gcc/tree-ssa-copyrename.c

   1 /* Rename SSA copies.
   2    Copyright (C) 2004 Free Software Foundation, Inc.
   3    Contributed by Andrew MacLeod <amacleod@redhat.com>
   4
   5 This file is part of GCC.
   6
   7 GCC is free software; you can redistribute it and/or modify
   8 it under the terms of the GNU General Public License as published by
   9 the Free Software Foundation; either version 2, or (at your option)
  10 any later version.
  11
  12 GCC is distributed in the hope that it will be useful,
  13 but WITHOUT ANY WARRANTY; without even the implied warranty of
  14 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  15 GNU General Public License for more details.
  16
  17 You should have received a copy of the GNU General Public License
  18 along with GCC; see the file COPYING.  If not, write to
  19 the Free Software Foundation, 51 Franklin Street, Fifth Floor,
  20 Boston, MA 02110-1301, USA.  */
  21
  22 #include "config.h"
  23 #include "system.h"
  24 #include "coretypes.h"
  25 #include "tm.h"
  26 #include "tree.h"
  27 #include "flags.h"
  28 #include "basic-block.h"
  29 #include "function.h"
  30 #include "diagnostic.h"
  31 #include "bitmap.h"
  32 #include "tree-flow.h"
  33 #include "tree-gimple.h"
  34 #include "tree-inline.h"
  35 #include "timevar.h"
  36 #include "hashtab.h"
  37 #include "tree-dump.h"
  38 #include "tree-ssa-live.h"
  39 #include "tree-pass.h"
  40 #include "langhooks.h"
  41
  42 /* The following routines implement the SSA copy renaming phase.
  43
  44    This optimization looks for copies between 2 SSA_NAMES, either through a
  45    direct copy, or an implicit one via a PHI node result and its arguments.
  46
  47    Each copy is examined to determine if it is possible to rename the base
  48    variable of one of the operands to the same variable as the other operand.
  49    i.e.
  50    T.3_5 = <blah>
  51    a_1 = T.3_5
  52
  53    If this copy couldn't be copy propagated, it could possibly remain in the
  54    program throughout the optimization phases.   After SSA->normal, it would
  55    become:
  56
  57    T.3 = <blah>
  58    a = T.3
  59
  60    Since T.3_5 is distinct from all other SSA versions of T.3, there is no
  61    fundamental reason why the base variable needs to be T.3, subject to
  62    certain restrictions.  This optimization attempts to determine if we can
  63    change the base variable on copies like this, and result in code such as:
  64
  65    a_5 = <blah>
  66    a_1 = a_5
  67
  68    This gives the SSA->normal pass a shot at coalescing a_1 and a_5. If it is
  69    possible, the copy goes away completely. If it isn't possible, a new temp
  70    will be created for a_5, and you will end up with the exact same code:
  71
  72    a.8 = <blah>
  73    a = a.8
  74
  75    The other benefit of performing this optimization relates to what variables
  76    are chosen in copies.  Gimplification of the program uses temporaries for
  77    a lot of things. expressions like
  78
  79    a_1 = <blah>
  80    <blah2> = a_1
  81
  82    get turned into
  83
  84    T.3_5 = <blah>
  85    a_1 = T.3_5
  86    <blah2> = a_1
  87
  88    Copy propagation is done in a forward direction, and if we can propagate
  89    through the copy, we end up with:
  90
  91    T.3_5 = <blah>
  92    <blah2> = T.3_5
  93
  94    The copy is gone, but so is all reference to the user variable 'a'. By
  95    performing this optimization, we would see the sequence:
  96
  97    a_5 = <blah>
  98    a_1 = a_5
  99    <blah2> = a_1
 100
 101    which copy propagation would then turn into:
 102
 103    a_5 = <blah>
 104    <blah2> = a_5
 105
 106    and so we still retain the user variable whenever possible.  */
 107
 108
 109 /* Coalesce the partitions in MAP representing VAR1 and VAR2 if it is valid.
 110    Choose a representative for the partition, and send debug info to DEBUG.  */
 111
 112 static void
 113 copy_rename_partition_coalesce (var_map map, tree var1, tree var2, FILE *debug)
 114 {
 115   int p1, p2, p3;
 116   tree root1, root2;
 117   tree rep1, rep2;
 118   var_ann_t ann1, ann2, ann3;
 119   bool ign1, ign2, abnorm;
 120
 121   gcc_assert (TREE_CODE (var1) == SSA_NAME);
 122   gcc_assert (TREE_CODE (var2) == SSA_NAME);
 123
 124   register_ssa_partition (map, var1, false);
 125   register_ssa_partition (map, var2, true);
 126
 127   p1 = partition_find (map->var_partition, SSA_NAME_VERSION (var1));
 128   p2 = partition_find (map->var_partition, SSA_NAME_VERSION (var2));
 129
 130   if (debug)
 131     {
 132       fprintf (debug, "Try : ");
 133       print_generic_expr (debug, var1, TDF_SLIM);
 134       fprintf (debug, "(P%d) & ", p1);
 135       print_generic_expr (debug, var2, TDF_SLIM);
 136       fprintf (debug, "(P%d)", p2);
 137     }
 138
 139   gcc_assert (p1 != NO_PARTITION);
 140   gcc_assert (p2 != NO_PARTITION);
 141
 142   rep1 = partition_to_var (map, p1);
 143   rep2 = partition_to_var (map, p2);
 144   root1 = SSA_NAME_VAR (rep1);
 145   root2 = SSA_NAME_VAR (rep2);
 146
 147   ann1 = var_ann (root1);
 148   ann2 = var_ann (root2);
 149
 150   if (p1 == p2)
 151     {
 152       if (debug)
 153         fprintf (debug, " : Already coalesced.\n");
 154       return;
 155     }
 156
 157   /* Don't coalesce if one of the variables occurs in an abnormal PHI.  */
 158   abnorm = (SSA_NAME_OCCURS_IN_ABNORMAL_PHI (rep1)
 159             || SSA_NAME_OCCURS_IN_ABNORMAL_PHI (rep2));
 160   if (abnorm)
 161     {
 162       if (debug)
 163         fprintf (debug, " : Abnormal PHI barrier.  No coalesce.\n");
 164       return;
 165     }
 166
 167   /* Partitions already have the same root, simply merge them.  */
 168   if (root1 == root2)
 169     {
 170       p1 = partition_union (map->var_partition, p1, p2);
 171       if (debug)
 172         fprintf (debug, " : Same root, coalesced --> P%d.\n", p1);
 173       return;
 174     }
 175
 176   /* Never attempt to coalesce 2 difference parameters.  */
 177   if (TREE_CODE (root1) == PARM_DECL && TREE_CODE (root2) == PARM_DECL)
 178     {
 179       if (debug)
 180         fprintf (debug, " : 2 different PARM_DECLS. No coalesce.\n");
 181       return;
 182     }
 183
 184   if ((TREE_CODE (root1) == RESULT_DECL) != (TREE_CODE (root2) == RESULT_DECL))
 185     {
 186       if (debug)
 187         fprintf (debug, " : One root a RESULT_DECL. No coalesce.\n");
 188       return;
 189     }
 190
 191   ign1 = TREE_CODE (root1) == VAR_DECL && DECL_IGNORED_P (root1);
 192   ign2 = TREE_CODE (root2) == VAR_DECL && DECL_IGNORED_P (root2);
 193
 194   /* Never attempt to coalesce 2 user variables unless one is an inline
 195      variable.  */
 196   if (!ign1 && !ign2)
 197     {
 198       if (DECL_FROM_INLINE (root2))
 199         ign2 = true;
 200       else if (DECL_FROM_INLINE (root1))
 201         ign1 = true;
 202       else
 203         {
 204           if (debug)
 205             fprintf (debug, " : 2 different USER vars. No coalesce.\n");
 206           return;
 207         }
 208     }
 209
 210   /* Don't coalesce if there are two different memory tags.  */
 211   if (ann1->type_mem_tag && ann2->type_mem_tag
 212       && ann1->type_mem_tag != ann2->type_mem_tag)
 213     {
 214       if (debug)
 215         fprintf (debug, " : 2 memory tags. No coalesce.\n");
 216       return;
 217     }
 218
 219   /* If both values have default defs, we can't coalesce.  If only one has a
 220      tag, make sure that variable is the new root partition.  */
 221   if (default_def (root1))
 222     {
 223       if (default_def (root2))
 224         {
 225           if (debug)
 226             fprintf (debug, " : 2 default defs. No coalesce.\n");
 227           return;
 228         }
 229       else
 230         {
 231           ign2 = true;
 232           ign1 = false;
 233         }
 234     }
 235   else if (default_def (root2))
 236     {
 237       ign1 = true;
 238       ign2 = false;
 239     }
 240
 241   /* Don't coalesce if the two variables aren't type compatible.  */
 242   if (!lang_hooks.types_compatible_p (TREE_TYPE (root1), TREE_TYPE (root2)))
 243     {
 244       if (debug)
 245         fprintf (debug, " : Incompatible types.  No coalesce.\n");
 246       return;
 247     }
 248
 249   /* Don't coalesce if the aliasing sets of the types are different.  */
 250   if (POINTER_TYPE_P (TREE_TYPE (root1))
 251       && POINTER_TYPE_P (TREE_TYPE (root2))
 252       && get_alias_set (TREE_TYPE (TREE_TYPE (root1)))
 253           != get_alias_set (TREE_TYPE (TREE_TYPE (root2))))
 254     {
 255       if (debug)
 256         fprintf (debug, " : 2 different aliasing sets. No coalesce.\n");
 257       return;
 258     }
 259
 260
 261   /* Merge the two partitions.  */
 262   p3 = partition_union (map->var_partition, p1, p2);
 263
 264   /* Set the root variable of the partition to the better choice, if there is
 265      one.  */
 266   if (!ign2)
 267     replace_ssa_name_symbol (partition_to_var (map, p3), root2);
 268   else if (!ign1)
 269     replace_ssa_name_symbol (partition_to_var (map, p3), root1);
 270
 271   /* Update the various flag widgitry of the current base representative.  */
 272   ann3 = var_ann (SSA_NAME_VAR (partition_to_var (map, p3)));
 273   if (ann1->type_mem_tag)
 274     ann3->type_mem_tag = ann1->type_mem_tag;
 275   else
 276     ann3->type_mem_tag = ann2->type_mem_tag;
 277
 278   if (debug)
 279     {
 280       fprintf (debug, " --> P%d ", p3);
 281       print_generic_expr (debug, SSA_NAME_VAR (partition_to_var (map, p3)),
 282                           TDF_SLIM);
 283       fprintf (debug, "\n");
 284     }
 285 }
 286
 287
 288 /* This function will make a pass through the IL, and attempt to coalesce any
 289    SSA versions which occur in PHI's or copies.  Coalescing is accomplished by
 290    changing the underlying root variable of all coalesced version.  This will
 291    then cause the SSA->normal pass to attempt to coalesce them all to the same
 292    variable.  */
 293
 294 static void
 295 rename_ssa_copies (void)
 296 {
 297   var_map map;
 298   basic_block bb;
 299   block_stmt_iterator bsi;
 300   tree phi, stmt, var, part_var;
 301   unsigned x;
 302   FILE *debug;
 303
 304   if (dump_file && (dump_flags & TDF_DETAILS))
 305     debug = dump_file;
 306   else
 307     debug = NULL;
 308
 309   map = init_var_map (num_ssa_names + 1);
 310
 311   FOR_EACH_BB (bb)
 312     {
 313       /* Scan for real copies.  */
 314       for (bsi = bsi_start (bb); !bsi_end_p (bsi); bsi_next (&bsi))
 315         {
 316           stmt = bsi_stmt (bsi);
 317           if (TREE_CODE (stmt) == MODIFY_EXPR)
 318             {
 319               tree lhs = TREE_OPERAND (stmt, 0);
 320               tree rhs = TREE_OPERAND (stmt, 1);
 321
 322               if (TREE_CODE (lhs) == SSA_NAME && TREE_CODE (rhs) == SSA_NAME)
 323                 copy_rename_partition_coalesce (map, lhs, rhs, debug);
 324             }
 325         }
 326     }
 327
 328   FOR_EACH_BB (bb)
 329     {
 330       /* Treat PHI nodes as copies between the result and each argument.  */
 331       for (phi = phi_nodes (bb); phi; phi = PHI_CHAIN (phi))
 332         {
 333           int i;
 334           tree res = PHI_RESULT (phi);
 335
 336           /* Do not process virtual SSA_NAMES.  */
 337           if (!is_gimple_reg (SSA_NAME_VAR (res)))
 338             continue;
 339
 340           for (i = 0; i < PHI_NUM_ARGS (phi); i++)
 341             {
 342               tree arg = PHI_ARG_DEF (phi, i);
 343               if (TREE_CODE (arg) == SSA_NAME)
 344                 copy_rename_partition_coalesce (map, res, arg, debug);
 345             }
 346         }
 347     }
 348
 349   if (debug)
 350     dump_var_map (debug, map);
 351
 352   /* Now one more pass to make all elements of a partition share the same
 353      root variable.  */
 354
 355   for (x = 1; x <= num_ssa_names; x++)
 356     {
 357       part_var = partition_to_var (map, x);
 358       if (!part_var)
 359         continue;
 360       var = map->partition_to_var[x];
 361       if (debug)
 362         {
 363           if (SSA_NAME_VAR (var) != SSA_NAME_VAR (part_var))
 364             {
 365               fprintf (debug, "Coalesced ");
 366               print_generic_expr (debug, var, TDF_SLIM);
 367               fprintf (debug, " to ");
 368               print_generic_expr (debug, part_var, TDF_SLIM);
 369               fprintf (debug, "\n");
 370             }
 371         }
 372       replace_ssa_name_symbol (var, SSA_NAME_VAR (part_var));
 373     }
 374
 375   delete_var_map (map);
 376 }
 377
 378 /* Return true if copy rename is to be performed.  */
 379
 380 static bool
 381 gate_copyrename (void)
 382 {
 383   return flag_tree_copyrename != 0;
 384 }
 385
 386 struct tree_opt_pass pass_rename_ssa_copies =
 387 {
 388   "copyrename",                         /* name */
 389   gate_copyrename,                      /* gate */
 390   rename_ssa_copies,                    /* execute */
 391   NULL,                                 /* sub */
 392   NULL,                                 /* next */
 393   0,                                    /* static_pass_number */
 394   TV_TREE_COPY_RENAME,                  /* tv_id */
 395   PROP_cfg | PROP_ssa | PROP_alias,     /* properties_required */
 396   0,                                    /* properties_provided */
 397   0,                                    /* properties_destroyed */
 398   0,                                    /* todo_flags_start */
 399   TODO_dump_func | TODO_verify_ssa,     /* todo_flags_finish */
 400   0                                     /* letter */
 401 };
 402