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 "hashtab.h"
  37 #include "tree-dump.h"
  38 #include "tree-ssa-live.h"
  39 #include "tree-pass.h"
  40 #include "langhooks.h"
  41
  42 extern void rename_ssa_copies (void);
  43
  44 /* The following routines implement the SSA copy renaming phase.
  45
  46    This optimization looks for copies between 2 SSA_NAMES, either through a
  47    direct copy, or an implicit one via a PHI node result and its arguments.
  48
  49    Each copy is examined to determine if it is possible to rename the base
  50    variable of one of the operands to the same variable as the other operand.
  51    i.e.
  52    T.3_5 = <blah>
  53    a_1 = T.3_5
  54
  55    If this copy couldn't be copy propagated, it could possibly remain in the
  56    program throughout the optimization phases.   After SSA->normal, it would
  57    become:
  58
  59    T.3 = <blah>
  60    a = T.3
  61
  62    Since T.3_5 is distinct from all other SSA versions of T.3, there is no
  63    fundamental reason why the base variable needs to be T.3, subject to
  64    certain restrictions.  This optimization attempts to determine if we can
  65    change the base variable on copies like this, and result in code such as:
  66
  67    a_5 = <blah>
  68    a_1 = a_5
  69
  70    This gives the SSA->normal pass a shot at coalescing a_1 and a_5. If it is
  71    possible, the copy goes away completely. If it isn't possible, a new temp
  72    will be created for a_5, and you will end up with the exact same code:
  73
  74    a.8 = <blah>
  75    a = a.8
  76
  77    The other benefit of performing this optimization relates to what variables
  78    are chosen in copies.  Gimplification of the program uses temporaries for
  79    a lot of things. expressions like
  80
  81    a_1 = <blah>
  82    <blah2> = a_1
  83
  84    get turned into
  85
  86    T.3_5 = <blah>
  87    a_1 = T.3_5
  88    <blah2> = a_1
  89
  90    Copy propagation is done in a forward direction, and if we can propagate
  91    through the copy, we end up with:
  92
  93    T.3_5 = <blah>
  94    <blah2> = T.3_5
  95
  96    The copy is gone, but so is all reference to the user variable 'a'. By
  97    performing this optimization, we would see the sequence:
  98
  99    a_5 = <blah>
 100    a_1 = a_5
 101    <blah2> = a_1
 102
 103    which copy propagation would then turn into:
 104
 105    a_5 = <blah>
 106    <blah2> = a_5
 107
 108    and so we still retain the user variable whenever possible.  */
 109
 110
 111 /* Coalesce the partitions in MAP representing VAR1 and VAR2 if it is valid.
 112    Choose a representative for the partition, and send debug info to DEBUG.  */
 113
 114 static void
 115 copy_rename_partition_coalesce (var_map map, tree var1, tree var2, FILE *debug)
 116 {
 117   int p1, p2, p3;
 118   tree root1, root2;
 119   var_ann_t ann1, ann2, ann3;
 120   bool ign1, ign2;
 121
 122   gcc_assert (TREE_CODE (var1) == SSA_NAME);
 123   gcc_assert (TREE_CODE (var2) == SSA_NAME);
 124
 125   register_ssa_partition (map, var1, false);
 126   register_ssa_partition (map, var2, true);
 127
 128   p1 = partition_find (map->var_partition, SSA_NAME_VERSION (var1));
 129   p2 = partition_find (map->var_partition, SSA_NAME_VERSION (var2));
 130
 131   if (debug)
 132     {
 133       fprintf (debug, "Try : ");
 134       print_generic_expr (debug, var1, TDF_SLIM);
 135       fprintf (debug, "(P%d) & ", p1);
 136       print_generic_expr (debug, var2, TDF_SLIM);
 137       fprintf (debug, "(P%d)", p2);
 138     }
 139
 140   gcc_assert (p1 != NO_PARTITION);
 141   gcc_assert (p2 != NO_PARTITION);
 142
 143   root1 = SSA_NAME_VAR (partition_to_var (map, p1));
 144   root2 = SSA_NAME_VAR (partition_to_var (map, p2));
 145
 146   if (DECL_HARD_REGISTER (root1) || DECL_HARD_REGISTER (root2))
 147     {
 148       if (debug)
 149         {
 150           if (DECL_HARD_REGISTER (root1))
 151             print_generic_expr (debug, var1, TDF_SLIM);
 152           else
 153             print_generic_expr (debug, var2, TDF_SLIM);
 154           fprintf (debug, " is a hardware register.  No Coalescing.\n");
 155         }
 156       return;
 157     }
 158
 159   ann1 = var_ann (root1);
 160   ann2 = var_ann (root2);
 161
 162   if (p1 == p2)
 163     {
 164       if (debug)
 165         fprintf (debug, " : Already coalesced.\n");
 166       return;
 167     }
 168
 169   /* Partitions already have the same root, simply merge them.  */
 170   if (root1 == root2)
 171     {
 172       p1 = partition_union (map->var_partition, p1, p2);
 173       if (debug)
 174         fprintf (debug, " : Same root, coalesced --> P%d.\n", p1);
 175       return;
 176     }
 177
 178   /* Never attempt to coalesce 2 difference parameters.  */
 179   if (TREE_CODE (root1) == PARM_DECL && TREE_CODE (root2) == PARM_DECL)
 180     {
 181       if (debug)
 182         fprintf (debug, " : 2 different PARM_DECLS. No coalesce.\n");
 183       return;
 184     }
 185
 186   if ((TREE_CODE (root1) == RESULT_DECL) != (TREE_CODE (root2) == RESULT_DECL))
 187     {
 188       if (debug)
 189         fprintf (debug, " : One root a RESULT_DECL. No coalesce.\n");
 190       return;
 191     }
 192
 193   ign1 = TREE_CODE (root1) == VAR_DECL && DECL_IGNORED_P (root1);
 194   ign2 = TREE_CODE (root2) == VAR_DECL && DECL_IGNORED_P (root2);
 195
 196   /* Never attempt to coalesce 2 user variables unless one is an inline
 197      variable.  */
 198   if (!ign1 && !ign2)
 199     {
 200       if (DECL_FROM_INLINE (root2))
 201         ign2 = true;
 202       else if (DECL_FROM_INLINE (root1))
 203         ign1 = true;
 204       else
 205         {
 206           if (debug)
 207             fprintf (debug, " : 2 different USER vars. No coalesce.\n");
 208           return;
 209         }
 210     }
 211
 212   /* Don't coalesce if there are two different memory tags.  */
 213   if (ann1->type_mem_tag && ann2->type_mem_tag
 214       && ann1->type_mem_tag != ann2->type_mem_tag)
 215     {
 216       if (debug)
 217         fprintf (debug, " : 2 memory tags. No coalesce.\n");
 218       return;
 219     }
 220
 221   /* If both values have default defs, we can't coalesce.  If only one has a
 222      tag, make sure that variable is the new root partition.  */
 223   if (default_def (root1))
 224     {
 225       if (default_def (root2))
 226         {
 227           if (debug)
 228             fprintf (debug, " : 2 default defs. No coalesce.\n");
 229           return;
 230         }
 231       else
 232         {
 233           ign2 = true;
 234           ign1 = false;
 235         }
 236     }
 237   else if (default_def (root2))
 238     {
 239       ign1 = true;
 240       ign2 = false;
 241     }
 242
 243   /* Don't coalesce if the two variables aren't type compatible.  */
 244   if (!lang_hooks.types_compatible_p (TREE_TYPE (root1), TREE_TYPE (root2)))
 245     {
 246       if (debug)
 247         fprintf (debug, " : Incompatible types.  No coalesce.\n");
 248       return;
 249     }
 250
 251   /* Merge the two partitions.  */
 252   p3 = partition_union (map->var_partition, p1, p2);
 253
 254   /* Set the root variable of the partition to the better choice, if there is
 255      one.  */
 256   if (!ign2)
 257     replace_ssa_name_symbol (partition_to_var (map, p3), root2);
 258   else if (!ign1)
 259     replace_ssa_name_symbol (partition_to_var (map, p3), root1);
 260
 261   /* Update the various flag widgitry of the current base representative.  */
 262   ann3 = var_ann (SSA_NAME_VAR (partition_to_var (map, p3)));
 263   if (ann1->type_mem_tag)
 264     ann3->type_mem_tag = ann1->type_mem_tag;
 265   else
 266     ann3->type_mem_tag = ann2->type_mem_tag;
 267
 268   if (debug)
 269     {
 270       fprintf (debug, " --> P%d ", p3);
 271       print_generic_expr (debug, SSA_NAME_VAR (partition_to_var (map, p3)),
 272                           TDF_SLIM);
 273       fprintf (debug, "\n");
 274     }
 275 }
 276
 277
 278 /* This function will make a pass through the IL, and attempt to coalesce any
 279    SSA versions which occur in PHI's or copies.  Coalescing is accomplished by
 280    changing the underlying root variable of all coalesced version.  This will
 281    then cause the SSA->normal pass to attempt to coalesce them all to the same
 282    variable.  */
 283
 284 void
 285 rename_ssa_copies (void)
 286 {
 287   var_map map;
 288   basic_block bb;
 289   block_stmt_iterator bsi;
 290   tree phi, stmt, var, part_var;
 291   unsigned x;
 292   FILE *debug;
 293
 294   if (dump_file && (dump_flags & TDF_DETAILS))
 295     debug = dump_file;
 296   else
 297     debug = NULL;
 298
 299   map = init_var_map (num_ssa_names + 1);
 300
 301   FOR_EACH_BB (bb)
 302     {
 303       /* Scan for real copies.  */
 304       for (bsi = bsi_start (bb); !bsi_end_p (bsi); bsi_next (&bsi))
 305         {
 306           stmt = bsi_stmt (bsi);
 307           if (TREE_CODE (stmt) == MODIFY_EXPR)
 308             {
 309               tree lhs = TREE_OPERAND (stmt, 0);
 310               tree rhs = TREE_OPERAND (stmt, 1);
 311
 312               if (TREE_CODE (lhs) == SSA_NAME && TREE_CODE (rhs) == SSA_NAME)
 313                 copy_rename_partition_coalesce (map, lhs, rhs, debug);
 314             }
 315         }
 316     }
 317
 318   FOR_EACH_BB (bb)
 319     {
 320       /* Treat PHI nodes as copies between the result and each argument.  */
 321       for (phi = phi_nodes (bb); phi; phi = PHI_CHAIN (phi))
 322         {
 323           int i;
 324           tree res = PHI_RESULT (phi);
 325
 326           /* Do not process virtual SSA_NAMES.  */
 327           if (!is_gimple_reg (SSA_NAME_VAR (res)))
 328             continue;
 329
 330           for (i = 0; i < PHI_NUM_ARGS (phi); i++)
 331             {
 332               tree arg = PHI_ARG_DEF (phi, i);
 333               if (TREE_CODE (arg) == SSA_NAME)
 334                 copy_rename_partition_coalesce (map, res, arg, debug);
 335             }
 336         }
 337     }
 338
 339   if (debug)
 340     dump_var_map (debug, map);
 341
 342   /* Now one more pass to make all elements of a partition share the same
 343      root variable.  */
 344
 345   for (x = 1; x <= num_ssa_names; x++)
 346     {
 347       part_var = partition_to_var (map, x);
 348       if (!part_var)
 349         continue;
 350       var = map->partition_to_var[x];
 351       if (debug)
 352         {
 353           if (SSA_NAME_VAR (var) != SSA_NAME_VAR (part_var))
 354             {
 355               fprintf (debug, "Coalesced ");
 356               print_generic_expr (debug, var, TDF_SLIM);
 357               fprintf (debug, " to ");
 358               print_generic_expr (debug, part_var, TDF_SLIM);
 359               fprintf (debug, "\n");
 360             }
 361         }
 362       replace_ssa_name_symbol (var, SSA_NAME_VAR (part_var));
 363     }
 364
 365   delete_var_map (map);
 366 }
 367
 368 /* Return true if copy rename is to be performed.  */
 369
 370 static bool
 371 gate_copyrename (void)
 372 {
 373   return flag_tree_copyrename != 0;
 374 }
 375
 376 struct tree_opt_pass pass_rename_ssa_copies =
 377 {
 378   "copyrename",                         /* name */
 379   gate_copyrename,                      /* gate */
 380   rename_ssa_copies,                    /* execute */
 381   NULL,                                 /* sub */
 382   NULL,                                 /* next */
 383   0,                                    /* static_pass_number */
 384   TV_TREE_COPY_RENAME,                  /* tv_id */
 385   PROP_cfg | PROP_ssa | PROP_alias,     /* properties_required */
 386   0,                                    /* properties_provided */
 387   0,                                    /* properties_destroyed */
 388   0,                                    /* todo_flags_start */
 389   TODO_dump_func | TODO_verify_ssa,     /* todo_flags_finish */
 390   0                                     /* letter */
 391 };
 392