gcc/tree-ssa-loop-ivcanon.c

   1 /* Induction variable canonicalization.
   2    Copyright (C) 2004, 2005 Free Software Foundation, Inc.
   3
   4 This file is part of GCC.
   5
   6 GCC is free software; you can redistribute it and/or modify it
   7 under the terms of the GNU General Public License as published by the
   8 Free Software Foundation; either version 2, or (at your option) any
   9 later version.
  10
  11 GCC is distributed in the hope that it will be useful, but WITHOUT
  12 ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  13 FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  14 for more details.
  15
  16 You should have received a copy of the GNU General Public License
  17 along with GCC; see the file COPYING.  If not, write to the Free
  18 Software Foundation, 59 Temple Place - Suite 330, Boston, MA
  19 02111-1307, USA.  */
  20
  21 /* This pass detects the loops that iterate a constant number of times,
  22    adds a canonical induction variable (step -1, tested against 0)
  23    and replaces the exit test.  This enables the less powerful rtl
  24    level analysis to use this information.
  25
  26    This might spoil the code in some cases (by increasing register pressure).
  27    Note that in the case the new variable is not needed, ivopts will get rid
  28    of it, so it might only be a problem when there are no other linear induction
  29    variables.  In that case the created optimization possibilities are likely
  30    to pay up.
  31
  32    Additionally in case we detect that it is beneficial to unroll the
  33    loop completely, we do it right here to expose the optimization
  34    possibilities to the following passes.  */
  35
  36 #include "config.h"
  37 #include "system.h"
  38 #include "coretypes.h"
  39 #include "tm.h"
  40 #include "tree.h"
  41 #include "rtl.h"
  42 #include "tm_p.h"
  43 #include "hard-reg-set.h"
  44 #include "basic-block.h"
  45 #include "output.h"
  46 #include "diagnostic.h"
  47 #include "tree-flow.h"
  48 #include "tree-dump.h"
  49 #include "cfgloop.h"
  50 #include "tree-pass.h"
  51 #include "ggc.h"
  52 #include "tree-chrec.h"
  53 #include "tree-scalar-evolution.h"
  54 #include "params.h"
  55 #include "flags.h"
  56 #include "tree-inline.h"
  57
  58 /* Adds a canonical induction variable to LOOP iterating NITER times.  EXIT
  59    is the exit edge whose condition is replaced.  */
  60
  61 static void
  62 create_canonical_iv (struct loop *loop, edge exit, tree niter)
  63 {
  64   edge in;
  65   tree cond, type, var;
  66   block_stmt_iterator incr_at;
  67   enum tree_code cmp;
  68
  69   if (dump_file && (dump_flags & TDF_DETAILS))
  70     {
  71       fprintf (dump_file, "Added canonical iv to loop %d, ", loop->num);
  72       print_generic_expr (dump_file, niter, TDF_SLIM);
  73       fprintf (dump_file, " iterations.\n");
  74     }
  75
  76   cond = last_stmt (exit->src);
  77   in = EDGE_SUCC (exit->src, 0);
  78   if (in == exit)
  79     in = EDGE_SUCC (exit->src, 1);
  80
  81   /* Note that we do not need to worry about overflows, since
  82      type of niter is always unsigned and all comparisons are
  83      just for equality/nonequality -- i.e. everything works
  84      with a modulo arithmetics.  */
  85
  86   type = TREE_TYPE (niter);
  87   niter = fold (build2 (PLUS_EXPR, type,
  88                         niter,
  89                         build_int_cst (type, 1)));
  90   incr_at = bsi_last (in->src);
  91   create_iv (niter,
  92              fold_convert (type, integer_minus_one_node),
  93              NULL_TREE, loop,
  94              &incr_at, false, NULL, &var);
  95
  96   cmp = (exit->flags & EDGE_TRUE_VALUE) ? EQ_EXPR : NE_EXPR;
  97   COND_EXPR_COND (cond) = build2 (cmp, boolean_type_node,
  98                                   var,
  99                                   build_int_cst (type, 0));
 100   update_stmt (cond);
 101 }
 102
 103 /* Computes an estimated number of insns in LOOP.  */
 104
 105 unsigned
 106 tree_num_loop_insns (struct loop *loop)
 107 {
 108   basic_block *body = get_loop_body (loop);
 109   block_stmt_iterator bsi;
 110   unsigned size = 1, i;
 111
 112   for (i = 0; i < loop->num_nodes; i++)
 113     for (bsi = bsi_start (body[i]); !bsi_end_p (bsi); bsi_next (&bsi))
 114       size += estimate_num_insns (bsi_stmt (bsi));
 115   free (body);
 116
 117   return size;
 118 }
 119
 120 /* Tries to unroll LOOP completely, i.e. NITER times.  LOOPS is the
 121    loop tree.  COMPLETELY_UNROLL is true if we should unroll the loop
 122    even if it may cause code growth.  EXIT is the exit of the loop
 123    that should be eliminated.  */
 124
 125 static bool
 126 try_unroll_loop_completely (struct loops *loops ATTRIBUTE_UNUSED,
 127                             struct loop *loop,
 128                             edge exit, tree niter,
 129                             bool completely_unroll)
 130 {
 131   unsigned HOST_WIDE_INT n_unroll, ninsns, max_unroll;
 132   tree old_cond, cond, dont_exit, do_exit;
 133
 134   if (loop->inner)
 135     return false;
 136
 137   if (!host_integerp (niter, 1))
 138     return false;
 139   n_unroll = tree_low_cst (niter, 1);
 140
 141   max_unroll = PARAM_VALUE (PARAM_MAX_COMPLETELY_PEEL_TIMES);
 142   if (n_unroll > max_unroll)
 143     return false;
 144
 145   if (n_unroll)
 146     {
 147       if (!completely_unroll)
 148         return false;
 149
 150       ninsns = tree_num_loop_insns (loop);
 151
 152       if (n_unroll * ninsns
 153           > (unsigned) PARAM_VALUE (PARAM_MAX_COMPLETELY_PEELED_INSNS))
 154         return false;
 155     }
 156
 157   if (exit->flags & EDGE_TRUE_VALUE)
 158     {
 159       dont_exit = boolean_false_node;
 160       do_exit = boolean_true_node;
 161     }
 162   else
 163     {
 164       dont_exit = boolean_true_node;
 165       do_exit = boolean_false_node;
 166     }
 167   cond = last_stmt (exit->src);
 168
 169   if (n_unroll)
 170     {
 171       old_cond = COND_EXPR_COND (cond);
 172       COND_EXPR_COND (cond) = dont_exit;
 173       update_stmt (cond);
 174
 175       if (!tree_duplicate_loop_to_header_edge (loop, loop_preheader_edge (loop),
 176                                                loops, n_unroll, NULL,
 177                                                NULL, NULL, NULL, 0))
 178         {
 179           COND_EXPR_COND (cond) = old_cond;
 180           update_stmt (cond);
 181           return false;
 182         }
 183     }
 184
 185   COND_EXPR_COND (cond) = do_exit;
 186   update_stmt (cond);
 187
 188   update_ssa (TODO_update_ssa);
 189
 190   if (dump_file && (dump_flags & TDF_DETAILS))
 191     fprintf (dump_file, "Unrolled loop %d completely.\n", loop->num);
 192
 193   return true;
 194 }
 195
 196 /* Adds a canonical induction variable to LOOP if suitable.  LOOPS is the loops
 197    tree.  CREATE_IV is true if we may create a new iv.  COMPLETELY_UNROLL is
 198    true if we should do complete unrolling even if it may cause the code
 199    growth.  If TRY_EVAL is true, we try to determine the number of iterations
 200    of a loop by direct evaluation.  Returns true if cfg is changed.  */
 201
 202 static bool
 203 canonicalize_loop_induction_variables (struct loops *loops, struct loop *loop,
 204                                        bool create_iv, bool completely_unroll,
 205                                        bool try_eval)
 206 {
 207   edge exit = NULL;
 208   tree niter;
 209
 210   niter = number_of_iterations_in_loop (loop);
 211   if (TREE_CODE (niter) == INTEGER_CST)
 212     {
 213       exit = loop->single_exit;
 214       if (!just_once_each_iteration_p (loop, exit->src))
 215         return false;
 216
 217       /* The result of number_of_iterations_in_loop is by one higher than
 218          we expect (i.e. it returns number of executions of the exit
 219          condition, not of the loop latch edge).  */
 220       niter = fold (build2 (MINUS_EXPR, TREE_TYPE (niter), niter,
 221                             build_int_cst (TREE_TYPE (niter), 1)));
 222     }
 223   else
 224     {
 225       /* If the loop has more than one exit, try checking all of them
 226          for # of iterations determinable through scev.  */
 227       if (!loop->single_exit)
 228         niter = find_loop_niter (loop, &exit);
 229
 230       /* Finally if everything else fails, try brute force evaluation.  */
 231       if (try_eval
 232           && (chrec_contains_undetermined (niter)
 233               || TREE_CODE (niter) != INTEGER_CST))
 234         niter = find_loop_niter_by_eval (loop, &exit);
 235
 236       if (chrec_contains_undetermined (niter)
 237           || TREE_CODE (niter) != INTEGER_CST)
 238         return false;
 239     }
 240
 241   if (dump_file && (dump_flags & TDF_DETAILS))
 242     {
 243       fprintf (dump_file, "Loop %d iterates ", loop->num);
 244       print_generic_expr (dump_file, niter, TDF_SLIM);
 245       fprintf (dump_file, " times.\n");
 246     }
 247
 248   if (try_unroll_loop_completely (loops, loop, exit, niter, completely_unroll))
 249     return true;
 250
 251   if (create_iv)
 252     create_canonical_iv (loop, exit, niter);
 253
 254   return false;
 255 }
 256
 257 /* The main entry point of the pass.  Adds canonical induction variables
 258    to the suitable LOOPS.  */
 259
 260 void
 261 canonicalize_induction_variables (struct loops *loops)
 262 {
 263   unsigned i;
 264   struct loop *loop;
 265   bool changed = false;
 266
 267   for (i = 1; i < loops->num; i++)
 268     {
 269       loop = loops->parray[i];
 270
 271       if (loop)
 272         changed |= canonicalize_loop_induction_variables (loops, loop,
 273                                                           true, false, true);
 274     }
 275
 276   /* Clean up the information about numbers of iterations, since brute force
 277      evaluation could reveal new information.  */
 278   scev_reset ();
 279
 280   if (changed)
 281     cleanup_tree_cfg_loop ();
 282 }
 283
 284 /* Unroll LOOPS completely if they iterate just few times.  */
 285
 286 void
 287 tree_unroll_loops_completely (struct loops *loops)
 288 {
 289   unsigned i;
 290   struct loop *loop;
 291   bool changed = false;
 292
 293   for (i = 1; i < loops->num; i++)
 294     {
 295       loop = loops->parray[i];
 296
 297       if (!loop)
 298         continue;
 299
 300       changed |= canonicalize_loop_induction_variables (loops, loop,
 301                                                         false, true,
 302                                                         !flag_tree_loop_ivcanon);
 303     }
 304
 305   /* Clean up the information about numbers of iterations, since complete
 306      unrolling might have invalidated it.  */
 307   scev_reset ();
 308
 309   if (changed)
 310     cleanup_tree_cfg_loop ();
 311 }