gcc/tree-ssa-copyrename.c

   1 /* Rename SSA copies.
   2    Copyright (C) 2004, 2006, 2007, 2008, 2009, 2010, 2011
   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 static struct
  44 {
  45   /* Number of copies coalesced.  */
  46   int coalesced;
  47 } stats;
  48
  49 /* The following routines implement the SSA copy renaming phase.
  50
  51    This optimization looks for copies between 2 SSA_NAMES, either through a
  52    direct copy, or an implicit one via a PHI node result and its arguments.
  53
  54    Each copy is examined to determine if it is possible to rename the base
  55    variable of one of the operands to the same variable as the other operand.
  56    i.e.
  57    T.3_5 = <blah>
  58    a_1 = T.3_5
  59
  60    If this copy couldn't be copy propagated, it could possibly remain in the
  61    program throughout the optimization phases.   After SSA->normal, it would
  62    become:
  63
  64    T.3 = <blah>
  65    a = T.3
  66
  67    Since T.3_5 is distinct from all other SSA versions of T.3, there is no
  68    fundamental reason why the base variable needs to be T.3, subject to
  69    certain restrictions.  This optimization attempts to determine if we can
  70    change the base variable on copies like this, and result in code such as:
  71
  72    a_5 = <blah>
  73    a_1 = a_5
  74
  75    This gives the SSA->normal pass a shot at coalescing a_1 and a_5. If it is
  76    possible, the copy goes away completely. If it isn't possible, a new temp
  77    will be created for a_5, and you will end up with the exact same code:
  78
  79    a.8 = <blah>
  80    a = a.8
  81
  82    The other benefit of performing this optimization relates to what variables
  83    are chosen in copies.  Gimplification of the program uses temporaries for
  84    a lot of things. expressions like
  85
  86    a_1 = <blah>
  87    <blah2> = a_1
  88
  89    get turned into
  90
  91    T.3_5 = <blah>
  92    a_1 = T.3_5
  93    <blah2> = a_1
  94
  95    Copy propagation is done in a forward direction, and if we can propagate
  96    through the copy, we end up with:
  97
  98    T.3_5 = <blah>
  99    <blah2> = T.3_5
 100
 101    The copy is gone, but so is all reference to the user variable 'a'. By
 102    performing this optimization, we would see the sequence:
 103
 104    a_5 = <blah>
 105    a_1 = a_5
 106    <blah2> = a_1
 107
 108    which copy propagation would then turn into:
 109
 110    a_5 = <blah>
 111    <blah2> = a_5
 112
 113    and so we still retain the user variable whenever possible.  */
 114
 115
 116 /* Coalesce the partitions in MAP representing VAR1 and VAR2 if it is valid.
 117    Choose a representative for the partition, and send debug info to DEBUG.  */
 118
 119 static bool
 120 copy_rename_partition_coalesce (var_map map, tree var1, tree var2, FILE *debug)
 121 {
 122   int p1, p2, p3;
 123   tree root1, root2;
 124   tree rep1, rep2;
 125   bool ign1, ign2, abnorm;
 126
 127   gcc_assert (TREE_CODE (var1) == SSA_NAME);
 128   gcc_assert (TREE_CODE (var2) == SSA_NAME);
 129
 130   register_ssa_partition (map, var1);
 131   register_ssa_partition (map, var2);
 132
 133   p1 = partition_find (map->var_partition, SSA_NAME_VERSION (var1));
 134   p2 = partition_find (map->var_partition, SSA_NAME_VERSION (var2));
 135
 136   if (debug)
 137     {
 138       fprintf (debug, "Try : ");
 139       print_generic_expr (debug, var1, TDF_SLIM);
 140       fprintf (debug, "(P%d) & ", p1);
 141       print_generic_expr (debug, var2, TDF_SLIM);
 142       fprintf (debug, "(P%d)", p2);
 143     }
 144
 145   gcc_assert (p1 != NO_PARTITION);
 146   gcc_assert (p2 != NO_PARTITION);
 147
 148   rep1 = partition_to_var (map, p1);
 149   rep2 = partition_to_var (map, p2);
 150   root1 = SSA_NAME_VAR (rep1);
 151   root2 = SSA_NAME_VAR (rep2);
 152
 153   if (p1 == p2)
 154     {
 155       if (debug)
 156         fprintf (debug, " : Already coalesced.\n");
 157       return false;
 158     }
 159
 160   /* Don't coalesce if one of the variables occurs in an abnormal PHI.  */
 161   abnorm = (SSA_NAME_OCCURS_IN_ABNORMAL_PHI (rep1)
 162             || SSA_NAME_OCCURS_IN_ABNORMAL_PHI (rep2));
 163   if (abnorm)
 164     {
 165       if (debug)
 166         fprintf (debug, " : Abnormal PHI barrier.  No coalesce.\n");
 167       return false;
 168     }
 169
 170   /* Partitions already have the same root, simply merge them.  */
 171   if (root1 == root2)
 172     {
 173       p1 = partition_union (map->var_partition, p1, p2);
 174       if (debug)
 175         fprintf (debug, " : Same root, coalesced --> P%d.\n", p1);
 176       return false;
 177     }
 178
 179   /* Never attempt to coalesce 2 different parameters.  */
 180   if (TREE_CODE (root1) == PARM_DECL && TREE_CODE (root2) == PARM_DECL)
 181     {
 182       if (debug)
 183         fprintf (debug, " : 2 different PARM_DECLS. No coalesce.\n");
 184       return false;
 185     }
 186
 187   if ((TREE_CODE (root1) == RESULT_DECL) != (TREE_CODE (root2) == RESULT_DECL))
 188     {
 189       if (debug)
 190         fprintf (debug, " : One root a RESULT_DECL. No coalesce.\n");
 191       return false;
 192     }
 193
 194   ign1 = TREE_CODE (root1) == VAR_DECL && DECL_IGNORED_P (root1);
 195   ign2 = TREE_CODE (root2) == VAR_DECL && DECL_IGNORED_P (root2);
 196
 197   /* Never attempt to coalesce 2 user variables unless one is an inline
 198      variable.  */
 199   if (!ign1 && !ign2)
 200     {
 201       if (DECL_FROM_INLINE (root2))
 202         ign2 = true;
 203       else if (DECL_FROM_INLINE (root1))
 204         ign1 = true;
 205       else
 206         {
 207           if (debug)
 208             fprintf (debug, " : 2 different USER vars. No coalesce.\n");
 209           return false;
 210         }
 211     }
 212
 213   /* If both values have default defs, we can't coalesce.  If only one has a
 214      tag, make sure that variable is the new root partition.  */
 215   if (gimple_default_def (cfun, root1))
 216     {
 217       if (gimple_default_def (cfun, root2))
 218         {
 219           if (debug)
 220             fprintf (debug, " : 2 default defs. No coalesce.\n");
 221           return false;
 222         }
 223       else
 224         {
 225           ign2 = true;
 226           ign1 = false;
 227         }
 228     }
 229   else if (gimple_default_def (cfun, root2))
 230     {
 231       ign1 = true;
 232       ign2 = false;
 233     }
 234
 235   /* Don't coalesce if the new chosen root variable would be read-only.
 236      If both ign1 && ign2, then the root var of the larger partition
 237      wins, so reject in that case if any of the root vars is TREE_READONLY.
 238      Otherwise reject only if the root var, on which replace_ssa_name_symbol
 239      will be called below, is readonly.  */
 240   if ((TREE_READONLY (root1) && ign2) || (TREE_READONLY (root2) && ign1))
 241     {
 242       if (debug)
 243         fprintf (debug, " : Readonly variable.  No coalesce.\n");
 244       return false;
 245     }
 246
 247   /* Don't coalesce if the two variables aren't type compatible .  */
 248   if (!types_compatible_p (TREE_TYPE (root1), TREE_TYPE (root2))
 249       /* There is a disconnect between the middle-end type-system and
 250          VRP, avoid coalescing enum types with different bounds.  */
 251       || ((TREE_CODE (TREE_TYPE (root1)) == ENUMERAL_TYPE
 252            || TREE_CODE (TREE_TYPE (root2)) == ENUMERAL_TYPE)
 253           && TREE_TYPE (root1) != TREE_TYPE (root2)))
 254     {
 255       if (debug)
 256         fprintf (debug, " : Incompatible types.  No coalesce.\n");
 257       return false;
 258     }
 259
 260   /* Merge the two partitions.  */
 261   p3 = partition_union (map->var_partition, p1, p2);
 262
 263   /* Set the root variable of the partition to the better choice, if there is
 264      one.  */
 265   if (!ign2)
 266     replace_ssa_name_symbol (partition_to_var (map, p3), root2);
 267   else if (!ign1)
 268     replace_ssa_name_symbol (partition_to_var (map, p3), root1);
 269
 270   if (debug)
 271     {
 272       fprintf (debug, " --> P%d ", p3);
 273       print_generic_expr (debug, SSA_NAME_VAR (partition_to_var (map, p3)),
 274                           TDF_SLIM);
 275       fprintf (debug, "\n");
 276     }
 277   return true;
 278 }
 279
 280
 281 /* This function will make a pass through the IL, and attempt to coalesce any
 282    SSA versions which occur in PHI's or copies.  Coalescing is accomplished by
 283    changing the underlying root variable of all coalesced version.  This will
 284    then cause the SSA->normal pass to attempt to coalesce them all to the same
 285    variable.  */
 286
 287 static unsigned int
 288 rename_ssa_copies (void)
 289 {
 290   var_map map;
 291   basic_block bb;
 292   gimple_stmt_iterator gsi;
 293   tree var, part_var;
 294   gimple stmt, phi;
 295   unsigned x;
 296   FILE *debug;
 297   bool updated = false;
 298
 299   if (dump_file && (dump_flags & TDF_DETAILS))
 300     debug = dump_file;
 301   else
 302     debug = NULL;
 303
 304   map = init_var_map (num_ssa_names);
 305
 306   FOR_EACH_BB (bb)
 307     {
 308       /* Scan for real copies.  */
 309       for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi))
 310         {
 311           stmt = gsi_stmt (gsi);
 312           if (gimple_assign_ssa_name_copy_p (stmt))
 313             {
 314               tree lhs = gimple_assign_lhs (stmt);
 315               tree rhs = gimple_assign_rhs1 (stmt);
 316
 317               updated |= copy_rename_partition_coalesce (map, lhs, rhs, debug);
 318             }
 319         }
 320     }
 321
 322   FOR_EACH_BB (bb)
 323     {
 324       /* Treat PHI nodes as copies between the result and each argument.  */
 325       for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); gsi_next (&gsi))
 326         {
 327           size_t i;
 328           tree res;
 329
 330           phi = gsi_stmt (gsi);
 331           res = gimple_phi_result (phi);
 332
 333           /* Do not process virtual SSA_NAMES.  */
 334           if (!is_gimple_reg (SSA_NAME_VAR (res)))
 335             continue;
 336
 337           for (i = 0; i < gimple_phi_num_args (phi); i++)
 338             {
 339               tree arg = gimple_phi_arg (phi, i)->def;
 340               if (TREE_CODE (arg) == SSA_NAME)
 341                 updated |= copy_rename_partition_coalesce (map, res, arg, debug);
 342             }
 343         }
 344     }
 345
 346   if (debug)
 347     dump_var_map (debug, map);
 348
 349   /* Now one more pass to make all elements of a partition share the same
 350      root variable.  */
 351
 352   for (x = 1; x < num_ssa_names; x++)
 353     {
 354       part_var = partition_to_var (map, x);
 355       if (!part_var)
 356         continue;
 357       var = ssa_name (x);
 358       if (debug)
 359         {
 360           if (SSA_NAME_VAR (var) != SSA_NAME_VAR (part_var))
 361             {
 362               fprintf (debug, "Coalesced ");
 363               print_generic_expr (debug, var, TDF_SLIM);
 364               fprintf (debug, " to ");
 365               print_generic_expr (debug, part_var, TDF_SLIM);
 366               fprintf (debug, "\n");
 367             }
 368         }
 369       stats.coalesced++;
 370       replace_ssa_name_symbol (var, SSA_NAME_VAR (part_var));
 371     }
 372
 373   statistics_counter_event (cfun, "copies coalesced",
 374                             stats.coalesced);
 375   delete_var_map (map);
 376   return updated ? TODO_remove_unused_locals : 0;
 377 }
 378
 379 /* Return true if copy rename is to be performed.  */
 380
 381 static bool
 382 gate_copyrename (void)
 383 {
 384   return flag_tree_copyrename != 0;
 385 }
 386
 387 struct gimple_opt_pass pass_rename_ssa_copies =
 388 {
 389  {
 390   GIMPLE_PASS,
 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  }
 404 };