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, tree stmt)
 119 {
 120   tree t;
 121   use_operand_p imm;
 122
 123   gcc_assert (DECL_P (var)
 124               || TREE_CODE (var) == INDIRECT_REF);
 125
 126   gcc_assert (!stmt
 127               || EXPR_P (stmt) || GIMPLE_STMT_P (stmt)
 128               || TREE_CODE (stmt) == PHI_NODE);
 129
 130   /* If our free list has an element, then use it.  */
 131   if (FREE_SSANAMES (fn))
 132     {
 133       t = FREE_SSANAMES (fn);
 134       FREE_SSANAMES (fn) = TREE_CHAIN (FREE_SSANAMES (fn));
 135 #ifdef GATHER_STATISTICS
 136       ssa_name_nodes_reused++;
 137 #endif
 138
 139       /* The node was cleared out when we put it on the free list, so
 140          there is no need to do so again here.  */
 141       gcc_assert (ssa_name (SSA_NAME_VERSION (t)) == NULL);
 142       VEC_replace (tree, SSANAMES (fn), SSA_NAME_VERSION (t), t);
 143     }
 144   else
 145     {
 146       t = make_node (SSA_NAME);
 147       SSA_NAME_VERSION (t) = VEC_length (tree, SSANAMES (fn));
 148       VEC_safe_push (tree, gc, SSANAMES (fn), t);
 149 #ifdef GATHER_STATISTICS
 150       ssa_name_nodes_created++;
 151 #endif
 152     }
 153
 154   TREE_TYPE (t) = TREE_TYPE (var);
 155   SSA_NAME_VAR (t) = var;
 156   SSA_NAME_DEF_STMT (t) = stmt;
 157   SSA_NAME_PTR_INFO (t) = NULL;
 158   SSA_NAME_IN_FREE_LIST (t) = 0;
 159   SSA_NAME_IS_DEFAULT_DEF (t) = 0;
 160   imm = &(SSA_NAME_IMM_USE_NODE (t));
 161   imm->use = NULL;
 162   imm->prev = imm;
 163   imm->next = imm;
 164   imm->stmt = t;
 165
 166   return t;
 167 }
 168
 169
 170 /* We no longer need the SSA_NAME expression VAR, release it so that
 171    it may be reused.
 172
 173    Note it is assumed that no calls to make_ssa_name will be made
 174    until all uses of the ssa name are released and that the only
 175    use of the SSA_NAME expression is to check its SSA_NAME_VAR.  All
 176    other fields must be assumed clobbered.  */
 177
 178 void
 179 release_ssa_name (tree var)
 180 {
 181   if (!var)
 182     return;
 183
 184   /* Never release the default definition for a symbol.  It's a
 185      special SSA name that should always exist once it's created.  */
 186   if (SSA_NAME_IS_DEFAULT_DEF (var))
 187     return;
 188
 189   /* If VAR has been registered for SSA updating, don't remove it.
 190      After update_ssa has run, the name will be released.  */
 191   if (name_registered_for_update_p (var))
 192     {
 193       release_ssa_name_after_update_ssa (var);
 194       return;
 195     }
 196
 197   /* release_ssa_name can be called multiple times on a single SSA_NAME.
 198      However, it should only end up on our free list one time.   We
 199      keep a status bit in the SSA_NAME node itself to indicate it has
 200      been put on the free list.
 201
 202      Note that once on the freelist you can not reference the SSA_NAME's
 203      defining statement.  */
 204   if (! SSA_NAME_IN_FREE_LIST (var))
 205     {
 206       tree saved_ssa_name_var = SSA_NAME_VAR (var);
 207       int saved_ssa_name_version = SSA_NAME_VERSION (var);
 208       use_operand_p imm = &(SSA_NAME_IMM_USE_NODE (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->stmt = var;
 223       /* First put back the right tree node so that the tree checking
 224          macros do not complain.  */
 225       TREE_SET_CODE (var, SSA_NAME);
 226
 227       /* Restore the version number.  */
 228       SSA_NAME_VERSION (var) = saved_ssa_name_version;
 229
 230       /* Hopefully this can go away once we have the new incremental
 231          SSA updating code installed.  */
 232       SSA_NAME_VAR (var) = saved_ssa_name_var;
 233
 234       /* Note this SSA_NAME is now in the first list.  */
 235       SSA_NAME_IN_FREE_LIST (var) = 1;
 236
 237       /* And finally link it into the free list.  */
 238       TREE_CHAIN (var) = FREE_SSANAMES (cfun);
 239       FREE_SSANAMES (cfun) = var;
 240     }
 241 }
 242
 243 /* Creates a duplicate of a ssa name NAME defined in statement STMT.  */
 244
 245 tree
 246 duplicate_ssa_name (tree name, tree stmt)
 247 {
 248   tree new_name = make_ssa_name (SSA_NAME_VAR (name), stmt);
 249   struct ptr_info_def *old_ptr_info = SSA_NAME_PTR_INFO (name);
 250
 251   if (old_ptr_info)
 252     duplicate_ssa_name_ptr_info (new_name, old_ptr_info);
 253
 254   return new_name;
 255 }
 256
 257
 258 /* Creates a duplicate of the ptr_info_def at PTR_INFO for use by
 259    the SSA name NAME.  */
 260
 261 void
 262 duplicate_ssa_name_ptr_info (tree name, struct ptr_info_def *ptr_info)
 263 {
 264   struct ptr_info_def *new_ptr_info;
 265
 266   gcc_assert (POINTER_TYPE_P (TREE_TYPE (name)));
 267   gcc_assert (!SSA_NAME_PTR_INFO (name));
 268
 269   if (!ptr_info)
 270     return;
 271
 272   new_ptr_info = GGC_NEW (struct ptr_info_def);
 273   *new_ptr_info = *ptr_info;
 274
 275   if (ptr_info->pt_vars)
 276     {
 277       new_ptr_info->pt_vars = BITMAP_GGC_ALLOC ();
 278       bitmap_copy (new_ptr_info->pt_vars, ptr_info->pt_vars);
 279     }
 280
 281   SSA_NAME_PTR_INFO (name) = new_ptr_info;
 282 }
 283
 284
 285 /* Release all the SSA_NAMEs created by STMT.  */
 286
 287 void
 288 release_defs (tree stmt)
 289 {
 290   tree def;
 291   ssa_op_iter iter;
 292
 293   /* Make sure that we are in SSA.  Otherwise, operand cache may point
 294      to garbage.  */
 295   gcc_assert (gimple_in_ssa_p (cfun));
 296
 297   FOR_EACH_SSA_TREE_OPERAND (def, stmt, iter, SSA_OP_ALL_DEFS)
 298     if (TREE_CODE (def) == SSA_NAME)
 299       release_ssa_name (def);
 300 }
 301
 302
 303 /* Replace the symbol associated with SSA_NAME with SYM.  */
 304
 305 void
 306 replace_ssa_name_symbol (tree ssa_name, tree sym)
 307 {
 308   SSA_NAME_VAR (ssa_name) = sym;
 309   TREE_TYPE (ssa_name) = TREE_TYPE (sym);
 310 }
 311
 312 /* Return SSA names that are unused to GGC memory.  This is used to keep
 313    footprint of compiler during interprocedural optimization.
 314    As a side effect the SSA_NAME_VERSION number reuse is reduced
 315    so this function should not be used too often.  */
 316 static unsigned int
 317 release_dead_ssa_names (void)
 318 {
 319   tree t, next;
 320   int n = 0;
 321   referenced_var_iterator rvi;
 322
 323   /* Current defs point to various dead SSA names that in turn points to dead
 324      statements so bunch of dead memory is held from releasing.  */
 325   FOR_EACH_REFERENCED_VAR (t, rvi)
 326     set_current_def (t, NULL);
 327   /* Now release the freelist.  */
 328   for (t = FREE_SSANAMES (cfun); t; t = next)
 329     {
 330       next = TREE_CHAIN (t);
 331       /* Dangling pointers might make GGC to still see dead SSA names, so it is
 332          important to unlink the list and avoid GGC from seeing all subsequent
 333          SSA names.  In longer run we want to have all dangling pointers here
 334          removed (since they usually go through dead statements that consume
 335          considerable amounts of memory).  */
 336       TREE_CHAIN (t) = NULL_TREE;
 337       n++;
 338     }
 339   FREE_SSANAMES (cfun) = NULL;
 340
 341   /* Cgraph edges has been invalidated and point to dead statement.  We need to
 342      remove them now and will rebuild it before next IPA pass.  */
 343   cgraph_node_remove_callees (cgraph_node (current_function_decl));
 344
 345   if (dump_file)
 346     fprintf (dump_file, "Released %i names, %.2f%%\n", n, n * 100.0 / num_ssa_names);
 347   return 0;
 348 }
 349
 350 struct gimple_opt_pass pass_release_ssa_names =
 351 {
 352  {
 353   GIMPLE_PASS,
 354   "release_ssa",                        /* name */
 355   NULL,                                 /* gate */
 356   release_dead_ssa_names,               /* execute */
 357   NULL,                                 /* sub */
 358   NULL,                                 /* next */
 359   0,                                    /* static_pass_number */
 360   0,                                    /* tv_id */
 361   PROP_ssa,                             /* properties_required */
 362   0,                                    /* properties_provided */
 363   0,                                    /* properties_destroyed */
 364   0,                                    /* todo_flags_start */
 365   TODO_dump_func                        /* todo_flags_finish */
 366  }
 367 };