplicate_ssa_name_ptr_info (new_name, old_ptr_info);
 296
 297   return new_name;
 298 }
 299
 300
 301 /* Release all the SSA_NAMEs created by STMT.  */
 302
 303 void
 304 release_defs (gimple stmt)
 305 {
 306   tree def;
 307   ssa_op_iter iter;
 308
 309   /* Make sure that we are in SSA.  Otherwise, operand cache may point
 310      to garbage.  */
 311   gcc_assert (gimple_in_ssa_p (cfun));
 312
 313   FOR_EACH_SSA_TREE_OPERAND (def, stmt, iter, SSA_OP_ALL_DEFS)
 314     if (TREE_CODE (def) == SSA_NAME)
 315       release_ssa_name (def);
 316 }
 317
 318
 319 /* Replace the symbol associated with SSA_NAME with SYM.  */
 320
 321 void
 322 replace_ssa_name_symbol (tree ssa_name, tree sym)
 323 {
 324   SSA_NAME_VAR (ssa_name) = sym;
 325   TREE_TYPE (ssa_name) = TREE_TYPE (sym);
 326 }
 327
 328 /* Return SSA names that are unused to GGC memory.  This is used to keep
 329    footprint of compiler during interprocedural optimization.
 330    As a side effect the SSA_NAME_VERSION number reuse is reduced
 331    so this function should not be used too often.  */
 332 static unsigned int
 333 release_dead_ssa_names (void)
 334 {
 335   tree t;
 336   int n = VEC_length (tree, FREE_SSANAMES (cfun));
 337   referenced_var_iterator rvi;
 338
 339   /* Current defs point to various dead SSA names that in turn point to
 340      eventually dead variables so a bunch of memory is held live.  */
 341   FOR_EACH_REFERENCED_VAR (cfun, t, rvi)
 342     set_current_def (t, NULL);
 343   /* Now release the freelist.  */
 344   VEC_free (tree, gc, FREE_SSANAMES (cfun));
 345   FREE_SSANAMES (cfun) = NULL;
 346
 347   statistics_counter_event (cfun, "SSA names released", n);
 348   if (dump_file)
 349     fprintf (dump_file, "Released %i names, %.2f%%\n",
 350              n, n * 100.0 / num_ssa_names);
 351   return 0;
 352 }
 353
 354 struct gimple_opt_pass pass_release_ssa_names =
 355 {
 356  {
 357   GIMPLE_PASS,
 358   "release_ssa",                        /* name */
 359   NULL,                                 /* gate */
 360   release_dead_ssa_names,               /* execute */
 361   NULL,                                 /* sub */
 362   NULL,                                 /* next */
 363   0,                                    /* static_pass_number */
 364   TV_TREE_SSA_OTHER,                    /* tv_id */
 365   PROP_ssa,                             /* properties_required */
 366   0,                                    /* properties_provided */
 367   0,                                    /* properties_destroyed */
 368   0,                                    /* todo_flags_start */
 369   TODO_dump_func                        /* todo_flags_finish */
 370  }
 371 };