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
  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 /* Array of all SSA_NAMEs used in the function.  */
  62 VEC(tree,gc) *ssa_names;
  63
  64 /* Free list of SSA_NAMEs.  This list is wiped at the end of each function
  65    after we leave SSA form.  */
  66 static GTY (()) tree free_ssanames;
  67
  68 /* Version numbers with special meanings.  We start allocating new version
  69    numbers after the special ones.  */
  70 #define UNUSED_NAME_VERSION 0
  71
  72 #ifdef GATHER_STATISTICS
  73 unsigned int ssa_name_nodes_reused;
  74 unsigned int ssa_name_nodes_created;
  75 #endif
  76
  77 /* Initialize management of SSA_NAMEs.  */
  78
  79 void
  80 init_ssanames (void)
  81 {
  82   ssa_names = VEC_alloc (tree, gc, 50);
  83
  84   /* Version 0 is special, so reserve the first slot in the table.  Though
  85      currently unused, we may use version 0 in alias analysis as part of
  86      the heuristics used to group aliases when the alias sets are too
  87      large.
  88
  89      We use VEC_quick_push here because we know that SSA_NAMES has at
  90      least 50 elements reserved in it.  */
  91   VEC_quick_push (tree, ssa_names, NULL_TREE);
  92   free_ssanames = NULL;
  93 }
  94
  95 /* Finalize management of SSA_NAMEs.  */
  96
  97 void
  98 fini_ssanames (void)
  99 {
 100   VEC_free (tree, gc, ssa_names);
 101   free_ssanames = 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    STMT may be an empty statement for artificial references (e.g., default
 117    definitions created when a variable is used without a preceding
 118    definition).  */
 119
 120 tree
 121 make_ssa_name (tree var, tree stmt)
 122 {
 123   tree t;
 124   use_operand_p imm;
 125
 126   gcc_assert (DECL_P (var)
 127               || TREE_CODE (var) == INDIRECT_REF);
 128
 129   gcc_assert (!stmt || EXPR_P (stmt) || TREE_CODE (stmt) == PHI_NODE);
 130
 131   /* If our free list has an element, then use it.  */
 132   if (free_ssanames)
 133     {
 134       t = free_ssanames;
 135       free_ssanames = TREE_CHAIN (free_ssanames);
 136 #ifdef GATHER_STATISTICS
 137       ssa_name_nodes_reused++;
 138 #endif
 139
 140       /* The node was cleared out when we put it on the free list, so
 141          there is no need to do so again here.  */
 142       gcc_assert (ssa_name (SSA_NAME_VERSION (t)) == NULL);
 143       VEC_replace (tree, ssa_names, SSA_NAME_VERSION (t), t);
 144     }
 145   else
 146     {
 147       t = make_node (SSA_NAME);
 148       SSA_NAME_VERSION (t) = num_ssa_names;
 149       VEC_safe_push (tree, gc, ssa_names, t);
 150 #ifdef GATHER_STATISTICS
 151       ssa_name_nodes_created++;
 152 #endif
 153     }
 154
 155   TREE_TYPE (t) = TREE_TYPE (var);
 156   SSA_NAME_VAR (t) = var;
 157   SSA_NAME_DEF_STMT (t) = stmt;
 158   SSA_NAME_PTR_INFO (t) = NULL;
 159   SSA_NAME_IN_FREE_LIST (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 (var == default_def (SSA_NAME_VAR (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, ssa_names, SSA_NAME_VERSION (var), NULL_TREE);
 217       memset (var, 0, tree_size (var));
 218
 219       imm->prev = imm;
 220       imm->next = imm;
 221       imm->stmt = var;
 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;
 238       free_ssanames = 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, tree 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_alloc (sizeof (struct ptr_info_def));
 272   *new_ptr_info = *ptr_info;
 273
 274   if (ptr_info->pt_vars)
 275     {
 276       new_ptr_info->pt_vars = BITMAP_GGC_ALLOC ();
 277       bitmap_copy (new_ptr_info->pt_vars, ptr_info->pt_vars);
 278     }
 279
 280   SSA_NAME_PTR_INFO (name) = new_ptr_info;
 281 }
 282
 283
 284 /* Release all the SSA_NAMEs created by STMT.  */
 285
 286 void
 287 release_defs (tree stmt)
 288 {
 289   tree def;
 290   ssa_op_iter iter;
 291
 292   /* Make sure that we are in SSA.  Otherwise, operand cache may point
 293      to garbage.  */
 294   gcc_assert (in_ssa_p);
 295
 296   FOR_EACH_SSA_TREE_OPERAND (def, stmt, iter, SSA_OP_ALL_DEFS)
 297     if (TREE_CODE (def) == SSA_NAME)
 298       release_ssa_name (def);
 299 }
 300
 301
 302 /* Replace the symbol associated with SSA_NAME with SYM.  */
 303
 304 void
 305 replace_ssa_name_symbol (tree ssa_name, tree sym)
 306 {
 307   SSA_NAME_VAR (ssa_name) = sym;
 308   TREE_TYPE (ssa_name) = TREE_TYPE (sym);
 309 }
 310
 311 #include "gt-tree-ssanames.h"