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
  29 /* Rewriting a function into SSA form can create a huge number of SSA_NAMEs,
  30    many of which may be thrown away shortly after their creation if jumps
  31    were threaded through PHI nodes.
  32
  33    While our garbage collection mechanisms will handle this situation, it
  34    is extremely wasteful to create nodes and throw them away, especially
  35    when the nodes can be reused.
  36
  37    For PR 8361, we can significantly reduce the number of nodes allocated
  38    and thus the total amount of memory allocated by managing SSA_NAMEs a
  39    little.  This additionally helps reduce the amount of work done by the
  40    garbage collector.  Similar results have been seen on a wider variety
  41    of tests (such as the compiler itself).
  42
  43    Right now we maintain our free list on a per-function basis.  It may
  44    or may not make sense to maintain the free list for the duration of
  45    a compilation unit.
  46
  47    External code should rely solely upon HIGHEST_SSA_VERSION and the
  48    externally defined functions.  External code should not know about
  49    the details of the free list management.
  50
  51    External code should also not assume the version number on nodes is
  52    monotonically increasing.  We reuse the version number when we
  53    reuse an SSA_NAME expression.  This helps keep arrays and bitmaps
  54    more compact.
  55
  56    We could also use a zone allocator for these objects since they have
  57    a very well defined lifetime.  If someone wants to experiment with that
  58    this is the place to try it.  */
  59
  60 /* Array of all SSA_NAMEs used in the function.  */
  61 VEC(tree,gc) *ssa_names;
  62
  63 /* Free list of SSA_NAMEs.  This list is wiped at the end of each function
  64    after we leave SSA form.  */
  65 static GTY (()) tree free_ssanames;
  66
  67 /* Version numbers with special meanings.  We start allocating new version
  68    numbers after the special ones.  */
  69 #define UNUSED_NAME_VERSION 0
  70
  71 #ifdef GATHER_STATISTICS
  72 unsigned int ssa_name_nodes_reused;
  73 unsigned int ssa_name_nodes_created;
  74 #endif
  75
  76 /* Initialize management of SSA_NAMEs.  */
  77
  78 void
  79 init_ssanames (void)
  80 {
  81   ssa_names = VEC_alloc (tree, gc, 50);
  82
  83   /* Version 0 is special, so reserve the first slot in the table.  Though
  84      currently unused, we may use version 0 in alias analysis as part of
  85      the heuristics used to group aliases when the alias sets are too
  86      large.
  87
  88      We use VEC_quick_push here because we know that SSA_NAMES has at
  89      least 50 elements reserved in it.  */
  90   VEC_quick_push (tree, ssa_names, NULL_TREE);
  91   free_ssanames = NULL;
  92 }
  93
  94 /* Finalize management of SSA_NAMEs.  */
  95
  96 void
  97 fini_ssanames (void)
  98 {
  99   VEC_free (tree, gc, ssa_names);
 100   free_ssanames = 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    STMT may be an empty statement for artificial references (e.g., default
 116    definitions created when a variable is used without a preceding
 117    definition).  */
 118
 119 tree
 120 make_ssa_name (tree var, tree stmt)
 121 {
 122   tree t;
 123   use_operand_p imm;
 124
 125   gcc_assert (DECL_P (var)
 126               || TREE_CODE (var) == INDIRECT_REF);
 127
 128   gcc_assert (!stmt || EXPR_P (stmt) || TREE_CODE (stmt) == PHI_NODE);
 129
 130   /* If our free list has an element, then use it.  */
 131   if (free_ssanames)
 132     {
 133       t = free_ssanames;
 134       free_ssanames = TREE_CHAIN (free_ssanames);
 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, ssa_names, SSA_NAME_VERSION (t), t);
 143     }
 144   else
 145     {
 146       t = make_node (SSA_NAME);
 147       SSA_NAME_VERSION (t) = num_ssa_names;
 148       VEC_safe_push (tree, gc, ssa_names, 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   imm = &(SSA_NAME_IMM_USE_NODE (t));
 160   imm->use = NULL;
 161   imm->prev = imm;
 162   imm->next = imm;
 163   imm->stmt = t;
 164
 165   return t;
 166 }
 167
 168
 169 /* We no longer need the SSA_NAME expression VAR, release it so that
 170    it may be reused.
 171
 172    Note it is assumed that no calls to make_ssa_name will be made
 173    until all uses of the ssa name are released and that the only
 174    use of the SSA_NAME expression is to check its SSA_NAME_VAR.  All
 175    other fields must be assumed clobbered.  */
 176
 177 void
 178 release_ssa_name (tree var)
 179 {
 180   if (!var)
 181     return;
 182
 183   /* Never release the default definition for a symbol.  It's a
 184      special SSA name that should always exist once it's created.  */
 185   if (var == default_def (SSA_NAME_VAR (var)))
 186     return;
 187
 188   /* If VAR has been registered for SSA updating, don't remove it.
 189      After update_ssa has run, the name will be released.  */
 190   if (name_registered_for_update_p (var))
 191     {
 192       release_ssa_name_after_update_ssa (var);
 193       return;
 194     }
 195
 196   /* release_ssa_name can be called multiple times on a single SSA_NAME.
 197      However, it should only end up on our free list one time.   We
 198      keep a status bit in the SSA_NAME node itself to indicate it has
 199      been put on the free list.
 200
 201      Note that once on the freelist you can not reference the SSA_NAME's
 202      defining statement.  */
 203   if (! SSA_NAME_IN_FREE_LIST (var))
 204     {
 205       tree saved_ssa_name_var = SSA_NAME_VAR (var);
 206       int saved_ssa_name_version = SSA_NAME_VERSION (var);
 207       use_operand_p imm = &(SSA_NAME_IMM_USE_NODE (var));
 208
 209 #ifdef ENABLE_CHECKING
 210       verify_imm_links (stderr, var);
 211 #endif
 212       while (imm->next != imm)
 213         delink_imm_use (imm->next);
 214
 215       VEC_replace (tree, ssa_names, SSA_NAME_VERSION (var), NULL_TREE);
 216       memset (var, 0, tree_size (var));
 217
 218       imm->prev = imm;
 219       imm->next = imm;
 220       imm->stmt = var;
 221       /* First put back the right tree node so that the tree checking
 222          macros do not complain.  */
 223       TREE_SET_CODE (var, SSA_NAME);
 224
 225       /* Restore the version number.  */
 226       SSA_NAME_VERSION (var) = saved_ssa_name_version;
 227
 228       /* Hopefully this can go away once we have the new incremental
 229          SSA updating code installed.  */
 230       SSA_NAME_VAR (var) = saved_ssa_name_var;
 231
 232       /* Note this SSA_NAME is now in the first list.  */
 233       SSA_NAME_IN_FREE_LIST (var) = 1;
 234
 235       /* And finally link it into the free list.  */
 236       TREE_CHAIN (var) = free_ssanames;
 237       free_ssanames = var;
 238     }
 239 }
 240
 241 /* Creates a duplicate of a ssa name NAME defined in statement STMT.  */
 242
 243 tree
 244 duplicate_ssa_name (tree name, tree stmt)
 245 {
 246   tree new_name = make_ssa_name (SSA_NAME_VAR (name), stmt);
 247   struct ptr_info_def *old_ptr_info = SSA_NAME_PTR_INFO (name);
 248
 249   if (old_ptr_info)
 250     duplicate_ssa_name_ptr_info (new_name, old_ptr_info);
 251
 252   return new_name;
 253 }
 254
 255
 256 /* Creates a duplicate of the ptr_info_def at PTR_INFO for use by
 257    the SSA name NAME.  */
 258
 259 void
 260 duplicate_ssa_name_ptr_info (tree name, struct ptr_info_def *ptr_info)
 261 {
 262   struct ptr_info_def *new_ptr_info;
 263
 264   gcc_assert (POINTER_TYPE_P (TREE_TYPE (name)));
 265   gcc_assert (!SSA_NAME_PTR_INFO (name));
 266
 267   if (!ptr_info)
 268     return;
 269
 270   new_ptr_info = ggc_alloc (sizeof (struct ptr_info_def));
 271   *new_ptr_info = *ptr_info;
 272
 273   if (ptr_info->pt_vars)
 274     {
 275       new_ptr_info->pt_vars = BITMAP_GGC_ALLOC ();
 276       bitmap_copy (new_ptr_info->pt_vars, ptr_info->pt_vars);
 277     }
 278
 279   SSA_NAME_PTR_INFO (name) = new_ptr_info;
 280 }
 281
 282
 283 /* Release all the SSA_NAMEs created by STMT.  */
 284
 285 void
 286 release_defs (tree stmt)
 287 {
 288   tree def;
 289   ssa_op_iter iter;
 290
 291   /* Make sure that we are in SSA.  Otherwise, operand cache may point
 292      to garbage.  */
 293   gcc_assert (in_ssa_p);
 294
 295   FOR_EACH_SSA_TREE_OPERAND (def, stmt, iter, SSA_OP_ALL_DEFS)
 296     if (TREE_CODE (def) == SSA_NAME)
 297       release_ssa_name (def);
 298 }
 299
 300
 301 /* Replace the symbol associated with SSA_NAME with SYM.  */
 302
 303 void
 304 replace_ssa_name_symbol (tree ssa_name, tree sym)
 305 {
 306   SSA_NAME_VAR (ssa_name) = sym;
 307   TREE_TYPE (ssa_name) = TREE_TYPE (sym);
 308 }
 309
 310 #include "gt-tree-ssanames.h"