gcc/tree-ssa-copyrename.c

   1 /* Rename SSA copies.
   2    Copyright (C) 2004, 2006, 2007, 2008, 2009, 2010
   3    Free Software Foundation, Inc.
   4    Contributed by Andrew MacLeod <amacleod@redhat.com>
   5
   6 This file is part of GCC.
   7
   8 GCC is free software; you can redistribute it and/or modify
   9 it under the terms of the GNU General Public License as published by
  10 the Free Software Foundation; either version 3, or (at your option)
  11 any later version.
  12
  13 GCC is distributed in the hope that it will be useful,
  14 but WITHOUT ANY WARRANTY; without even the implied warranty of
  15 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  16 GNU General Public License for more details.
  17
  18 You should have received a copy of the GNU General Public License
  19 along with GCC; see the file COPYING3.  If not see
  20 <http://www.gnu.org/licenses/>.  */
  21
  22 #include "config.h"
  23 #include "system.h"
  24 #include "coretypes.h"
  25 #include "tm.h"
  26 #include "tree.h"
  27 #include "gimple.h"
  28 #include "flags.h"
  29 #include "basic-block.h"
  30 #include "function.h"
  31 #include "tree-pretty-print.h"
  32 #include "bitmap.h"
  33 #include "tree-flow.h"
  34 #include "gimple.h"
  35 #include "tree-inline.h"
  36 #include "timevar.h"
  37 #include "hashtab.h"
  38 #include "tree-dump.h"
  39 #include "tree-ssa-live.h"
  40 #include "tree-pass.h"
  41 #include "langhooks.h"
  42
  43 /* The following routines implement the SSA copy renaming phase.
  44
  45    This optimization looks for copies between 2 SSA_NAMES, either through a
  46    direct copy, or an implicit one via a PHI node result and its arguments.
  47
  48    Each copy is examined to determine if it is possible to rename the base
  49    variable of one of the operands to the same variable as the other operand.
  50    i.e.
  51    T.3_5 = <blah>
  52    a_1 = T.3_5
  53
  54    If this copy couldn't be copy propagated, it could possibly remain in the
  55    program throughout the optimization phases.   After SSA->normal, it would
  56    become:
  57
  58    T.3 = <blah>
  59    a = T.3
  60
  61    Since T.3_5 is distinct from all other SSA versions of T.3, there is no
  62    fundamental reason why the base variable needs to be T.3, subject to
  63    certain restrictions.  This optimization attempts to determine if we can
  64    change the base variable on copies like this, and result in code such as:
  65
  66    a_5 = <blah>
  67    a_1 = a_5
  68
  69    This gives the SSA->normal pass a shot at coalescing a_1 and a_5. If it is
  70    possible, the copy goes away completely. If it isn't possible, a new temp
  71    will be created for a_5, and you will end up with the exact same code:
  72
  73    a.8 = <blah>
  74    a = a.8
  75
  76    The other benefit of performing this optimization relates to what variables
  77    are chosen in copies.  Gimplification of the program uses temporaries for
  78    a lot of things. expressions like
  79
  80    a_1 = <blah>
  81    <blah2> = a_1
  82
  83    get turned into
  84
  85    T.3_5 = <blah>
  86    a_1 = T.3_5
  87    <blah2> = a_1
  88
  89    Copy propagation is done in a forward direction, and if we can propagate
  90    through the copy, we end up with:
  91
  92    T.3_5 = <blah>
  93    <blah2> = T.3_5
  94
  95    The copy is gone, but so is all reference to the user variable 'a'. By
  96    performing this optimization, we would see the sequence:
  97
  98    a_5 = <blah>
  99    a_1 = a_5
 100    <blah2> = a_1
 101
 102    which copy propagation would then turn into:
 103
 104    a_5 = <blah>
 105    <blah2> = a_5
 106
 107    and so we still retain the user variable whenever possible.  */
 108
 109
 110 /* Coalesce the partitions in MAP representing VAR1 and VAR2 if it is valid.
 111    Choose a representative for the partition, and send debug info to DEBUG.  */
 112
 113 static bool
 114 copy_rename_partition_coalesce (var_map map, tree var1, tree var2, FILE *debug)
 115 {
 116   int p1, p2, p3;
 117   tree root1, root2;
 118   tree rep1, rep2;
 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);
 125   register_ssa_partition (map, var2);
 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   if (p1 == p2)
 148     {
 149       if (debug)
 150         fprintf (debug, " : Already coalesced.\n");
 151       return false;
 152     }
 153
 154   /* Don't coalesce if one of the variables occurs in an abnormal PHI.  */
 155   abnorm = (SSA_NAME_OCCURS_IN_ABNORMAL_PHI (rep1)
 156             || SSA_NAME_OCCURS_IN_ABNORMAL_PHI (rep2));
 157   if (abnorm)
 158     {
 159       if (debug)
 160         fprintf (debug, " : Abnormal PHI barrier.  No coalesce.\n");
 161       return false;
 162     }
 163
 164   /* Partitions already have the same root, simply merge them.  */
 165   if (root1 == root2)
 166     {
 167       p1 = partition_union (map->var_partition, p1, p2);
 168       if (debug)
 169         fprintf (debug, " : Same root, coalesced --> P%d.\n", p1);
 170       return false;
 171     }
 172
 173   /* Never attempt to coalesce 2 difference parameters.  */
 174   if (TREE_CODE (root1) == PARM_DECL && TREE_CODE (root2) == PARM_DECL)
 175     {
 176       if (debug)
 177         fprintf (debug, " : 2 different PARM_DECLS. No coalesce.\n");
 178       return false;
 179     }
 180
 181   if ((TREE_CODE (root1) == RESULT_DECL) != (TREE_CODE (root2) == RESULT_DECL))
 182     {
 183       if (debug)
 184         fprintf (debug, " : One root a RESULT_DECL. No coalesce.\n");
 185       return false;
 186     }
 187
 188   ign1 = TREE_CODE (root1) == VAR_DECL && DECL_IGNORED_P (root1);
 189   ign2 = TREE_CODE (root2) == VAR_DECL && DECL_IGNORED_P (root2);
 190
 191   /* Never attempt to coalesce 2 user variables unless one is an inline
 192      variable.  */
 193   if (!ign1 && !ign2)
 194     {
 195       if (DECL_FROM_INLINE (root2))
 196         ign2 = true;
 197       else if (DECL_FROM_INLINE (root1))
 198         ign1 = true;
 199       else
 200         {
 201           if (debug)
 202             fprintf (debug, " : 2 different USER vars. No coalesce.\n");
 203           return false;
 204         }
 205     }
 206
 207   /* If both values have default defs, we can't coalesce.  If only one has a
 208      tag, make sure that variable is the new root partition.  */
 209   if (gimple_default_def (cfun, root1))
 210     {
 211       if (gimple_default_def (cfun, root2))
 212         {
 213           if (debug)
 214             fprintf (debug, " : 2 default defs. No coalesce.\n");
 215           return false;
 216         }
 217       else
 218         {
 219           ign2 = true;
 220           ign1 = false;
 221         }
 222     }
 223   else if (gimple_default_def (cfun, root2))
 224     {
 225       ign1 = true;
 226       ign2 = false;
 227     }
 228
 229   /* Don't coalesce if the two variables are not of the same type.  */
 230   if (TREE_TYPE (root1) != TREE_TYPE (root2))
 231     {
 232       if (debug)
 233         fprintf (debug, " : Different types.  No coalesce.\n");
 234       return false;
 235     }
 236
 237   /* Merge the two partitions.  */
 238   p3 = partition_union (map->var_partition, p1, p2);
 239
 240   /* Set the root variable of the partition to the better choice, if there is
 241      one.  */
 242   if (!ign2)
 243     replace_ssa_name_symbol (partition_to_var (map, p3), root2);
 244   else if (!ign1)
 245     replace_ssa_name_symbol (partition_to_var (map, p3), root1);
 246
 247   if (debug)
 248     {
 249       fprintf (debug, " --> P%d ", p3);
 250       print_generic_expr (debug, SSA_NAME_VAR (partition_to_var (map, p3)),
 251                           TDF_SLIM);
 252       fprintf (debug, "\n");
 253     }
 254   return true;
 255 }
 256
 257
 258 /* This function will make a pass through the IL, and attempt to coalesce any
 259    SSA versions which occur in PHI's or copies.  Coalescing is accomplished by
 260    changing the underlying root variable of all coalesced version.  This will
 261    then cause the SSA->normal pass to attempt to coalesce them all to the same
 262    variable.  */
 263
 264 static unsigned int
 265 rename_ssa_copies (void)
 266 {
 267   var_map map;
 268   basic_block bb;
 269   gimple_stmt_iterator gsi;
 270   tree var, part_var;
 271   gimple stmt, phi;
 272   unsigned x;
 273   FILE *debug;
 274   bool updated = false;
 275
 276   if (dump_file && (dump_flags & TDF_DETAILS))
 277     debug = dump_file;
 278   else
 279     debug = NULL;
 280
 281   map = init_var_map (num_ssa_names);
 282
 283   FOR_EACH_BB (bb)
 284     {
 285       /* Scan for real copies.  */
 286       for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi))
 287         {
 288           stmt = gsi_stmt (gsi);
 289           if (gimple_assign_ssa_name_copy_p (stmt))
 290             {
 291               tree lhs = gimple_assign_lhs (stmt);
 292               tree rhs = gimple_assign_rhs1 (stmt);
 293
 294               updated |= copy_rename_partition_coalesce (map, lhs, rhs, debug);
 295             }
 296         }
 297     }
 298
 299   FOR_EACH_BB (bb)
 300     {
 301       /* Treat PHI nodes as copies between the result and each argument.  */
 302       for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); gsi_next (&gsi))
 303         {
 304           size_t i;
 305           tree res;
 306
 307           phi = gsi_stmt (gsi);
 308           res = gimple_phi_result (phi);
 309
 310           /* Do not process virtual SSA_NAMES.  */
 311           if (!is_gimple_reg (SSA_NAME_VAR (res)))
 312             continue;
 313
 314           for (i = 0; i < gimple_phi_num_args (phi); i++)
 315             {
 316               tree arg = gimple_phi_arg (phi, i)->def;
 317               if (TREE_CODE (arg) == SSA_NAME)
 318                 updated |= copy_rename_partition_coalesce (map, res, arg, debug);
 319             }
 320         }
 321     }
 322
 323   if (debug)
 324     dump_var_map (debug, map);
 325
 326   /* Now one more pass to make all elements of a partition share the same
 327      root variable.  */
 328
 329   for (x = 1; x < num_ssa_names; x++)
 330     {
 331       part_var = partition_to_var (map, x);
 332       if (!part_var)
 333         continue;
 334       var = ssa_name (x);
 335       if (debug)
 336         {
 337           if (SSA_NAME_VAR (var) != SSA_NAME_VAR (part_var))
 338             {
 339               fprintf (debug, "Coalesced ");
 340               print_generic_expr (debug, var, TDF_SLIM);
 341               fprintf (debug, " to ");
 342               print_generic_expr (debug, part_var, TDF_SLIM);
 343               fprintf (debug, "\n");
 344             }
 345         }
 346       replace_ssa_name_symbol (var, SSA_NAME_VAR (part_var));
 347     }
 348
 349   delete_var_map (map);
 350   return updated ? TODO_remove_unused_locals : 0;
 351 }
 352
 353 /* Return true if copy rename is to be performed.  */
 354
 355 static bool
 356 gate_copyrename (void)
 357 {
 358   return flag_tree_copyrename != 0;
 359 }
 360
 361 struct gimple_opt_pass pass_rename_ssa_copies =
 362 {
 363  {
 364   GIMPLE_PASS,
 365   "copyrename",                         /* name */
 366   gate_copyrename,                      /* gate */
 367   rename_ssa_copies,                    /* execute */
 368   NULL,                                 /* sub */
 369   NULL,                                 /* next */
 370   0,                                    /* static_pass_number */
 371   TV_TREE_COPY_RENAME,                  /* tv_id */
 372   PROP_cfg | PROP_ssa,                  /* properties_required */
 373   0,                                    /* properties_provided */
 374   0,                                    /* properties_destroyed */
 375   0,                                    /* todo_flags_start */
 376   TODO_dump_func | TODO_verify_ssa      /* todo_flags_finish */
 377  }
 378 };