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 "varray.h"
  27 #include "ggc.h"
  28 #include "tree-flow.h"
  29 #include "tree-pass.h"
  30
  31 /* Rewriting a function into SSA form can create a huge number of SSA_NAMEs,
  32    many of which may be thrown away shortly after their creation if jumps
  33    were threaded through PHI nodes.
  34
  35    While our garbage collection mechanisms will handle this situation, it
  36    is extremely wasteful to create nodes and throw them away, especially
  37    when the nodes can be reused.
  38
  39    For PR 8361, we can significantly reduce the number of nodes allocated
  40    and thus the total amount of memory allocated by managing SSA_NAMEs a
  41    little.  This additionally helps reduce the amount of work done by the
  42    garbage collector.  Similar results have been seen on a wider variety
  43    of tests (such as the compiler itself).
  44
  45    Right now we maintain our free list on a per-function basis.  It may
  46    or may not make sense to maintain the free list for the duration of
  47    a compilation unit.
  48
  49    External code should rely solely upon HIGHEST_SSA_VERSION and the
  50    externally defined functions.  External code should not know about
  51    the details of the free list management.
  52
  53    External code should also not assume the version number on nodes is
  54    monotonically increasing.  We reuse the version number when we
  55    reuse an SSA_NAME expression.  This helps keep arrays and bitmaps
  56    more compact.
  57
  58    We could also use a zone allocator for these objects since they have
  59    a very well defined lifetime.  If someone wants to experiment with that
  60    this is the place to try it.  */
  61
  62 /* Version numbers with special meanings.  We start allocating new version
  63    numbers after the special ones.  */
  64 #define UNUSED_NAME_VERSION 0
  65
  66 #ifdef GATHER_STATISTICS
  67 unsigned int ssa_name_nodes_reused;
  68 unsigned int ssa_name_nodes_created;
  69 #endif
  70
  71 /* Initialize management of SSA_NAMEs to default SIZE.  If SIZE is
  72    zero use default.  */
  73
  74 void
  75 init_ssanames (struct function *fn, int size)
  76 {
  77   if (size < 50)
  78     size = 50;
  79
  80   SSANAMES (fn) = VEC_alloc (tree, gc, size);
  81
  82   /* Version 0 is special, so reserve the first slot in the table.  Though
  83      currently unused, we may use version 0 in alias analysis as part of
  84      the heuristics used to group aliases when the alias sets are too
  85      large.
  86
  87      We use VEC_quick_push here because we know that SSA_NAMES has at
  88      least 50 elements reserved in it.  */
  89   VEC_quick_push (tree, SSANAMES (fn), NULL_TREE);
  90   FREE_SSANAMES (fn) = NULL;
  91
  92   SYMS_TO_RENAME (fn) = BITMAP_GGC_ALLOC ();
  93 }
  94
  95 /* Finalize management of SSA_NAMEs.  */
  96
  97 void
  98 fini_ssanames (void)
  99 {
 100   VEC_free (tree, gc, SSANAMES (cfun));
 101   FREE_SSANAMES (cfun) = NULL;
 102 }
 103
 104 /* Dump some simple statistics regarding the re-use of SSA_NAME nodes.  */
 105
 106 #ifdef GATHER_STATISTICS
 107 void
 108 ssanames_print_statistics (void)
 109 {
 110   fprintf (stderr, "SSA_NAME nodes allocated: %u\n", ssa_name_nodes_created);
 111   fprintf (stderr, "SSA_NAME nodes reused: %u\n", ssa_name_nodes_reused);
 112 }
 113 #endif
 114
 115 /* Return an SSA_NAME node for variable VAR defined in statement STMT
 116    in function FN.  STMT may be an empty statement for artificial
 117    references (e.g., default definitions created when a variable is
 118    used without a preceding definition).  */
 119
 120 tree
 121 make_ssa_name_fn (struct function *fn, tree var, gimple stmt)
 122 {
 123   tree t;
 124   use_operand_p imm;
 125
 126   gcc_assert (DECL_P (var));
 127
 128   /* If our free list has an element, then use it.  */
 129   if (FREE_SSANAMES (fn))
 130     {
 131       t = FREE_SSANAMES (fn);
 132       FREE_SSANAMES (fn) = TREE_CHAIN (FREE_SSANAMES (fn));
 133 #ifdef GATHER_STATISTICS
 134       ssa_name_nodes_reused++;
 135 #endif
 136
 137       /* The node was cleared out when we put it on the free list, so
 138          there is no need to do so again here.  */
 139       gcc_assert (ssa_name (SSA_NAME_VERSION (t)) == NULL);
 140       VEC_replace (tree, SSANAMES (fn), SSA_NAME_VERSION (t), t);
 141     }
 142   else
 143     {
 144       t = make_node (SSA_NAME);
 145       SSA_NAME_VERSION (t) = VEC_length (tree, SSANAMES (fn));
 146       VEC_safe_push (tree, gc, SSANAMES (fn), t);
 147 #ifdef GATHER_STATISTICS
 148       ssa_name_nodes_created++;
 149 #endif
 150     }
 151
 152   TREE_TYPE (t) = TREE_TYPE (var);
 153   SSA_NAME_VAR (t) = var;
 154   SSA_NAME_DEF_STMT (t) = stmt;
 155   SSA_NAME_PTR_INFO (t) = NULL;
 156   SSA_NAME_IN_FREE_LIST (t) = 0;
 157   SSA_NAME_IS_DEFAULT_DEF (t) = 0;
 158   imm = &(SSA_NAME_IMM_USE_NODE (t));
 159   imm->use = NULL;
 160   imm->prev = imm;
 161   imm->next = imm;
 162   imm->loc.ssa_name = t;
 163
 164   return t;
 165 }
 166
 167
 168 /* We no longer need the SSA_NAME expression VAR, release it so that
 169    it may be reused.
 170
 171    Note it is assumed that no calls to make_ssa_name will be made
 172    until all uses of the ssa name are released and that the only
 173    use of the SSA_NAME expression is to check its SSA_NAME_VAR.  All
 174    other fields must be assumed clobbered.  */
 175
 176 void
 177 release_ssa_name (tree var)
 178 {
 179   if (!var)
 180     return;
 181
 182   /* Never release the default definition for a symbol.  It's a
 183      special SSA name that should always exist once it's created.  */
 184   if (SSA_NAME_IS_DEFAULT_DEF (var))
 185     return;
 186
 187   /* If VAR has been registered for SSA updating, don't remove it.
 188      After update_ssa has run, the name will be released.  */
 189   if (name_registered_for_update_p (var))
 190     {
 191       release_ssa_name_after_update_ssa (var);
 192       return;
 193     }
 194
 195   /* release_ssa_name can be called multiple times on a single SSA_NAME.
 196      However, it should only end up on our free list one time.   We
 197      keep a status bit in the SSA_NAME node itself to indicate it has
 198      been put on the free list.
 199
 200      Note that once on the freelist you can not reference the SSA_NAME's
 201      defining statement.  */
 202   if (! SSA_NAME_IN_FREE_LIST (var))
 203     {
 204       tree saved_ssa_name_var = SSA_NAME_VAR (var);
 205       int saved_ssa_name_version = SSA_NAME_VERSION (var);
 206       use_operand_p imm = &(SSA_NAME_IMM_USE_NODE (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 link it into the free list.  */
 237       TREE_CHAIN (var) = FREE_SSANAMES (cfun);
 238       FREE_SSANAMES (cfun) = var;
 239     }
 240 }
 241
 242 /* Creates a duplicate of a ssa name NAME defined in statement STMT.  */
 243
 244 tree
 245 duplicate_ssa_name (tree name, gimple stmt)
 246 {
 247   tree new_name = make_ssa_name (SSA_NAME_VAR (name), stmt);
 248   struct ptr_info_def *old_ptr_info = SSA_NAME_PTR_INFO (name);
 249
 250   if (old_ptr_info)
 251     duplicate_ssa_name_ptr_info (new_name, old_ptr_info);
 252
 253   return new_name;
 254 }
 255
 256
 257 /* Creates a duplicate of the ptr_info_def at PTR_INFO for use by
 258    the SSA name NAME.  */
 259
 260 void
 261 duplicate_ssa_name_ptr_info (tree name, struct ptr_info_def *ptr_info)
 262 {
 263   struct ptr_info_def *new_ptr_info;
 264
 265   gcc_assert (POINTER_TYPE_P (TREE_TYPE (name)));
 266   gcc_assert (!SSA_NAME_PTR_INFO (name));
 267
 268   if (!ptr_info)
 269     return;
 270
 271   new_ptr_info = GGC_NEW (struct ptr_info_def);
 272   *new_ptr_info = *ptr_info;
 273
 274   SSA_NAME_PTR_INFO (name) = new_ptr_info;
 275 }
 276
 277
 278 /* Release all the SSA_NAMEs created by STMT.  */
 279
 280 void
 281 release_defs (gimple stmt)
 282 {
 283   tree def;
 284   ssa_op_iter iter;
 285
 286   /* Make sure that we are in SSA.  Otherwise, operand cache may point
 287      to garbage.  */
 288   gcc_assert (gimple_in_ssa_p (cfun));
 289
 290   FOR_EACH_SSA_TREE_OPERAND (def, stmt, iter, SSA_OP_ALL_DEFS)
 291     if (TREE_CODE (def) == SSA_NAME)
 292       release_ssa_name (def);
 293 }
 294
 295
 296 /* Replace the symbol associated with SSA_NAME with SYM.  */
 297
 298 void
 299 replace_ssa_name_symbol (tree ssa_name, tree sym)
 300 {
 301   SSA_NAME_VAR (ssa_name) = sym;
 302   TREE_TYPE (ssa_name) = TREE_TYPE (sym);
 303 }
 304
 305 /* Return SSA names that are unused to GGC memory.  This is used to keep
 306    footprint of compiler during interprocedural optimization.
 307    As a side effect the SSA_NAME_VERSION number reuse is reduced
 308    so this function should not be used too often.  */
 309 static unsigned int
 310 release_dead_ssa_names (void)
 311 {
 312   tree t, next;
 313   int n = 0;
 314   referenced_var_iterator rvi;
 315
 316   /* Current defs point to various dead SSA names that in turn points to dead
 317      statements so bunch of dead memory is held from releasing.  */
 318   FOR_EACH_REFERENCED_VAR (t, rvi)
 319     set_current_def (t, NULL);
 320   /* Now release the freelist.  */
 321   for (t = FREE_SSANAMES (cfun); t; t = next)
 322     {
 323       next = TREE_CHAIN (t);
 324       /* Dangling pointers might make GGC to still see dead SSA names, so it is
 325          important to unlink the list and avoid GGC from seeing all subsequent
 326          SSA names.  In longer run we want to have all dangling pointers here
 327          removed (since they usually go through dead statements that consume
 328          considerable amounts of memory).  */
 329       TREE_CHAIN (t) = NULL_TREE;
 330       n++;
 331     }
 332   FREE_SSANAMES (cfun) = NULL;
 333
 334   /* Cgraph edges has been invalidated and point to dead statement.  We need to
 335      remove them now and will rebuild it before next IPA pass.  */
 336   cgraph_node_remove_callees (cgraph_node (current_function_decl));
 337
 338   if (dump_file)
 339     fprintf (dump_file, "Released %i names, %.2f%%\n", n, n * 100.0 / num_ssa_names);
 340   return 0;
 341 }
 342
 343 struct gimple_opt_pass pass_release_ssa_names =
 344 {
 345  {
 346   GIMPLE_PASS,
 347   "release_ssa",                        /* name */
 348   NULL,                                 /* gate */
 349   release_dead_ssa_names,               /* execute */
 350   NULL,                                 /* sub */
 351   NULL,                                 /* next */
 352   0,                                    /* static_pass_number */
 353   TV_NONE,                              /* tv_id */
 354   PROP_ssa,                             /* properties_required */
 355   0,                                    /* properties_provided */
 356   0,                                    /* properties_destroyed */
 357   0,                                    /* todo_flags_start */
 358   TODO_dump_func                        /* todo_flags_finish */
 359  }
 360 };