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 "tree-alias-common.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 extern void rename_ssa_copies (void);
  44
  45 /* The following routines implement the SSA copy renaming phase.
  46
  47    This optimization looks for copies between 2 SSA_NAMES, either through a
  48    direct copy, or an implicit one via a PHI node result and its arguments.
  49
  50    Each copy is examined to determine if it is possible to rename the base
  51    variable of one of the operands to the same variable as the other operand.
  52    ie.
  53    T.3_5 = <blah>
  54    a_1 = T.3_5
  55
  56    If this copy couldn't be copy propagated, it could possibly remain in the
  57    program throughout the optimization phases.   After SSA->normal, it would
  58    become:
  59
  60    T.3 = <blah>
  61    a = T.3
  62
  63    Since T.3_5 is distinct from all other SSA versions of T.3, there is no
  64    fundamental reason why the base variable needs to be T.3, subject to
  65    certain restrictions.  This optimization attempts to determine if we can
  66    change the base variable on copies like this, and result in code such as:
  67
  68    a_5 = <blah>
  69    a_1 = a_5
  70
  71    This gives the SSA->normal pass a shot at coalescing a_1 and a_5. If it is
  72    possible, the copy goes away completely. If it isn't possible, a new temp
  73    will be created for a_5, and you will end up with the exact same code:
  74
  75    a.8 = <blah>
  76    a = a.8
  77
  78    The other benefit of performing this optimization relates to what variables
  79    are chosen in copies.  Gimplification of the program uses temporaries for
  80    a lot of things. expressions like
  81
  82    a_1 = <blah>
  83    <blah2> = a_1
  84
  85    get turned into
  86
  87    T.3_5 = <blah>
  88    a_1 = T.3_5
  89    <blah2> = a_1
  90
  91    Copy propagation is done in a forward direction, and if we can propagate
  92    through the copy, we end up with:
  93
  94    T.3_5 = <blah>
  95    <blah2> = T.3_5
  96
  97    The copy is gone, but so is all reference to the user variable 'a'. By
  98    performing this optimization, we would see the sequence:
  99
 100    a_5 = <blah>
 101    a_1 = a_5
 102    <blah2> = a_1
 103
 104    which copy propagation would then turn into:
 105
 106    a_5 = <blah>
 107    <blah2> = a_5
 108
 109    and so we still retain the user variable whenever possible.  */
 110
 111
 112 /* Coalesce the partitions in MAP representing VAR1 and VAR2 if it is valid.
 113    Choose a representative for the partition, and send debug info to DEBUG.  */
 114
 115 static void
 116 copy_rename_partition_coalesce (var_map map, tree var1, tree var2, FILE *debug)
 117 {
 118   int p1, p2, p3;
 119   tree root1, root2;
 120   var_ann_t ann1, ann2, ann3;
 121   bool gimp1, gimp2;
 122
 123 #ifdef ENABLE_CHECKING
 124   if (TREE_CODE (var1) != SSA_NAME || TREE_CODE (var2) != SSA_NAME)
 125     abort ();
 126 #endif
 127
 128   register_ssa_partition (map, var1, false);
 129   register_ssa_partition (map, var2, true);
 130
 131   p1 = partition_find (map->var_partition, SSA_NAME_VERSION (var1));
 132   p2 = partition_find (map->var_partition, SSA_NAME_VERSION (var2));
 133
 134   if (debug)
 135     {
 136       fprintf (debug, "Try : ");
 137       print_generic_expr (debug, var1, TDF_SLIM);
 138       fprintf (debug, "(P%d) & ", p1);
 139       print_generic_expr (debug, var2, TDF_SLIM);
 140       fprintf (debug, "(P%d)", p2);
 141     }
 142
 143 #ifdef ENABLE_CHECKING
 144   if (p1 == NO_PARTITION || p2 == NO_PARTITION)
 145     abort ();
 146 #endif
 147
 148   root1 = SSA_NAME_VAR (partition_to_var (map, p1));
 149   root2 = SSA_NAME_VAR (partition_to_var (map, p2));
 150
 151   if (DECL_HARD_REGISTER (root1) || DECL_HARD_REGISTER (root2))
 152     {
 153       if (debug)
 154         {
 155           if (DECL_HARD_REGISTER (root1))
 156             print_generic_expr (debug, var1, TDF_SLIM);
 157           else
 158             print_generic_expr (debug, var2, TDF_SLIM);
 159           fprintf (debug, " is a hardware register.  No Coalescing.\n");
 160         }
 161       return;
 162     }
 163
 164   ann1 = var_ann (root1);
 165   ann2 = var_ann (root2);
 166
 167   if (p1 == p2)
 168     {
 169       if (debug)
 170         fprintf (debug, " : Already coalesced.\n");
 171       return;
 172     }
 173
 174   /* Partitions already have the same root, simply merge them.  */
 175   if (root1 == root2)
 176     {
 177       p1 = partition_union (map->var_partition, p1, p2);
 178       if (debug)
 179         fprintf (debug, " : Same root, coalesced --> P%d.\n", p1);
 180       return;
 181     }
 182
 183   /* Never attempt to coalesce 2 difference parameters.  */
 184   if (TREE_CODE (root1) == PARM_DECL && TREE_CODE (root2) == PARM_DECL)
 185     {
 186       if (debug)
 187         fprintf (debug, " : 2 different PARM_DECLS. No coalesce.\n");
 188       return;
 189     }
 190
 191   if ((TREE_CODE (root1) == RESULT_DECL) != (TREE_CODE (root2) == RESULT_DECL))
 192     {
 193       if (debug)
 194         fprintf (debug, " : One root a RESULT_DECL. No coalesce.\n");
 195       return;
 196     }
 197
 198   gimp1 = is_gimple_tmp_var (root1);
 199   gimp2 = is_gimple_tmp_var (root2);
 200
 201   /* Never attempt to coalesce 2 user variables unless one is an inline
 202      variable.  */
 203   if (!gimp1 && !gimp2)
 204     {
 205       if (DECL_FROM_INLINE (root2))
 206         gimp2 = true;
 207       else
 208         if (DECL_FROM_INLINE (root1))
 209           gimp1 = true;
 210         else
 211           {
 212             if (debug)
 213               fprintf (debug, " : 2 different USER vars. No coalesce.\n");
 214             return;
 215           }
 216     }
 217
 218
 219   /* Don't coalesce if there are two different memory tags.  */
 220   if (ann1->type_mem_tag && ann2->type_mem_tag
 221       && ann1->type_mem_tag != ann2->type_mem_tag)
 222     {
 223       if (debug)
 224         fprintf (debug, " : 2 memory tags. No coalesce.\n");
 225       return;
 226     }
 227
 228   /* If both values have default defs, we can't coalesce.  If only one has a
 229      tag, make sure that variable is the new root partition.  */
 230   if (default_def (root1))
 231     {
 232       if (default_def (root2))
 233         {
 234           if (debug)
 235             fprintf (debug, " : 2 default defs. No coalesce.\n");
 236           return;
 237         }
 238       else
 239         {
 240           gimp2 = true;
 241           gimp1 = false;
 242         }
 243     }
 244   else
 245     if (default_def (root2))
 246       {
 247         gimp1 = true;
 248         gimp2 = false;
 249       }
 250
 251   /* Don't coalesce if the two variables aren't type compatible.  */
 252   if (!lang_hooks.types_compatible_p (TREE_TYPE (root1), TREE_TYPE (root2)))
 253     {
 254       if (debug)
 255         fprintf (debug, " : Incompatible types.  No coalesce.\n");
 256       return;
 257     }
 258
 259   /* Merge the two partitions.  */
 260   p3 = partition_union (map->var_partition, p1, p2);
 261
 262   /* Set the root variable of the partition to the better choice, if there is
 263      one.  */
 264   if (!gimp2)
 265     replace_ssa_name_symbol (partition_to_var (map, p3), root2);
 266   else
 267     if (!gimp1)
 268       replace_ssa_name_symbol (partition_to_var (map, p3), root1);
 269
 270   /* Update the various flag widgitry of the current base representative.  */
 271   ann3 = var_ann (SSA_NAME_VAR (partition_to_var (map, p3)));
 272   if (ann1->type_mem_tag)
 273     ann3->type_mem_tag = ann1->type_mem_tag;
 274   else
 275     ann3->type_mem_tag = ann2->type_mem_tag;
 276
 277   if (debug)
 278     {
 279       fprintf (debug, " --> P%d ", p3);
 280       print_generic_expr (debug, SSA_NAME_VAR (partition_to_var (map, p3)),
 281                           TDF_SLIM);
 282       fprintf (debug, "\n");
 283     }
 284 }
 285
 286
 287 /* This function will make a pass through the IL, and attempt to coalesce any
 288    SSA versions which occur in PHI's or copies.  Coalescing is accomplished by
 289    changing the underlying root variable of all coalesced version.  This will
 290    then cause the SSA->normal pass to attempt to coalesce them all to the same
 291    variable.  */
 292
 293 void
 294 rename_ssa_copies (void)
 295 {
 296   var_map map;
 297   basic_block bb;
 298   block_stmt_iterator bsi;
 299   tree phi, stmt, var, part_var;
 300   unsigned x;
 301   FILE *debug;
 302
 303   if (dump_file && (dump_flags & TDF_DETAILS))
 304     debug = dump_file;
 305   else
 306     debug = NULL;
 307
 308   map = init_var_map (num_ssa_names + 1);
 309
 310   FOR_EACH_BB (bb)
 311     {
 312       /* Scan for real copies.  */
 313       for (bsi = bsi_start (bb); !bsi_end_p (bsi); bsi_next (&bsi))
 314         {
 315           stmt = bsi_stmt (bsi);
 316           if (TREE_CODE (stmt) == MODIFY_EXPR)
 317             {
 318               tree lhs = TREE_OPERAND (stmt, 0);
 319               tree rhs = TREE_OPERAND (stmt, 1);
 320
 321               if (TREE_CODE (lhs) == SSA_NAME && TREE_CODE (rhs) == SSA_NAME)
 322                 copy_rename_partition_coalesce (map, lhs, rhs, debug);
 323             }
 324         }
 325     }
 326
 327   FOR_EACH_BB (bb)
 328     {
 329       /* Treat PHI nodes as copies between the result and each argument.  */
 330       for (phi = phi_nodes (bb); phi; phi = PHI_CHAIN (phi))
 331         {
 332           int i;
 333           tree res = PHI_RESULT (phi);
 334
 335           /* Do not process virtual SSA_NAMES.  */
 336           if (!is_gimple_reg (SSA_NAME_VAR (res)))
 337             continue;
 338
 339           for (i = 0; i < PHI_NUM_ARGS (phi); i++)
 340             {
 341               tree arg = PHI_ARG_DEF (phi, i);
 342               if (TREE_CODE (arg) == SSA_NAME)
 343                 copy_rename_partition_coalesce (map, res, arg, debug);
 344             }
 345         }
 346     }
 347
 348   if (debug)
 349     dump_var_map (debug, map);
 350
 351   /* Now one more pass to make all elements of a partition share the same
 352      root variable.  */
 353
 354   for (x = 1; x <= num_ssa_names; x++)
 355     {
 356       part_var = partition_to_var (map, x);
 357       if (!part_var)
 358         continue;
 359       var = map->partition_to_var[x];
 360       if (debug)
 361         {
 362           if (SSA_NAME_VAR (var) != SSA_NAME_VAR (part_var))
 363             {
 364               fprintf (debug, "Coalesced ");
 365               print_generic_expr (debug, var, TDF_SLIM);
 366               fprintf (debug, " to ");
 367               print_generic_expr (debug, part_var, TDF_SLIM);
 368               fprintf (debug, "\n");
 369             }
 370         }
 371       replace_ssa_name_symbol (var, SSA_NAME_VAR (part_var));
 372     }
 373
 374   delete_var_map (map);
 375 }
 376
 377 /* Return true if copy rename is to be performed.  */
 378
 379 static bool
 380 gate_copyrename (void)
 381 {
 382   return flag_tree_copyrename != 0;
 383 }
 384
 385 struct tree_opt_pass pass_rename_ssa_copies =
 386 {
 387   "copyrename",                         /* name */
 388   gate_copyrename,                      /* gate */
 389   rename_ssa_copies,                    /* execute */
 390   NULL,                                 /* sub */
 391   NULL,                                 /* next */
 392   0,                                    /* static_pass_number */
 393   TV_TREE_COPY_RENAME,                  /* tv_id */
 394   PROP_cfg | PROP_ssa | PROP_alias,     /* properties_required */
 395   0,                                    /* properties_provided */
 396   0,                                    /* properties_destroyed */
 397   0,                                    /* todo_flags_start */
 398   TODO_dump_func | TODO_verify_ssa      /* todo_flags_finish */
 399 };
 400