gcc/tree-ssanames.c

   1 /* Generic routines for manipulating SSA_NAME expressions
   2    Copyright (C) 2003, 2004, 2005 Free Software Foundation, Inc.
   3
   4 This file is part of GCC.
   5
   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 2, or (at your option)
   9 any later version.
  10
  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.
  15
  16 You should have received a copy of the GNU General Public License
  17 along with GCC; see the file COPYING.  If not, write to
  18 the Free Software Foundation, 51 Franklin Street, Fifth Floor,
  19 Boston, MA 02110-1301, USA.  */
  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.  */
  72
  73 void
  74 init_ssanames (void)
  75 {
  76   SSANAMES (cfun) = VEC_alloc (tree, gc, 50);
  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 (cfun), NULL_TREE);
  86   FREE_SSANAMES (cfun) = NULL;
  87 }
  88
  89 /* Finalize management of SSA_NAMEs.  */
  90
  91 void
  92 fini_ssanames (void)
  93 {
  94   VEC_free (tree, gc, SSANAMES (cfun));
  95   FREE_SSANAMES (cfun) = NULL;
  96 }
  97
  98 /* Dump some simple statistics regarding the re-use of SSA_NAME nodes.  */
  99
 100 #ifdef GATHER_STATISTICS
 101 void
 102 ssanames_print_statistics (void)
 103 {
 104   fprintf (stderr, "SSA_NAME nodes allocated: %u\n", ssa_name_nodes_created);
 105   fprintf (stderr, "SSA_NAME nodes reused: %u\n", ssa_name_nodes_reused);
 106 }
 107 #endif
 108
 109 /* Return an SSA_NAME node for variable VAR defined in statement STMT.
 110    STMT may be an empty statement for artificial references (e.g., default
 111    definitions created when a variable is used without a preceding
 112    definition).  */
 113
 114 tree
 115 make_ssa_name (tree var, tree stmt)
 116 {
 117   tree t;
 118   use_operand_p imm;
 119
 120   gcc_assert (DECL_P (var)
 121               || TREE_CODE (var) == INDIRECT_REF);
 122
 123   gcc_assert (!stmt
 124               || EXPR_P (stmt) || GIMPLE_STMT_P (stmt)
 125               || TREE_CODE (stmt) == PHI_NODE);
 126
 127   /* If our free list has an element, then use it.  */
 128   if (FREE_SSANAMES (cfun))
 129     {
 130       t = FREE_SSANAMES (cfun);
 131       FREE_SSANAMES (cfun) = TREE_CHAIN (FREE_SSANAMES (cfun));
 132 #ifdef GATHER_STATISTICS
 133       ssa_name_nodes_reused++;
 134 #endif
 135
 136       /* The node was cleared out when we put it on the free list, so
 137          there is no need to do so again here.  */
 138       gcc_assert (ssa_name (SSA_NAME_VERSION (t)) == NULL);
 139       VEC_replace (tree, SSANAMES (cfun), SSA_NAME_VERSION (t), t);
 140     }
 141   else
 142     {
 143       t = make_node (SSA_NAME);
 144       SSA_NAME_VERSION (t) = num_ssa_names;
 145       VEC_safe_push (tree, gc, SSANAMES (cfun), t);
 146 #ifdef GATHER_STATISTICS
 147       ssa_name_nodes_created++;
 148 #endif
 149     }
 150
 151   TREE_TYPE (t) = TREE_TYPE (var);
 152   SSA_NAME_VAR (t) = var;
 153   SSA_NAME_DEF_STMT (t) = stmt;
 154   SSA_NAME_PTR_INFO (t) = NULL;
 155   SSA_NAME_IN_FREE_LIST (t) = 0;
 156   SSA_NAME_IS_DEFAULT_DEF (t) = 0;
 157   imm = &(SSA_NAME_IMM_USE_NODE (t));
 158   imm->use = NULL;
 159   imm->prev = imm;
 160   imm->next = imm;
 161   imm->stmt = t;
 162
 163   return t;
 164 }
 165
 166
 167 /* We no longer need the SSA_NAME expression VAR, release it so that
 168    it may be reused.
 169
 170    Note it is assumed that no calls to make_ssa_name will be made
 171    until all uses of the ssa name are released and that the only
 172    use of the SSA_NAME expression is to check its SSA_NAME_VAR.  All
 173    other fields must be assumed clobbered.  */
 174
 175 void
 176 release_ssa_name (tree var)
 177 {
 178   if (!var)
 179     return;
 180
 181   /* Never release the default definition for a symbol.  It's a
 182      special SSA name that should always exist once it's created.  */
 183   if (SSA_NAME_IS_DEFAULT_DEF (var))
 184     return;
 185
 186   /* If VAR has been registered for SSA updating, don't remove it.
 187      After update_ssa has run, the name will be released.  */
 188   if (name_registered_for_update_p (var))
 189     {
 190       release_ssa_name_after_update_ssa (var);
 191       return;
 192     }
 193
 194   /* release_ssa_name can be called multiple times on a single SSA_NAME.
 195      However, it should only end up on our free list one time.   We
 196      keep a status bit in the SSA_NAME node itself to indicate it has
 197      been put on the free list.
 198
 199      Note that once on the freelist you can not reference the SSA_NAME's
 200      defining statement.  */
 201   if (! SSA_NAME_IN_FREE_LIST (var))
 202     {
 203       tree saved_ssa_name_var = SSA_NAME_VAR (var);
 204       int saved_ssa_name_version = SSA_NAME_VERSION (var);
 205       use_operand_p imm = &(SSA_NAME_IMM_USE_NODE (var));
 206
 207 #ifdef ENABLE_CHECKING
 208       verify_imm_links (stderr, var);
 209 #endif
 210       while (imm->next != imm)
 211         delink_imm_use (imm->next);
 212
 213       VEC_replace (tree, SSANAMES (cfun),
 214                    SSA_NAME_VERSION (var), NULL_TREE);
 215       memset (var, 0, tree_size (var));
 216
 217       imm->prev = imm;
 218       imm->next = imm;
 219       imm->stmt = var;
 220       /* First put back the right tree node so that the tree checking
 221          macros do not complain.  */
 222       TREE_SET_CODE (var, SSA_NAME);
 223
 224       /* Restore the version number.  */
 225       SSA_NAME_VERSION (var) = saved_ssa_name_version;
 226
 227       /* Hopefully this can go away once we have the new incremental
 228          SSA updating code installed.  */
 229       SSA_NAME_VAR (var) = saved_ssa_name_var;
 230
 231       /* Note this SSA_NAME is now in the first list.  */
 232       SSA_NAME_IN_FREE_LIST (var) = 1;
 233
 234       /* And finally link it into the free list.  */
 235       TREE_CHAIN (var) = FREE_SSANAMES (cfun);
 236       FREE_SSANAMES (cfun) = var;
 237     }
 238 }
 239
 240 /* Creates a duplicate of a ssa name NAME defined in statement STMT.  */
 241
 242 tree
 243 duplicate_ssa_name (tree name, tree stmt)
 244 {
 245   tree new_name = make_ssa_name (SSA_NAME_VAR (name), stmt);
 246   struct ptr_info_def *old_ptr_info = SSA_NAME_PTR_INFO (name);
 247
 248   if (old_ptr_info)
 249     duplicate_ssa_name_ptr_info (new_name, old_ptr_info);
 250
 251   return new_name;
 252 }
 253
 254
 255 /* Creates a duplicate of the ptr_info_def at PTR_INFO for use by
 256    the SSA name NAME.  */
 257
 258 void
 259 duplicate_ssa_name_ptr_info (tree name, struct ptr_info_def *ptr_info)
 260 {
 261   struct ptr_info_def *new_ptr_info;
 262
 263   gcc_assert (POINTER_TYPE_P (TREE_TYPE (name)));
 264   gcc_assert (!SSA_NAME_PTR_INFO (name));
 265
 266   if (!ptr_info)
 267     return;
 268
 269   new_ptr_info = ggc_alloc (sizeof (struct ptr_info_def));
 270   *new_ptr_info = *ptr_info;
 271
 272   if (ptr_info->pt_vars)
 273     {
 274       new_ptr_info->pt_vars = BITMAP_GGC_ALLOC ();
 275       bitmap_copy (new_ptr_info->pt_vars, ptr_info->pt_vars);
 276     }
 277
 278   SSA_NAME_PTR_INFO (name) = new_ptr_info;
 279 }
 280
 281
 282 /* Release all the SSA_NAMEs created by STMT.  */
 283
 284 void
 285 release_defs (tree stmt)
 286 {
 287   tree def;
 288   ssa_op_iter iter;
 289
 290   /* Make sure that we are in SSA.  Otherwise, operand cache may point
 291      to garbage.  */
 292   gcc_assert (gimple_in_ssa_p (cfun));
 293
 294   FOR_EACH_SSA_TREE_OPERAND (def, stmt, iter, SSA_OP_ALL_DEFS)
 295     if (TREE_CODE (def) == SSA_NAME)
 296       release_ssa_name (def);
 297 }
 298
 299
 300 /* Replace the symbol associated with SSA_NAME with SYM.  */
 301
 302 void
 303 replace_ssa_name_symbol (tree ssa_name, tree sym)
 304 {
 305   SSA_NAME_VAR (ssa_name) = sym;
 306   TREE_TYPE (ssa_name) = TREE_TYPE (sym);
 307 }
 308
 309 /* Return SSA names that are unused to GGC memory.  This is used to keep
 310    footprint of compiler during interprocedural optimization.
 311    As a side effect the SSA_NAME_VERSION number reuse is reduced
 312    so this function should not be used too often.  */
 313 static unsigned int
 314 release_dead_ssa_names (void)
 315 {
 316   tree t, next;
 317   int n = 0;
 318   referenced_var_iterator rvi;
 319
 320   /* Current defs point to various dead SSA names that in turn points to dead
 321      statements so bunch of dead memory is held from releasing.  */
 322   FOR_EACH_REFERENCED_VAR (t, rvi)
 323     set_current_def (t, NULL);
 324   /* Now release the freelist.  */
 325   for (t = FREE_SSANAMES (cfun); t; t = next)
 326     {
 327       next = TREE_CHAIN (t);
 328       /* Dangling pointers might make GGC to still see dead SSA names, so it is
 329          important to unlink the list and avoid GGC from seeing all subsequent
 330          SSA names.  In longer run we want to have all dangling pointers here
 331          removed (since they usually go through dead statements that consume
 332          considerable amounts of memory).  */
 333       TREE_CHAIN (t) = NULL_TREE;
 334       n++;
 335     }
 336   FREE_SSANAMES (cfun) = NULL;
 337
 338   /* Cgraph edges has been invalidated and point to dead statement.  We need to
 339      remove them now and will rebuild it before next IPA pass.  */
 340   cgraph_node_remove_callees (cgraph_node (current_function_decl));
 341
 342   if (dump_file)
 343     fprintf (dump_file, "Released %i names, %.2f%%\n", n, n * 100.0 / num_ssa_names);
 344   return 0;
 345 }
 346
 347 struct tree_opt_pass pass_release_ssa_names =
 348 {
 349   "release_ssa",                        /* name */
 350   NULL,                                 /* gate */
 351   release_dead_ssa_names,               /* execute */
 352   NULL,                                 /* sub */
 353   NULL,                                 /* next */
 354   0,                                    /* static_pass_number */
 355   0,                                    /* tv_id */
 356   PROP_ssa,                             /* properties_required */
 357   0,                                    /* properties_provided */
 358   0,                                    /* properties_destroyed */
 359   0,                                    /* todo_flags_start */
 360   0,                                    /* todo_flags_finish */
 361   0                                     /* letter */
 362 };