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, 59 Temple Place - Suite 330,
  20 Boston, MA 02111-1307, 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 extern void rename_ssa_copies (void);
  43
  44 /* The following routines implement the SSA copy renaming phase.
  45
  46    This optimization looks for copies between 2 SSA_NAMES, either through a
  47    direct copy, or an implicit one via a PHI node result and its arguments.
  48
  49    Each copy is examined to determine if it is possible to rename the base
  50    variable of one of the operands to the same variable as the other operand.
  51    i.e.
  52    T.3_5 = <blah>
  53    a_1 = T.3_5
  54
  55    If this copy couldn't be copy propagated, it could possibly remain in the
  56    program throughout the optimization phases.   After SSA->normal, it would
  57    become:
  58
  59    T.3 = <blah>
  60    a = T.3
  61
  62    Since T.3_5 is distinct from all other SSA versions of T.3, there is no
  63    fundamental reason why the base variable needs to be T.3, subject to
  64    certain restrictions.  This optimization attempts to determine if we can
  65    change the base variable on copies like this, and result in code such as:
  66
  67    a_5 = <blah>
  68    a_1 = a_5
  69
  70    This gives the SSA->normal pass a shot at coalescing a_1 and a_5. If it is
  71    possible, the copy goes away completely. If it isn't possible, a new temp
  72    will be created for a_5, and you will end up with the exact same code:
  73
  74    a.8 = <blah>
  75    a = a.8
  76
  77    The other benefit of performing this optimization relates to what variables
  78    are chosen in copies.  Gimplification of the program uses temporaries for
  79    a lot of things. expressions like
  80
  81    a_1 = <blah>
  82    <blah2> = a_1
  83
  84    get turned into
  85
  86    T.3_5 = <blah>
  87    a_1 = T.3_5
  88    <blah2> = a_1
  89
  90    Copy propagation is done in a forward direction, and if we can propagate
  91    through the copy, we end up with:
  92
  93    T.3_5 = <blah>
  94    <blah2> = T.3_5
  95
  96    The copy is gone, but so is all reference to the user variable 'a'. By
  97    performing this optimization, we would see the sequence:
  98
  99    a_5 = <blah>
 100    a_1 = a_5
 101    <blah2> = a_1
 102
 103    which copy propagation would then turn into:
 104
 105    a_5 = <blah>
 106    <blah2> = a_5
 107
 108    and so we still retain the user variable whenever possible.  */
 109
 110
 111 /* Coalesce the partitions in MAP representing VAR1 and VAR2 if it is valid.
 112    Choose a representative for the partition, and send debug info to DEBUG.  */
 113
 114 static void
 115 copy_rename_partition_coalesce (var_map map, tree var1, tree var2, FILE *debug)
 116 {
 117   int p1, p2, p3;
 118   tree root1, root2;
 119   tree rep1, rep2;
 120   var_ann_t ann1, ann2, ann3;
 121   bool ign1, ign2, abnorm;
 122
 123   gcc_assert (TREE_CODE (var1) == SSA_NAME);
 124   gcc_assert (TREE_CODE (var2) == SSA_NAME);
 125
 126   register_ssa_partition (map, var1, false);
 127   register_ssa_partition (map, var2, true);
 128
 129   p1 = partition_find (map->var_partition, SSA_NAME_VERSION (var1));
 130   p2 = partition_find (map->var_partition, SSA_NAME_VERSION (var2));
 131
 132   if (debug)
 133     {
 134       fprintf (debug, "Try : ");
 135       print_generic_expr (debug, var1, TDF_SLIM);
 136       fprintf (debug, "(P%d) & ", p1);
 137       print_generic_expr (debug, var2, TDF_SLIM);
 138       fprintf (debug, "(P%d)", p2);
 139     }
 140
 141   gcc_assert (p1 != NO_PARTITION);
 142   gcc_assert (p2 != NO_PARTITION);
 143
 144   rep1 = partition_to_var (map, p1);
 145   rep2 = partition_to_var (map, p2);
 146   root1 = SSA_NAME_VAR (rep1);
 147   root2 = SSA_NAME_VAR (rep2);
 148
 149   if (DECL_HARD_REGISTER (root1) || DECL_HARD_REGISTER (root2))
 150     {
 151       if (debug)
 152         {
 153           if (DECL_HARD_REGISTER (root1))
 154             print_generic_expr (debug, var1, TDF_SLIM);
 155           else
 156             print_generic_expr (debug, var2, TDF_SLIM);
 157           fprintf (debug, " is a hardware register.  No Coalescing.\n");
 158         }
 159       return;
 160     }
 161
 162   ann1 = var_ann (root1);
 163   ann2 = var_ann (root2);
 164
 165   if (p1 == p2)
 166     {
 167       if (debug)
 168         fprintf (debug, " : Already coalesced.\n");
 169       return;
 170     }
 171
 172   /* Partitions already have the same root, simply merge them.  */
 173   if (root1 == root2)
 174     {
 175       p1 = partition_union (map->var_partition, p1, p2);
 176       if (debug)
 177         fprintf (debug, " : Same root, coalesced --> P%d.\n", p1);
 178       return;
 179     }
 180
 181   /* Never attempt to coalesce 2 difference parameters.  */
 182   if (TREE_CODE (root1) == PARM_DECL && TREE_CODE (root2) == PARM_DECL)
 183     {
 184       if (debug)
 185         fprintf (debug, " : 2 different PARM_DECLS. No coalesce.\n");
 186       return;
 187     }
 188
 189   if ((TREE_CODE (root1) == RESULT_DECL) != (TREE_CODE (root2) == RESULT_DECL))
 190     {
 191       if (debug)
 192         fprintf (debug, " : One root a RESULT_DECL. No coalesce.\n");
 193       return;
 194     }
 195
 196   ign1 = TREE_CODE (root1) == VAR_DECL && DECL_IGNORED_P (root1);
 197   ign2 = TREE_CODE (root2) == VAR_DECL && DECL_IGNORED_P (root2);
 198
 199   /* Never attempt to coalesce 2 user variables unless one is an inline
 200      variable.  */
 201   if (!ign1 && !ign2)
 202     {
 203       if (DECL_FROM_INLINE (root2))
 204         ign2 = true;
 205       else if (DECL_FROM_INLINE (root1))
 206         ign1 = true;
 207       else
 208         {
 209           if (debug)
 210             fprintf (debug, " : 2 different USER vars. No coalesce.\n");
 211           return;
 212         }
 213     }
 214
 215   /* Don't coalesce if there are two different memory tags.  */
 216   if (ann1->type_mem_tag && ann2->type_mem_tag
 217       && ann1->type_mem_tag != ann2->type_mem_tag)
 218     {
 219       if (debug)
 220         fprintf (debug, " : 2 memory tags. No coalesce.\n");
 221       return;
 222     }
 223
 224   /* If both values have default defs, we can't coalesce.  If only one has a
 225      tag, make sure that variable is the new root partition.  */
 226   if (default_def (root1))
 227     {
 228       if (default_def (root2))
 229         {
 230           if (debug)
 231             fprintf (debug, " : 2 default defs. No coalesce.\n");
 232           return;
 233         }
 234       else
 235         {
 236           ign2 = true;
 237           ign1 = false;
 238         }
 239     }
 240   else if (default_def (root2))
 241     {
 242       ign1 = true;
 243       ign2 = false;
 244     }
 245
 246   /* Don't coalesce if the two variables aren't type compatible.  */
 247   if (!lang_hooks.types_compatible_p (TREE_TYPE (root1), TREE_TYPE (root2)))
 248     {
 249       if (debug)
 250         fprintf (debug, " : Incompatible types.  No coalesce.\n");
 251       return;
 252     }
 253
 254   /* Don't coalesce if one of the variables occurs in an abnormal PHI.  */
 255   abnorm = (SSA_NAME_OCCURS_IN_ABNORMAL_PHI (rep1)
 256             || SSA_NAME_OCCURS_IN_ABNORMAL_PHI (rep2));
 257   if (abnorm)
 258     {
 259       if (debug)
 260         fprintf (debug, " : Abnormal PHI barrier.  No coalesce.\n");
 261       return;
 262     }
 263
 264   /* Merge the two partitions.  */
 265   p3 = partition_union (map->var_partition, p1, p2);
 266
 267   /* Set the root variable of the partition to the better choice, if there is
 268      one.  */
 269   if (!ign2)
 270     replace_ssa_name_symbol (partition_to_var (map, p3), root2);
 271   else if (!ign1)
 272     replace_ssa_name_symbol (partition_to_var (map, p3), root1);
 273
 274   /* Update the various flag widgitry of the current base representative.  */
 275   ann3 = var_ann (SSA_NAME_VAR (partition_to_var (map, p3)));
 276   if (ann1->type_mem_tag)
 277     ann3->type_mem_tag = ann1->type_mem_tag;
 278   else
 279     ann3->type_mem_tag = ann2->type_mem_tag;
 280
 281   if (debug)
 282     {
 283       fprintf (debug, " --> P%d ", p3);
 284       print_generic_expr (debug, SSA_NAME_VAR (partition_to_var (map, p3)),
 285                           TDF_SLIM);
 286       fprintf (debug, "\n");
 287     }
 288 }
 289
 290
 291 /* This function will make a pass through the IL, and attempt to coalesce any
 292    SSA versions which occur in PHI's or copies.  Coalescing is accomplished by
 293    changing the underlying root variable of all coalesced version.  This will
 294    then cause the SSA->normal pass to attempt to coalesce them all to the same
 295    variable.  */
 296
 297 void
 298 rename_ssa_copies (void)
 299 {
 300   var_map map;
 301   basic_block bb;
 302   block_stmt_iterator bsi;
 303   tree phi, stmt, var, part_var;
 304   unsigned x;
 305   FILE *debug;
 306
 307   if (dump_file && (dump_flags & TDF_DETAILS))
 308     debug = dump_file;
 309   else
 310     debug = NULL;
 311
 312   map = init_var_map (num_ssa_names + 1);
 313
 314   FOR_EACH_BB (bb)
 315     {
 316       /* Scan for real copies.  */
 317       for (bsi = bsi_start (bb); !bsi_end_p (bsi); bsi_next (&bsi))
 318         {
 319           stmt = bsi_stmt (bsi);
 320           if (TREE_CODE (stmt) == MODIFY_EXPR)
 321             {
 322               tree lhs = TREE_OPERAND (stmt, 0);
 323               tree rhs = TREE_OPERAND (stmt, 1);
 324
 325               if (TREE_CODE (lhs) == SSA_NAME && TREE_CODE (rhs) == SSA_NAME)
 326                 copy_rename_partition_coalesce (map, lhs, rhs, debug);
 327             }
 328         }
 329     }
 330
 331   FOR_EACH_BB (bb)
 332     {
 333       /* Treat PHI nodes as copies between the result and each argument.  */
 334       for (phi = phi_nodes (bb); phi; phi = PHI_CHAIN (phi))
 335         {
 336           int i;
 337           tree res = PHI_RESULT (phi);
 338
 339           /* Do not process virtual SSA_NAMES.  */
 340           if (!is_gimple_reg (SSA_NAME_VAR (res)))
 341             continue;
 342
 343           for (i = 0; i < PHI_NUM_ARGS (phi); i++)
 344             {
 345               tree arg = PHI_ARG_DEF (phi, i);
 346               if (TREE_CODE (arg) == SSA_NAME)
 347                 copy_rename_partition_coalesce (map, res, arg, debug);
 348             }
 349         }
 350     }
 351
 352   if (debug)
 353     dump_var_map (debug, map);
 354
 355   /* Now one more pass to make all elements of a partition share the same
 356      root variable.  */
 357
 358   for (x = 1; x <= num_ssa_names; x++)
 359     {
 360       part_var = partition_to_var (map, x);
 361       if (!part_var)
 362         continue;
 363       var = map->partition_to_var[x];
 364       if (debug)
 365         {
 366           if (SSA_NAME_VAR (var) != SSA_NAME_VAR (part_var))
 367             {
 368               fprintf (debug, "Coalesced ");
 369               print_generic_expr (debug, var, TDF_SLIM);
 370               fprintf (debug, " to ");
 371               print_generic_expr (debug, part_var, TDF_SLIM);
 372               fprintf (debug, "\n");
 373             }
 374         }
 375       replace_ssa_name_symbol (var, SSA_NAME_VAR (part_var));
 376     }
 377
 378   delete_var_map (map);
 379 }
 380
 381 /* Return true if copy rename is to be performed.  */
 382
 383 static bool
 384 gate_copyrename (void)
 385 {
 386   return flag_tree_copyrename != 0;
 387 }
 388
 389 struct tree_opt_pass pass_rename_ssa_copies =
 390 {
 391   "copyrename",                         /* name */
 392   gate_copyrename,                      /* gate */
 393   rename_ssa_copies,                    /* execute */
 394   NULL,                                 /* sub */
 395   NULL,                                 /* next */
 396   0,                                    /* static_pass_number */
 397   TV_TREE_COPY_RENAME,                  /* tv_id */
 398   PROP_cfg | PROP_ssa | PROP_alias,     /* properties_required */
 399   0,                                    /* properties_provided */
 400   0,                                    /* properties_destroyed */
 401   0,                                    /* todo_flags_start */
 402   TODO_dump_func | TODO_verify_ssa,     /* todo_flags_finish */
 403   0                                     /* letter */
 404 };
 405