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 "ggc.h"
  27 #include "tree-flow.h"
  28 #include "tree-pass.h"
  29
  30 /* Rewriting a function into SSA form can create a huge number of SSA_NAMEs,
  31    many of which may be thrown away shortly after their creation if jumps
  32    were threaded through PHI nodes.
  33
  34    While our garbage collection mechanisms will handle this situation, it
  35    is extremely wasteful to create nodes and throw them away, especially
  36    when the nodes can be reused.
  37
  38    For PR 8361, we can significantly reduce the number of nodes allocated
  39    and thus the total amount of memory allocated by managing SSA_NAMEs a
  40    little.  This additionally helps reduce the amount of work done by the
  41    garbage collector.  Similar results have been seen on a wider variety
  42    of tests (such as the compiler itself).
  43
  44    Right now we maintain our free list on a per-function basis.  It may
  45    or may not make sense to maintain the free list for the duration of
  46    a compilation unit.
  47
  48    External code should rely solely upon HIGHEST_SSA_VERSION and the
  49    externally defined functions.  External code should not know about
  50    the details of the free list management.
  51
  52    External code should also not assume the version number on nodes is
  53    monotonically increasing.  We reuse the version number when we
  54    reuse an SSA_NAME expression.  This helps keep arrays and bitmaps
  55    more compact.
  56
  57    We could also use a zone allocator for these objects since they have
  58    a very well defined lifetime.  If someone wants to experiment with that
  59    this is the place to try it.  */
  60
  61 /* Version numbers with special meanings.  We start allocating new version
  62    numbers after the special ones.  */
  63 #define UNUSED_NAME_VERSION 0
  64
  65 #ifdef GATHER_STATISTICS
  66 unsigned int ssa_name_nodes_reused;
  67 unsigned int ssa_name_nodes_created;
  68 #endif
  69
  70 /* Initialize management of SSA_NAMEs to default SIZE.  If SIZE is
  71    zero use default.  */
  72
  73 void
  74 init_ssanames (struct function *fn, int size)
  75 {
  76   if (size < 50)
  77     size = 50;
  78
  79   SSANAMES (fn) = VEC_alloc (tree, gc, size);
  80
  81   /* Version 0 is special, so reserve the first slot in the table.  Though
  82      currently unused, we may use version 0 in alias analysis as part of
  83      the heuristics used to group aliases when the alias sets are too
  84      large.
  85
  86      We use VEC_quick_push here because we know that SSA_NAMES has at
  87      least 50 elements reserved in it.  */
  88   VEC_quick_push (tree, SSANAMES (fn), NULL_TREE);
  89   FREE_SSANAMES (fn) = NULL;
  90
  91   SYMS_TO_RENAME (fn) = BITMAP_GGC_ALLOC ();
  92 }
  93
  94 /* Finalize management of SSA_NAMEs.  */
  95
  96 void
  97 fini_ssanames (void)
  98 {
  99   VEC_free (tree, gc, SSANAMES (cfun));
 100   FREE_SSANAMES (cfun) = NULL;
 101 }
 102
 103 /* Dump some simple statistics regarding the re-use of SSA_NAME nodes.  */
 104
 105 #ifdef GATHER_STATISTICS
 106 void
 107 ssanames_print_statistics (void)
 108 {
 109   fprintf (stderr, "SSA_NAME nodes allocated: %u\n", ssa_name_nodes_created);
 110   fprintf (stderr, "SSA_NAME nodes reused: %u\n", ssa_name_nodes_reused);
 111 }
 112 #endif
 113
 114 /* Return an SSA_NAME node for variable VAR defined in statement STMT
 115    in function FN.  STMT may be an empty statement for artificial
 116    references (e.g., default definitions created when a variable is
 117    used without a preceding definition).  */
 118
 119 tree
 120 make_ssa_name_fn (struct function *fn, tree var, gimple stmt)
 121 {
 122   tree t;
 123   use_operand_p imm;
 124
 125   gcc_assert (DECL_P (var));
 126
 127   /* If our free list has an element, then use it.  */
 128   if (FREE_SSANAMES (fn))
 129     {
 130       t = FREE_SSANAMES (fn);
 131       FREE_SSANAMES (fn) = TREE_CHAIN (FREE_SSANAMES (fn));
 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 (fn), SSA_NAME_VERSION (t), t);
 140     }
 141   else
 142     {
 143       t = make_node (SSA_NAME);
 144       SSA_NAME_VERSION (t) = VEC_length (tree, SSANAMES (fn));
 145       VEC_safe_push (tree, gc, SSANAMES (fn), 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->loc.ssa_name = 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       if (MAY_HAVE_DEBUG_STMTS)
 208         insert_debug_temp_for_var_def (NULL, var);
 209
 210 #ifdef ENABLE_CHECKING
 211       verify_imm_links (stderr, var);
 212 #endif
 213       while (imm->next != imm)
 214         delink_imm_use (imm->next);
 215
 216       VEC_replace (tree, SSANAMES (cfun),
 217                    SSA_NAME_VERSION (var), NULL_TREE);
 218       memset (var, 0, tree_size (var));
 219
 220       imm->prev = imm;
 221       imm->next = imm;
 222       imm->loc.ssa_name = var;
 223
 224       /* First put back the right tree node so that the tree checking
 225          macros do not complain.  */
 226       TREE_SET_CODE (var, SSA_NAME);
 227
 228       /* Restore the version number.  */
 229       SSA_NAME_VERSION (var) = saved_ssa_name_version;
 230
 231       /* Hopefully this can go away once we have the new incremental
 232          SSA updating code installed.  */
 233       SSA_NAME_VAR (var) = saved_ssa_name_var;
 234
 235       /* Note this SSA_NAME is now in the first list.  */
 236       SSA_NAME_IN_FREE_LIST (var) = 1;
 237
 238       /* And finally link it into the free list.  */
 239       TREE_CHAIN (var) = FREE_SSANAMES (cfun);
 240       FREE_SSANAMES (cfun) = var;
 241     }
 242 }
 243
 244 /* Creates a duplicate of a ssa name NAME defined in statement STMT.  */
 245
 246 tree
 247 duplicate_ssa_name (tree name, gimple stmt)
 248 {
 249   tree new_name = make_ssa_name (SSA_NAME_VAR (name), stmt);
 250   struct ptr_info_def *old_ptr_info = SSA_NAME_PTR_INFO (name);
 251
 252   if (old_ptr_info)
 253     duplicate_ssa_name_ptr_info (new_name, old_ptr_info);
 254
 255   return new_name;
 256 }
 257
 258
 259 /* Creates a duplicate of the ptr_info_def at PTR_INFO for use by
 260    the SSA name NAME.  */
 261
 262 void
 263 duplicate_ssa_name_ptr_info (tree name, struct ptr_info_def *ptr_info)
 264 {
 265   struct ptr_info_def *new_ptr_info;
 266
 267   gcc_assert (POINTER_TYPE_P (TREE_TYPE (name)));
 268   gcc_assert (!SSA_NAME_PTR_INFO (name));
 269
 270   if (!ptr_info)
 271     return;
 272
 273   new_ptr_info = GGC_NEW (struct ptr_info_def);
 274   *new_ptr_info = *ptr_info;
 275
 276   SSA_NAME_PTR_INFO (name) = new_ptr_info;
 277 }
 278
 279
 280 /* Release all the SSA_NAMEs created by STMT.  */
 281
 282 void
 283 release_defs (gimple stmt)
 284 {
 285   tree def;
 286   ssa_op_iter iter;
 287
 288   /* Make sure that we are in SSA.  Otherwise, operand cache may point
 289      to garbage.  */
 290   gcc_assert (gimple_in_ssa_p (cfun));
 291
 292   FOR_EACH_SSA_TREE_OPERAND (def, stmt, iter, SSA_OP_ALL_DEFS)
 293     if (TREE_CODE (def) == SSA_NAME)
 294       release_ssa_name (def);
 295 }
 296
 297
 298 /* Replace the symbol associated with SSA_NAME with SYM.  */
 299
 300 void
 301 replace_ssa_name_symbol (tree ssa_name, tree sym)
 302 {
 303   SSA_NAME_VAR (ssa_name) = sym;
 304   TREE_TYPE (ssa_name) = TREE_TYPE (sym);
 305 }
 306
 307 /* Return SSA names that are unused to GGC memory.  This is used to keep
 308    footprint of compiler during interprocedural optimization.
 309    As a side effect the SSA_NAME_VERSION number reuse is reduced
 310    so this function should not be used too often.  */
 311 static unsigned int
 312 release_dead_ssa_names (void)
 313 {
 314   tree t, next;
 315   int n = 0;
 316   referenced_var_iterator rvi;
 317
 318   /* Current defs point to various dead SSA names that in turn points to dead
 319      statements so bunch of dead memory is held from releasing.  */
 320   FOR_EACH_REFERENCED_VAR (t, rvi)
 321     set_current_def (t, NULL);
 322   /* Now release the freelist.  */
 323   for (t = FREE_SSANAMES (cfun); t; t = next)
 324     {
 325       next = TREE_CHAIN (t);
 326       /* Dangling pointers might make GGC to still see dead SSA names, so it is
 327          important to unlink the list and avoid GGC from seeing all subsequent
 328          SSA names.  In longer run we want to have all dangling pointers here
 329          removed (since they usually go through dead statements that consume
 330          considerable amounts of memory).  */
 331       TREE_CHAIN (t) = NULL_TREE;
 332       n++;
 333     }
 334   FREE_SSANAMES (cfun) = NULL;
 335
 336   /* Cgraph edges has been invalidated and point to dead statement.  We need to
 337      remove them now and will rebuild it before next IPA pass.  */
 338   cgraph_node_remove_callees (cgraph_node (current_function_decl));
 339
 340   if (dump_file)
 341     fprintf (dump_file, "Released %i names, %.2f%%\n", n, n * 100.0 / num_ssa_names);
 342   return 0;
 343 }
 344
 345 struct gimple_opt_pass pass_release_ssa_names =
 346 {
 347  {
 348   GIMPLE_PASS,
 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   TV_NONE,                              /* tv_id */
 356   PROP_ssa,                             /* properties_required */
 357   0,                                    /* properties_provided */
 358   0,                                    /* properties_destroyed */
 359   0,                                    /* todo_flags_start */
 360   TODO_dump_func                        /* todo_flags_finish */
 361  }
 362 };