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, 59 Temple Place - Suite 330, Boston, MA
  19 02111-1307, 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   gcov_type entry_count, body_count, total_count;
 135
 136   loops = loop_optimizer_init (dump_file);
 137   if (!loops)
 138     return;
 139   rewrite_into_loop_closed_ssa (NULL);
 140
 141   /* We do not try to keep the information about irreducible regions
 142      up-to-date.  */
 143   loops->state &= ~LOOPS_HAVE_MARKED_IRREDUCIBLE_REGIONS;
 144
 145 #ifdef ENABLE_CHECKING
 146   verify_loop_structure (loops);
 147 #endif
 148
 149   bbs = xmalloc (sizeof (basic_block) * n_basic_blocks);
 150   copied_bbs = xmalloc (sizeof (basic_block) * n_basic_blocks);
 151   bbs_size = n_basic_blocks;
 152
 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_succ_edge (loop_split_edge_with (exit, NULL));
 203
 204       entry = loop_preheader_edge (loop);
 205       entry_count = entry->src->count;
 206       body_count = exit->dest->count;
 207
 208       if (!tree_duplicate_sese_region (entry, exit, bbs, n_bbs, copied_bbs))
 209         {
 210           fprintf (dump_file, "Duplication failed.\n");
 211           continue;
 212         }
 213
 214       /* Fix profiling info.  Scaling is done in gcov_type arithmetic to
 215          avoid losing information; this is slow, but is done at most
 216          once per loop.  We special case 0 to avoid division by 0;
 217          probably other special cases exist.  */
 218       total_count = body_count + entry_count;
 219       if (total_count == 0LL)
 220         {
 221           scale_bbs_frequencies_int (bbs, n_bbs, 0, 1);
 222           scale_bbs_frequencies_int (copied_bbs, n_bbs, 0, 1);
 223         }
 224       else
 225         {
 226           scale_bbs_frequencies_gcov_type (bbs, n_bbs, body_count, total_count);
 227           scale_bbs_frequencies_gcov_type (copied_bbs, n_bbs, entry_count,
 228                                            total_count);
 229         }
 230
 231       /* Ensure that the latch and the preheader is simple (we know that they
 232          are not now, since there was the loop exit condition.  */
 233       loop_split_edge_with (loop_preheader_edge (loop), NULL);
 234       loop_split_edge_with (loop_latch_edge (loop), NULL);
 235     }
 236
 237   free (bbs);
 238   free (copied_bbs);
 239
 240   loop_optimizer_finalize (loops, NULL);
 241 }
 242
 243 static bool
 244 gate_ch (void)
 245 {
 246   return flag_tree_ch != 0;
 247 }
 248
 249 struct tree_opt_pass pass_ch =
 250 {
 251   "ch",                                 /* name */
 252   gate_ch,                              /* gate */
 253   copy_loop_headers,                    /* execute */
 254   NULL,                                 /* sub */
 255   NULL,                                 /* next */
 256   0,                                    /* static_pass_number */
 257   TV_TREE_CH,                           /* tv_id */
 258   PROP_cfg | PROP_ssa,                  /* properties_required */
 259   0,                                    /* properties_provided */
 260   0,                                    /* properties_destroyed */
 261   0,                                    /* todo_flags_start */
 262   TODO_cleanup_cfg | TODO_dump_func
 263   | TODO_verify_ssa,                    /* todo_flags_finish */
 264   0                                     /* letter */
 265 };