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 /* 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.  */
  71
  72 void
  73 init_ssanames (void)
  74 {
  75   SSANAMES (cfun) = VEC_alloc (tree, gc, 50);
  76
  77   /* Version 0 is special, so reserve the first slot in the table.  Though
  78      currently unused, we may use version 0 in alias analysis as part of
  79      the heuristics used to group aliases when the alias sets are too
  80      large.
  81
  82      We use VEC_quick_push here because we know that SSA_NAMES has at
  83      least 50 elements reserved in it.  */
  84   VEC_quick_push (tree, SSANAMES (cfun), NULL_TREE);
  85   FREE_SSANAMES (cfun) = NULL;
  86 }
  87
  88 /* Finalize management of SSA_NAMEs.  */
  89
  90 void
  91 fini_ssanames (void)
  92 {
  93   VEC_free (tree, gc, SSANAMES (cfun));
  94   FREE_SSANAMES (cfun) = NULL;
  95 }
  96
  97 /* Dump some simple statistics regarding the re-use of SSA_NAME nodes.  */
  98
  99 #ifdef GATHER_STATISTICS
 100 void
 101 ssanames_print_statistics (void)
 102 {
 103   fprintf (stderr, "SSA_NAME nodes allocated: %u\n", ssa_name_nodes_created);
 104   fprintf (stderr, "SSA_NAME nodes reused: %u\n", ssa_name_nodes_reused);
 105 }
 106 #endif
 107
 108 /* Return an SSA_NAME node for variable VAR defined in statement STMT.
 109    STMT may be an empty statement for artificial references (e.g., default
 110    definitions created when a variable is used without a preceding
 111    definition).  */
 112
 113 tree
 114 make_ssa_name (tree var, tree stmt)
 115 {
 116   tree t;
 117   use_operand_p imm;
 118
 119   gcc_assert (DECL_P (var)
 120               || TREE_CODE (var) == INDIRECT_REF);
 121
 122   gcc_assert (!stmt || EXPR_P (stmt) || TREE_CODE (stmt) == PHI_NODE);
 123
 124   /* If our free list has an element, then use it.  */
 125   if (FREE_SSANAMES (cfun))
 126     {
 127       t = FREE_SSANAMES (cfun);
 128       FREE_SSANAMES (cfun) = TREE_CHAIN (FREE_SSANAMES (cfun));
 129 #ifdef GATHER_STATISTICS
 130       ssa_name_nodes_reused++;
 131 #endif
 132
 133       /* The node was cleared out when we put it on the free list, so
 134          there is no need to do so again here.  */
 135       gcc_assert (ssa_name (SSA_NAME_VERSION (t)) == NULL);
 136       VEC_replace (tree, SSANAMES (cfun), SSA_NAME_VERSION (t), t);
 137     }
 138   else
 139     {
 140       t = make_node (SSA_NAME);
 141       SSA_NAME_VERSION (t) = num_ssa_names;
 142       VEC_safe_push (tree, gc, SSANAMES (cfun), t);
 143 #ifdef GATHER_STATISTICS
 144       ssa_name_nodes_created++;
 145 #endif
 146     }
 147
 148   TREE_TYPE (t) = TREE_TYPE (var);
 149   SSA_NAME_VAR (t) = var;
 150   SSA_NAME_DEF_STMT (t) = stmt;
 151   SSA_NAME_PTR_INFO (t) = NULL;
 152   SSA_NAME_IN_FREE_LIST (t) = 0;
 153   imm = &(SSA_NAME_IMM_USE_NODE (t));
 154   imm->use = NULL;
 155   imm->prev = imm;
 156   imm->next = imm;
 157   imm->stmt = t;
 158
 159   return t;
 160 }
 161
 162
 163 /* We no longer need the SSA_NAME expression VAR, release it so that
 164    it may be reused.
 165
 166    Note it is assumed that no calls to make_ssa_name will be made
 167    until all uses of the ssa name are released and that the only
 168    use of the SSA_NAME expression is to check its SSA_NAME_VAR.  All
 169    other fields must be assumed clobbered.  */
 170
 171 void
 172 release_ssa_name (tree var)
 173 {
 174   if (!var)
 175     return;
 176
 177   /* Never release the default definition for a symbol.  It's a
 178      special SSA name that should always exist once it's created.  */
 179   if (var == gimple_default_def (cfun, SSA_NAME_VAR (var)))
 180     return;
 181
 182   /* If VAR has been registered for SSA updating, don't remove it.
 183      After update_ssa has run, the name will be released.  */
 184   if (name_registered_for_update_p (var))
 185     {
 186       release_ssa_name_after_update_ssa (var);
 187       return;
 188     }
 189
 190   /* release_ssa_name can be called multiple times on a single SSA_NAME.
 191      However, it should only end up on our free list one time.   We
 192      keep a status bit in the SSA_NAME node itself to indicate it has
 193      been put on the free list.
 194
 195      Note that once on the freelist you can not reference the SSA_NAME's
 196      defining statement.  */
 197   if (! SSA_NAME_IN_FREE_LIST (var))
 198     {
 199       tree saved_ssa_name_var = SSA_NAME_VAR (var);
 200       int saved_ssa_name_version = SSA_NAME_VERSION (var);
 201       use_operand_p imm = &(SSA_NAME_IMM_USE_NODE (var));
 202
 203 #ifdef ENABLE_CHECKING
 204       verify_imm_links (stderr, var);
 205 #endif
 206       while (imm->next != imm)
 207         delink_imm_use (imm->next);
 208
 209       VEC_replace (tree, SSANAMES (cfun),
 210                    SSA_NAME_VERSION (var), NULL_TREE);
 211       memset (var, 0, tree_size (var));
 212
 213       imm->prev = imm;
 214       imm->next = imm;
 215       imm->stmt = var;
 216       /* First put back the right tree node so that the tree checking
 217          macros do not complain.  */
 218       TREE_SET_CODE (var, SSA_NAME);
 219
 220       /* Restore the version number.  */
 221       SSA_NAME_VERSION (var) = saved_ssa_name_version;
 222
 223       /* Hopefully this can go away once we have the new incremental
 224          SSA updating code installed.  */
 225       SSA_NAME_VAR (var) = saved_ssa_name_var;
 226
 227       /* Note this SSA_NAME is now in the first list.  */
 228       SSA_NAME_IN_FREE_LIST (var) = 1;
 229
 230       /* And finally link it into the free list.  */
 231       TREE_CHAIN (var) = FREE_SSANAMES (cfun);
 232       FREE_SSANAMES (cfun) = var;
 233     }
 234 }
 235
 236 /* Creates a duplicate of a ssa name NAME defined in statement STMT.  */
 237
 238 tree
 239 duplicate_ssa_name (tree name, tree stmt)
 240 {
 241   tree new_name = make_ssa_name (SSA_NAME_VAR (name), stmt);
 242   struct ptr_info_def *old_ptr_info = SSA_NAME_PTR_INFO (name);
 243
 244   if (old_ptr_info)
 245     duplicate_ssa_name_ptr_info (new_name, old_ptr_info);
 246
 247   return new_name;
 248 }
 249
 250
 251 /* Creates a duplicate of the ptr_info_def at PTR_INFO for use by
 252    the SSA name NAME.  */
 253
 254 void
 255 duplicate_ssa_name_ptr_info (tree name, struct ptr_info_def *ptr_info)
 256 {
 257   struct ptr_info_def *new_ptr_info;
 258
 259   gcc_assert (POINTER_TYPE_P (TREE_TYPE (name)));
 260   gcc_assert (!SSA_NAME_PTR_INFO (name));
 261
 262   if (!ptr_info)
 263     return;
 264
 265   new_ptr_info = ggc_alloc (sizeof (struct ptr_info_def));
 266   *new_ptr_info = *ptr_info;
 267
 268   if (ptr_info->pt_vars)
 269     {
 270       new_ptr_info->pt_vars = BITMAP_GGC_ALLOC ();
 271       bitmap_copy (new_ptr_info->pt_vars, ptr_info->pt_vars);
 272     }
 273
 274   SSA_NAME_PTR_INFO (name) = new_ptr_info;
 275 }
 276
 277
 278 /* Release all the SSA_NAMEs created by STMT.  */
 279
 280 void
 281 release_defs (tree stmt)
 282 {
 283   tree def;
 284   ssa_op_iter iter;
 285
 286   /* Make sure that we are in SSA.  Otherwise, operand cache may point
 287      to garbage.  */
 288   gcc_assert (gimple_in_ssa_p (cfun));
 289
 290   FOR_EACH_SSA_TREE_OPERAND (def, stmt, iter, SSA_OP_ALL_DEFS)
 291     if (TREE_CODE (def) == SSA_NAME)
 292       release_ssa_name (def);
 293 }
 294
 295
 296 /* Replace the symbol associated with SSA_NAME with SYM.  */
 297
 298 void
 299 replace_ssa_name_symbol (tree ssa_name, tree sym)
 300 {
 301   SSA_NAME_VAR (ssa_name) = sym;
 302   TREE_TYPE (ssa_name) = TREE_TYPE (sym);
 303 }