gnu/dist/gcc4/gcc/tree-ssa-loop-ch.c

   1 /* Loop header copying on trees.
   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, 51 Franklin Street, Fifth Floor, Boston, MA
  19 02110-1301, USA.  */
  20
  21 #include "config.h"
  22 #include "system.h"
  23 #include "coretypes.h"
  24 #include "tm.h"
  25 #include "tree.h"
  26 #include "rtl.h"
  27 #include "tm_p.h"
  28 #include "hard-reg-set.h"
  29 #include "basic-block.h"
  30 #include "output.h"
  31 #include "diagnostic.h"
  32 #include "tree-flow.h"
  33 #include "tree-dump.h"
  34 #include "tree-pass.h"
  35 #include "timevar.h"
  36 #include "cfgloop.h"
  37 #include "tree-inline.h"
  38 #include "flags.h"
  39 #include "tree-inline.h"
  40
  41 /* Duplicates headers of loops if they are small enough, so that the statements
  42    in the loop body are always executed when the loop is entered.  This
  43    increases effectiveness of code motion optimizations, and reduces the need
  44    for loop preconditioning.  */
  45
  46 /* Check whether we should duplicate HEADER of LOOP.  At most *LIMIT
  47    instructions should be duplicated, limit is decreased by the actual
  48    amount.  */
  49
  50 static bool
  51 should_duplicate_loop_header_p (basic_block header, struct loop *loop,
  52                                 int *limit)
  53 {
  54   block_stmt_iterator bsi;
  55   tree last;
  56
  57   /* Do not copy one block more than once (we do not really want to do
  58      loop peeling here).  */
  59   if (header->aux)
  60     return false;
  61
  62   gcc_assert (EDGE_COUNT (header->succs) > 0);
  63   if (single_succ_p (header))
  64     return false;
  65   if (flow_bb_inside_loop_p (loop, EDGE_SUCC (header, 0)->dest)
  66       && flow_bb_inside_loop_p (loop, EDGE_SUCC (header, 1)->dest))
  67     return false;
  68
  69   /* If this is not the original loop header, we want it to have just
  70      one predecessor in order to match the && pattern.  */
  71   if (header != loop->header && !single_pred_p (header))
  72     return false;
  73
  74   last = last_stmt (header);
  75   if (TREE_CODE (last) != COND_EXPR)
  76     return false;
  77
  78   /* Approximately copy the conditions that used to be used in jump.c --
  79      at most 20 insns and no calls.  */
  80   for (bsi = bsi_start (header); !bsi_end_p (bsi); bsi_next (&bsi))
  81     {
  82       last = bsi_stmt (bsi);
  83
  84       if (TREE_CODE (last) == LABEL_EXPR)
  85         continue;
  86
  87       if (get_call_expr_in (last))
  88         return false;
  89
  90       *limit -= estimate_num_insns (last);
  91       if (*limit < 0)
  92         return false;
  93     }
  94
  95   return true;
  96 }
  97
  98 /* Checks whether LOOP is a do-while style loop.  */
  99
 100 static bool
 101 do_while_loop_p (struct loop *loop)
 102 {
 103   tree stmt = last_stmt (loop->latch);
 104
 105   /* If the latch of the loop is not empty, it is not a do-while loop.  */
 106   if (stmt
 107       && TREE_CODE (stmt) != LABEL_EXPR)
 108     return false;
 109
 110   /* If the header contains just a condition, it is not a do-while loop.  */
 111   stmt = last_and_only_stmt (loop->header);
 112   if (stmt
 113       && TREE_CODE (stmt) == COND_EXPR)
 114     return false;
 115
 116   return true;
 117 }
 118
 119 /* For all loops, copy the condition at the end of the loop body in front
 120    of the loop.  This is beneficial since it increases efficiency of
 121    code motion optimizations.  It also saves one jump on entry to the loop.  */
 122
 123 static void
 124 copy_loop_headers (void)
 125 {
 126   struct loops *loops;
 127   unsigned i;
 128   struct loop *loop;
 129   basic_block header;
 130   edge exit, entry;
 131   basic_block *bbs, *copied_bbs;
 132   unsigned n_bbs;
 133   unsigned bbs_size;
 134   bool copied_p;
 135
 136   loops = loop_optimizer_init (dump_file);
 137   if (!loops)
 138     return;
 139
 140   /* We do not try to keep the information about irreducible regions
 141      up-to-date.  */
 142   loops->state &= ~LOOPS_HAVE_MARKED_IRREDUCIBLE_REGIONS;
 143
 144 #ifdef ENABLE_CHECKING
 145   verify_loop_structure (loops);
 146 #endif
 147
 148   bbs = xmalloc (sizeof (basic_block) * n_basic_blocks);
 149   copied_bbs = xmalloc (sizeof (basic_block) * n_basic_blocks);
 150   bbs_size = n_basic_blocks;
 151
 152   copied_p = 0;
 153   for (i = 1; i < loops->num; i++)
 154     {
 155       /* Copy at most 20 insns.  */
 156       int limit = 20;
 157
 158       loop = loops->parray[i];
 159       if (!loop)
 160         continue;
 161       header = loop->header;
 162
 163       /* If the loop is already a do-while style one (either because it was
 164          written as such, or because jump threading transformed it into one),
 165          we might be in fact peeling the first iteration of the loop.  This
 166          in general is not a good idea.  */
 167       if (do_while_loop_p (loop))
 168         continue;
 169
 170       /* Iterate the header copying up to limit; this takes care of the cases
 171          like while (a && b) {...}, where we want to have both of the conditions
 172          copied.  TODO -- handle while (a || b) - like cases, by not requiring
 173          the header to have just a single successor and copying up to
 174          postdominator.  */
 175
 176       exit = NULL;
 177       n_bbs = 0;
 178       while (should_duplicate_loop_header_p (header, loop, &limit))
 179         {
 180           /* Find a successor of header that is inside a loop; i.e. the new
 181              header after the condition is copied.  */
 182           if (flow_bb_inside_loop_p (loop, EDGE_SUCC (header, 0)->dest))
 183             exit = EDGE_SUCC (header, 0);
 184           else
 185             exit = EDGE_SUCC (header, 1);
 186           bbs[n_bbs++] = header;
 187           gcc_assert (bbs_size > n_bbs);
 188           header = exit->dest;
 189         }
 190
 191       if (!exit)
 192         continue;
 193
 194       if (dump_file && (dump_flags & TDF_DETAILS))
 195         fprintf (dump_file,
 196                  "Duplicating header of the loop %d up to edge %d->%d.\n",
 197                  loop->num, exit->src->index, exit->dest->index);
 198
 199       /* Ensure that the header will have just the latch as a predecessor
 200          inside the loop.  */
 201       if (!single_pred_p (exit->dest))
 202         exit = single_pred_edge (loop_split_edge_with (exit, NULL));
 203
 204       entry = loop_preheader_edge (loop);
 205
 206       if (tree_duplicate_sese_region (entry, exit, bbs, n_bbs, copied_bbs))
 207         copied_p = true;
 208       else
 209         {
 210           fprintf (dump_file, "Duplication failed.\n");
 211           continue;
 212         }
 213
 214       /* Ensure that the latch and the preheader is simple (we know that they
 215          are not now, since there was the loop exit condition.  */
 216       loop_split_edge_with (loop_preheader_edge (loop), NULL);
 217       loop_split_edge_with (loop_latch_edge (loop), NULL);
 218     }
 219
 220   if (copied_p)
 221     update_ssa (TODO_update_ssa);
 222
 223   free (bbs);
 224   free (copied_bbs);
 225
 226   loop_optimizer_finalize (loops, NULL);
 227 }
 228
 229 static bool
 230 gate_ch (void)
 231 {
 232   return flag_tree_ch != 0;
 233 }
 234
 235 struct tree_opt_pass pass_ch =
 236 {
 237   "ch",                                 /* name */
 238   gate_ch,                              /* gate */
 239   copy_loop_headers,                    /* execute */
 240   NULL,                                 /* sub */
 241   NULL,                                 /* next */
 242   0,                                    /* static_pass_number */
 243   TV_TREE_CH,                           /* tv_id */
 244   PROP_cfg | PROP_ssa,                  /* properties_required */
 245   0,                                    /* properties_provided */
 246   0,                                    /* properties_destroyed */
 247   0,                                    /* todo_flags_start */
 248   TODO_cleanup_cfg | TODO_dump_func
 249   | TODO_verify_ssa,                    /* todo_flags_finish */
 250   0                                     /* letter */
 251 };