gcc/tree-ssanames.c

   1 /* Generic routines for manipulating SSA_NAME expressions
   2    Copyright (C) 2003, 2004, 2005, 2007 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 3, 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 COPYING3.  If not see
  18 <http://www.gnu.org/licenses/>.  */
  19
  20 #include "config.h"
  21 #include "system.h"
  22 #include "coretypes.h"
  23 #include "tm.h"
  24 #include "tree.h"
  25 #include "varray.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
  92 /* Finalize management of SSA_NAMEs.  */
  93
  94 void
  95 fini_ssanames (void)
  96 {
  97   VEC_free (tree, gc, SSANAMES (cfun));
  98   FREE_SSANAMES (cfun) = NULL;
  99 }
 100
 101 /* Dump some simple statistics regarding the re-use of SSA_NAME nodes.  */
 102
 103 #ifdef GATHER_STATISTICS
 104 void
 105 ssanames_print_statistics (void)
 106 {
 107   fprintf (stderr, "SSA_NAME nodes allocated: %u\n", ssa_name_nodes_created);
 108   fprintf (stderr, "SSA_NAME nodes reused: %u\n", ssa_name_nodes_reused);
 109 }
 110 #endif
 111
 112 /* Return an SSA_NAME node for variable VAR defined in statement STMT
 113    in function FN.  STMT may be an empty statement for artificial
 114    references (e.g., default definitions created when a variable is
 115    used without a preceding definition).  */
 116
 117 tree
 118 make_ssa_name_fn (struct function *fn, tree var, gimple stmt)
 119 {
 120   tree t;
 121   use_operand_p imm;
 122
 123   gcc_assert (DECL_P (var));
 124
 125   /* If our free list has an element, then use it.  */
 126   if (FREE_SSANAMES (fn))
 127     {
 128       t = FREE_SSANAMES (fn);
 129       FREE_SSANAMES (fn) = TREE_CHAIN (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 #ifdef ENABLE_CHECKING
 206       verify_imm_links (stderr, var);
 207 #endif
 208       while (imm->next != imm)
 209         delink_imm_use (imm->next);
 210
 211       VEC_replace (tree, SSANAMES (cfun),
 212                    SSA_NAME_VERSION (var), NULL_TREE);
 213       memset (var, 0, tree_size (var));
 214
 215       imm->prev = imm;
 216       imm->next = imm;
 217       imm->loc.ssa_name = var;
 218
 219       /* First put back the right tree node so that the tree checking
 220          macros do not complain.  */
 221       TREE_SET_CODE (var, SSA_NAME);
 222
 223       /* Restore the version number.  */
 224       SSA_NAME_VERSION (var) = saved_ssa_name_version;
 225
 226       /* Hopefully this can go away once we have the new incremental
 227          SSA updating code installed.  */
 228       SSA_NAME_VAR (var) = saved_ssa_name_var;
 229
 230       /* Note this SSA_NAME is now in the first list.  */
 231       SSA_NAME_IN_FREE_LIST (var) = 1;
 232
 233       /* And finally link it into the free list.  */
 234       TREE_CHAIN (var) = FREE_SSANAMES (cfun);
 235       FREE_SSANAMES (cfun) = var;
 236     }
 237 }
 238
 239 /* Creates a duplicate of a ssa name NAME defined in statement STMT.  */
 240
 241 tree
 242 duplicate_ssa_name (tree name, gimple stmt)
 243 {
 244   tree new_name = make_ssa_name (SSA_NAME_VAR (name), stmt);
 245   struct ptr_info_def *old_ptr_info = SSA_NAME_PTR_INFO (name);
 246
 247   if (old_ptr_info)
 248     duplicate_ssa_name_ptr_info (new_name, old_ptr_info);
 249
 250   return new_name;
 251 }
 252
 253
 254 /* Creates a duplicate of the ptr_info_def at PTR_INFO for use by
 255    the SSA name NAME.  */
 256
 257 void
 258 duplicate_ssa_name_ptr_info (tree name, struct ptr_info_def *ptr_info)
 259 {
 260   struct ptr_info_def *new_ptr_info;
 261
 262   gcc_assert (POINTER_TYPE_P (TREE_TYPE (name)));
 263   gcc_assert (!SSA_NAME_PTR_INFO (name));
 264
 265   if (!ptr_info)
 266     return;
 267
 268   new_ptr_info = GGC_NEW (struct ptr_info_def);
 269   *new_ptr_info = *ptr_info;
 270
 271   if (ptr_info->pt_vars)
 272     {
 273       new_ptr_info->pt_vars = BITMAP_GGC_ALLOC ();
 274       bitmap_copy (new_ptr_info->pt_vars, ptr_info->pt_vars);
 275     }
 276
 277   SSA_NAME_PTR_INFO (name) = new_ptr_info;
 278 }
 279
 280
 281 /* Release all the SSA_NAMEs created by STMT.  */
 282
 283 void
 284 release_defs (gimple stmt)
 285 {
 286   tree def;
 287   ssa_op_iter iter;
 288
 289   /* Make sure that we are in SSA.  Otherwise, operand cache may point
 290      to garbage.  */
 291   gcc_assert (gimple_in_ssa_p (cfun));
 292
 293   FOR_EACH_SSA_TREE_OPERAND (def, stmt, iter, SSA_OP_ALL_DEFS)
 294     if (TREE_CODE (def) == SSA_NAME)
 295       release_ssa_name (def);
 296 }
 297
 298
 299 /* Replace the symbol associated with SSA_NAME with SYM.  */
 300
 301 void
 302 replace_ssa_name_symbol (tree ssa_name, tree sym)
 303 {
 304   SSA_NAME_VAR (ssa_name) = sym;
 305   TREE_TYPE (ssa_name) = TREE_TYPE (sym);
 306 }
 307
 308 /* Return SSA names that are unused to GGC memory.  This is used to keep
 309    footprint of compiler during interprocedural optimization.
 310    As a side effect the SSA_NAME_VERSION number reuse is reduced
 311    so this function should not be used too often.  */
 312 static unsigned int
 313 release_dead_ssa_names (void)
 314 {
 315   tree t, next;
 316   int n = 0;
 317   referenced_var_iterator rvi;
 318
 319   /* Current defs point to various dead SSA names that in turn points to dead
 320      statements so bunch of dead memory is held from releasing.  */
 321   FOR_EACH_REFERENCED_VAR (t, rvi)
 322     set_current_def (t, NULL);
 323   /* Now release the freelist.  */
 324   for (t = FREE_SSANAMES (cfun); t; t = next)
 325     {
 326       next = TREE_CHAIN (t);
 327       /* Dangling pointers might make GGC to still see dead SSA names, so it is
 328          important to unlink the list and avoid GGC from seeing all subsequent
 329          SSA names.  In longer run we want to have all dangling pointers here
 330          removed (since they usually go through dead statements that consume
 331          considerable amounts of memory).  */
 332       TREE_CHAIN (t) = NULL_TREE;
 333       n++;
 334     }
 335   FREE_SSANAMES (cfun) = NULL;
 336
 337   /* Cgraph edges has been invalidated and point to dead statement.  We need to
 338      remove them now and will rebuild it before next IPA pass.  */
 339   cgraph_node_remove_callees (cgraph_node (current_function_decl));
 340
 341   if (dump_file)
 342     fprintf (dump_file, "Released %i names, %.2f%%\n", n, n * 100.0 / num_ssa_names);
 343   return 0;
 344 }
 345
 346 struct gimple_opt_pass pass_release_ssa_names =
 347 {
 348  {
 349   GIMPLE_PASS,
 350   "release_ssa",                        /* name */
 351   NULL,                                 /* gate */
 352   release_dead_ssa_names,               /* execute */
 353   NULL,                                 /* sub */
 354   NULL,                                 /* next */
 355   0,                                    /* static_pass_number */
 356   0,                                    /* tv_id */
 357   PROP_ssa,                             /* properties_required */
 358   0,                                    /* properties_provided */
 359   0,                                    /* properties_destroyed */
 360   0,                                    /* todo_flags_start */
 361   TODO_dump_func                        /* todo_flags_finish */
 362  }
 363 };