gcc/tree-ssa-copyrename.c

   1 /* Rename SSA copies.
   2    Copyright (C) 2004, 2006, 2007, 2008 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 "gimple.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 "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 bool
 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   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   if (p1 == p2)
 147     {
 148       if (debug)
 149         fprintf (debug, " : Already coalesced.\n");
 150       return false;
 151     }
 152
 153   /* Don't coalesce if one of the variables occurs in an abnormal PHI.  */
 154   abnorm = (SSA_NAME_OCCURS_IN_ABNORMAL_PHI (rep1)
 155             || SSA_NAME_OCCURS_IN_ABNORMAL_PHI (rep2));
 156   if (abnorm)
 157     {
 158       if (debug)
 159         fprintf (debug, " : Abnormal PHI barrier.  No coalesce.\n");
 160       return false;
 161     }
 162
 163   /* Partitions already have the same root, simply merge them.  */
 164   if (root1 == root2)
 165     {
 166       p1 = partition_union (map->var_partition, p1, p2);
 167       if (debug)
 168         fprintf (debug, " : Same root, coalesced --> P%d.\n", p1);
 169       return false;
 170     }
 171
 172   /* Never attempt to coalesce 2 difference parameters.  */
 173   if (TREE_CODE (root1) == PARM_DECL && TREE_CODE (root2) == PARM_DECL)
 174     {
 175       if (debug)
 176         fprintf (debug, " : 2 different PARM_DECLS. No coalesce.\n");
 177       return false;
 178     }
 179
 180   if ((TREE_CODE (root1) == RESULT_DECL) != (TREE_CODE (root2) == RESULT_DECL))
 181     {
 182       if (debug)
 183         fprintf (debug, " : One root a RESULT_DECL. No coalesce.\n");
 184       return false;
 185     }
 186
 187   ign1 = TREE_CODE (root1) == VAR_DECL && DECL_IGNORED_P (root1);
 188   ign2 = TREE_CODE (root2) == VAR_DECL && DECL_IGNORED_P (root2);
 189
 190   /* Never attempt to coalesce 2 user variables unless one is an inline
 191      variable.  */
 192   if (!ign1 && !ign2)
 193     {
 194       if (DECL_FROM_INLINE (root2))
 195         ign2 = true;
 196       else if (DECL_FROM_INLINE (root1))
 197         ign1 = true;
 198       else
 199         {
 200           if (debug)
 201             fprintf (debug, " : 2 different USER vars. No coalesce.\n");
 202           return false;
 203         }
 204     }
 205
 206   /* If both values have default defs, we can't coalesce.  If only one has a
 207      tag, make sure that variable is the new root partition.  */
 208   if (gimple_default_def (cfun, root1))
 209     {
 210       if (gimple_default_def (cfun, root2))
 211         {
 212           if (debug)
 213             fprintf (debug, " : 2 default defs. No coalesce.\n");
 214           return false;
 215         }
 216       else
 217         {
 218           ign2 = true;
 219           ign1 = false;
 220         }
 221     }
 222   else if (gimple_default_def (cfun, root2))
 223     {
 224       ign1 = true;
 225       ign2 = false;
 226     }
 227
 228   /* Don't coalesce if the two variables aren't type compatible.  */
 229   if (!types_compatible_p (TREE_TYPE (root1), TREE_TYPE (root2)))
 230     {
 231       if (debug)
 232         fprintf (debug, " : Incompatible types.  No coalesce.\n");
 233       return false;
 234     }
 235
 236   /* Merge the two partitions.  */
 237   p3 = partition_union (map->var_partition, p1, p2);
 238
 239   /* Set the root variable of the partition to the better choice, if there is
 240      one.  */
 241   if (!ign2)
 242     replace_ssa_name_symbol (partition_to_var (map, p3), root2);
 243   else if (!ign1)
 244     replace_ssa_name_symbol (partition_to_var (map, p3), root1);
 245
 246   if (debug)
 247     {
 248       fprintf (debug, " --> P%d ", p3);
 249       print_generic_expr (debug, SSA_NAME_VAR (partition_to_var (map, p3)),
 250                           TDF_SLIM);
 251       fprintf (debug, "\n");
 252     }
 253   return true;
 254 }
 255
 256
 257 /* This function will make a pass through the IL, and attempt to coalesce any
 258    SSA versions which occur in PHI's or copies.  Coalescing is accomplished by
 259    changing the underlying root variable of all coalesced version.  This will
 260    then cause the SSA->normal pass to attempt to coalesce them all to the same
 261    variable.  */
 262
 263 static unsigned int
 264 rename_ssa_copies (void)
 265 {
 266   var_map map;
 267   basic_block bb;
 268   gimple_stmt_iterator gsi;
 269   tree var, part_var;
 270   gimple stmt, phi;
 271   unsigned x;
 272   FILE *debug;
 273   bool updated = false;
 274
 275   if (dump_file && (dump_flags & TDF_DETAILS))
 276     debug = dump_file;
 277   else
 278     debug = NULL;
 279
 280   map = init_var_map (num_ssa_names);
 281
 282   FOR_EACH_BB (bb)
 283     {
 284       /* Scan for real copies.  */
 285       for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi))
 286         {
 287           stmt = gsi_stmt (gsi);
 288           if (gimple_assign_ssa_name_copy_p (stmt))
 289             {
 290               tree lhs = gimple_assign_lhs (stmt);
 291               tree rhs = gimple_assign_rhs1 (stmt);
 292
 293               updated |= copy_rename_partition_coalesce (map, lhs, rhs, debug);
 294             }
 295         }
 296     }
 297
 298   FOR_EACH_BB (bb)
 299     {
 300       /* Treat PHI nodes as copies between the result and each argument.  */
 301       for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); gsi_next (&gsi))
 302         {
 303           size_t i;
 304           tree res;
 305
 306           phi = gsi_stmt (gsi);
 307           res = gimple_phi_result (phi);
 308
 309           /* Do not process virtual SSA_NAMES.  */
 310           if (!is_gimple_reg (SSA_NAME_VAR (res)))
 311             continue;
 312
 313           for (i = 0; i < gimple_phi_num_args (phi); i++)
 314             {
 315               tree arg = gimple_phi_arg (phi, i)->def;
 316               if (TREE_CODE (arg) == SSA_NAME)
 317                 updated |= copy_rename_partition_coalesce (map, res, arg, debug);
 318             }
 319         }
 320     }
 321
 322   if (debug)
 323     dump_var_map (debug, map);
 324
 325   /* Now one more pass to make all elements of a partition share the same
 326      root variable.  */
 327
 328   for (x = 1; x < num_ssa_names; x++)
 329     {
 330       part_var = partition_to_var (map, x);
 331       if (!part_var)
 332         continue;
 333       var = ssa_name (x);
 334       if (debug)
 335         {
 336           if (SSA_NAME_VAR (var) != SSA_NAME_VAR (part_var))
 337             {
 338               fprintf (debug, "Coalesced ");
 339               print_generic_expr (debug, var, TDF_SLIM);
 340               fprintf (debug, " to ");
 341               print_generic_expr (debug, part_var, TDF_SLIM);
 342               fprintf (debug, "\n");
 343             }
 344         }
 345       replace_ssa_name_symbol (var, SSA_NAME_VAR (part_var));
 346     }
 347
 348   delete_var_map (map);
 349   return updated ? TODO_remove_unused_locals : 0;
 350 }
 351
 352 /* Return true if copy rename is to be performed.  */
 353
 354 static bool
 355 gate_copyrename (void)
 356 {
 357   return flag_tree_copyrename != 0;
 358 }
 359
 360 struct gimple_opt_pass pass_rename_ssa_copies =
 361 {
 362  {
 363   GIMPLE_PASS,
 364   "copyrename",                         /* name */
 365   gate_copyrename,                      /* gate */
 366   rename_ssa_copies,                    /* execute */
 367   NULL,                                 /* sub */
 368   NULL,                                 /* next */
 369   0,                                    /* static_pass_number */
 370   TV_TREE_COPY_RENAME,                  /* tv_id */
 371   PROP_cfg | PROP_ssa,                  /* properties_required */
 372   0,                                    /* properties_provided */
 373   0,                                    /* properties_destroyed */
 374   0,                                    /* todo_flags_start */
 375   TODO_dump_func | TODO_verify_ssa      /* todo_flags_finish */
 376  }
 377 };