gcc/tree-ssa-copyrename.c

   1 /* Rename SSA copies.
   2    Copyright (C) 2004, 2007 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 3, 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 COPYING3.  If not see
  19 <http://www.gnu.org/licenses/>.  */
  20
  21 #include "config.h"
  22 #include "system.h"
  23 #include "coretypes.h"
  24 #include "tm.h"
  25 #include "tree.h"
  26 #include "flags.h"
  27 #include "basic-block.h"
  28 #include "function.h"
  29 #include "diagnostic.h"
  30 #include "bitmap.h"
  31 #include "tree-flow.h"
  32 #include "tree-gimple.h"
  33 #include "tree-inline.h"
  34 #include "timevar.h"
  35 #include "hashtab.h"
  36 #include "tree-dump.h"
  37 #include "tree-ssa-live.h"
  38 #include "tree-pass.h"
  39 #include "langhooks.h"
  40
  41 /* The following routines implement the SSA copy renaming phase.
  42
  43    This optimization looks for copies between 2 SSA_NAMES, either through a
  44    direct copy, or an implicit one via a PHI node result and its arguments.
  45
  46    Each copy is examined to determine if it is possible to rename the base
  47    variable of one of the operands to the same variable as the other operand.
  48    i.e.
  49    T.3_5 = <blah>
  50    a_1 = T.3_5
  51
  52    If this copy couldn't be copy propagated, it could possibly remain in the
  53    program throughout the optimization phases.   After SSA->normal, it would
  54    become:
  55
  56    T.3 = <blah>
  57    a = T.3
  58
  59    Since T.3_5 is distinct from all other SSA versions of T.3, there is no
  60    fundamental reason why the base variable needs to be T.3, subject to
  61    certain restrictions.  This optimization attempts to determine if we can
  62    change the base variable on copies like this, and result in code such as:
  63
  64    a_5 = <blah>
  65    a_1 = a_5
  66
  67    This gives the SSA->normal pass a shot at coalescing a_1 and a_5. If it is
  68    possible, the copy goes away completely. If it isn't possible, a new temp
  69    will be created for a_5, and you will end up with the exact same code:
  70
  71    a.8 = <blah>
  72    a = a.8
  73
  74    The other benefit of performing this optimization relates to what variables
  75    are chosen in copies.  Gimplification of the program uses temporaries for
  76    a lot of things. expressions like
  77
  78    a_1 = <blah>
  79    <blah2> = a_1
  80
  81    get turned into
  82
  83    T.3_5 = <blah>
  84    a_1 = T.3_5
  85    <blah2> = a_1
  86
  87    Copy propagation is done in a forward direction, and if we can propagate
  88    through the copy, we end up with:
  89
  90    T.3_5 = <blah>
  91    <blah2> = T.3_5
  92
  93    The copy is gone, but so is all reference to the user variable 'a'. By
  94    performing this optimization, we would see the sequence:
  95
  96    a_5 = <blah>
  97    a_1 = a_5
  98    <blah2> = a_1
  99
 100    which copy propagation would then turn into:
 101
 102    a_5 = <blah>
 103    <blah2> = a_5
 104
 105    and so we still retain the user variable whenever possible.  */
 106
 107
 108 /* Coalesce the partitions in MAP representing VAR1 and VAR2 if it is valid.
 109    Choose a representative for the partition, and send debug info to DEBUG.  */
 110
 111 static bool
 112 copy_rename_partition_coalesce (var_map map, tree var1, tree var2, FILE *debug)
 113 {
 114   int p1, p2, p3;
 115   tree root1, root2;
 116   tree rep1, rep2;
 117   var_ann_t ann1, ann2, ann3;
 118   bool ign1, ign2, abnorm;
 119
 120   gcc_assert (TREE_CODE (var1) == SSA_NAME);
 121   gcc_assert (TREE_CODE (var2) == SSA_NAME);
 122
 123   register_ssa_partition (map, var1);
 124   register_ssa_partition (map, var2);
 125
 126   p1 = partition_find (map->var_partition, SSA_NAME_VERSION (var1));
 127   p2 = partition_find (map->var_partition, SSA_NAME_VERSION (var2));
 128
 129   if (debug)
 130     {
 131       fprintf (debug, "Try : ");
 132       print_generic_expr (debug, var1, TDF_SLIM);
 133       fprintf (debug, "(P%d) & ", p1);
 134       print_generic_expr (debug, var2, TDF_SLIM);
 135       fprintf (debug, "(P%d)", p2);
 136     }
 137
 138   gcc_assert (p1 != NO_PARTITION);
 139   gcc_assert (p2 != NO_PARTITION);
 140
 141   rep1 = partition_to_var (map, p1);
 142   rep2 = partition_to_var (map, p2);
 143   root1 = SSA_NAME_VAR (rep1);
 144   root2 = SSA_NAME_VAR (rep2);
 145
 146   ann1 = var_ann (root1);
 147   ann2 = var_ann (root2);
 148
 149   if (p1 == p2)
 150     {
 151       if (debug)
 152         fprintf (debug, " : Already coalesced.\n");
 153       return false;
 154     }
 155
 156   /* Don't coalesce if one of the variables occurs in an abnormal PHI.  */
 157   abnorm = (SSA_NAME_OCCURS_IN_ABNORMAL_PHI (rep1)
 158             || SSA_NAME_OCCURS_IN_ABNORMAL_PHI (rep2));
 159   if (abnorm)
 160     {
 161       if (debug)
 162         fprintf (debug, " : Abnormal PHI barrier.  No coalesce.\n");
 163       return false;
 164     }
 165
 166   /* Partitions already have the same root, simply merge them.  */
 167   if (root1 == root2)
 168     {
 169       p1 = partition_union (map->var_partition, p1, p2);
 170       if (debug)
 171         fprintf (debug, " : Same root, coalesced --> P%d.\n", p1);
 172       return false;
 173     }
 174
 175   /* Never attempt to coalesce 2 difference parameters.  */
 176   if (TREE_CODE (root1) == PARM_DECL && TREE_CODE (root2) == PARM_DECL)
 177     {
 178       if (debug)
 179         fprintf (debug, " : 2 different PARM_DECLS. No coalesce.\n");
 180       return false;
 181     }
 182
 183   if ((TREE_CODE (root1) == RESULT_DECL) != (TREE_CODE (root2) == RESULT_DECL))
 184     {
 185       if (debug)
 186         fprintf (debug, " : One root a RESULT_DECL. No coalesce.\n");
 187       return false;
 188     }
 189
 190   ign1 = TREE_CODE (root1) == VAR_DECL && DECL_IGNORED_P (root1);
 191   ign2 = TREE_CODE (root2) == VAR_DECL && DECL_IGNORED_P (root2);
 192
 193   /* Never attempt to coalesce 2 user variables unless one is an inline
 194      variable.  */
 195   if (!ign1 && !ign2)
 196     {
 197       if (DECL_FROM_INLINE (root2))
 198         ign2 = true;
 199       else if (DECL_FROM_INLINE (root1))
 200         ign1 = true;
 201       else
 202         {
 203           if (debug)
 204             fprintf (debug, " : 2 different USER vars. No coalesce.\n");
 205           return false;
 206         }
 207     }
 208
 209   /* Don't coalesce if there are two different memory tags.  */
 210   if (ann1->symbol_mem_tag
 211       && ann2->symbol_mem_tag
 212       && ann1->symbol_mem_tag != ann2->symbol_mem_tag)
 213     {
 214       if (debug)
 215         fprintf (debug, " : 2 memory tags. No coalesce.\n");
 216       return false;
 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 (gimple_default_def (cfun, root1))
 222     {
 223       if (gimple_default_def (cfun, root2))
 224         {
 225           if (debug)
 226             fprintf (debug, " : 2 default defs. No coalesce.\n");
 227           return false;
 228         }
 229       else
 230         {
 231           ign2 = true;
 232           ign1 = false;
 233         }
 234     }
 235   else if (gimple_default_def (cfun, root2))
 236     {
 237       ign1 = true;
 238       ign2 = false;
 239     }
 240
 241   /* Don't coalesce if the two variables aren't type compatible.  */
 242   if (!types_compatible_p (TREE_TYPE (root1), TREE_TYPE (root2)))
 243     {
 244       if (debug)
 245         fprintf (debug, " : Incompatible types.  No coalesce.\n");
 246       return false;
 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 false;
 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->symbol_mem_tag)
 274     ann3->symbol_mem_tag = ann1->symbol_mem_tag;
 275   else
 276     ann3->symbol_mem_tag = ann2->symbol_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   return true;
 286 }
 287
 288
 289 /* This function will make a pass through the IL, and attempt to coalesce any
 290    SSA versions which occur in PHI's or copies.  Coalescing is accomplished by
 291    changing the underlying root variable of all coalesced version.  This will
 292    then cause the SSA->normal pass to attempt to coalesce them all to the same
 293    variable.  */
 294
 295 static unsigned int
 296 rename_ssa_copies (void)
 297 {
 298   var_map map;
 299   basic_block bb;
 300   block_stmt_iterator bsi;
 301   tree phi, stmt, var, part_var;
 302   unsigned x;
 303   FILE *debug;
 304   bool updated = false;
 305
 306   if (dump_file && (dump_flags & TDF_DETAILS))
 307     debug = dump_file;
 308   else
 309     debug = NULL;
 310
 311   map = init_var_map (num_ssa_names + 1);
 312
 313   FOR_EACH_BB (bb)
 314     {
 315       /* Scan for real copies.  */
 316       for (bsi = bsi_start (bb); !bsi_end_p (bsi); bsi_next (&bsi))
 317         {
 318           stmt = bsi_stmt (bsi);
 319           if (TREE_CODE (stmt) == GIMPLE_MODIFY_STMT)
 320             {
 321               tree lhs = GIMPLE_STMT_OPERAND (stmt, 0);
 322               tree rhs = GIMPLE_STMT_OPERAND (stmt, 1);
 323
 324               if (TREE_CODE (lhs) == SSA_NAME && TREE_CODE (rhs) == SSA_NAME)
 325                 updated |= copy_rename_partition_coalesce (map, lhs, rhs, debug);
 326             }
 327         }
 328     }
 329
 330   FOR_EACH_BB (bb)
 331     {
 332       /* Treat PHI nodes as copies between the result and each argument.  */
 333       for (phi = phi_nodes (bb); phi; phi = PHI_CHAIN (phi))
 334         {
 335           int i;
 336           tree res = PHI_RESULT (phi);
 337
 338           /* Do not process virtual SSA_NAMES.  */
 339           if (!is_gimple_reg (SSA_NAME_VAR (res)))
 340             continue;
 341
 342           for (i = 0; i < PHI_NUM_ARGS (phi); i++)
 343             {
 344               tree arg = PHI_ARG_DEF (phi, i);
 345               if (TREE_CODE (arg) == SSA_NAME)
 346                 updated |= copy_rename_partition_coalesce (map, res, arg, debug);
 347             }
 348         }
 349     }
 350
 351   if (debug)
 352     dump_var_map (debug, map);
 353
 354   /* Now one more pass to make all elements of a partition share the same
 355      root variable.  */
 356
 357   for (x = 1; x <= num_ssa_names; x++)
 358     {
 359       part_var = partition_to_var (map, x);
 360       if (!part_var)
 361         continue;
 362       var = map->partition_to_var[x];
 363       if (debug)
 364         {
 365           if (SSA_NAME_VAR (var) != SSA_NAME_VAR (part_var))
 366             {
 367               fprintf (debug, "Coalesced ");
 368               print_generic_expr (debug, var, TDF_SLIM);
 369               fprintf (debug, " to ");
 370               print_generic_expr (debug, part_var, TDF_SLIM);
 371               fprintf (debug, "\n");
 372             }
 373         }
 374       replace_ssa_name_symbol (var, SSA_NAME_VAR (part_var));
 375     }
 376
 377   delete_var_map (map);
 378   return updated ? TODO_remove_unused_locals : 0;
 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,                  /* 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