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
 135   loops = loop_optimizer_init (dump_file);
 136   if (!loops)
 137     return;
 138
 139   /* We do not try to keep the information about irreducible regions
 140      up-to-date.  */
 141   loops->state &= ~LOOPS_HAVE_MARKED_IRREDUCIBLE_REGIONS;
 142
 143 #ifdef ENABLE_CHECKING
 144   verify_loop_structure (loops);
 145 #endif
 146
 147   bbs = xmalloc (sizeof (basic_block) * n_basic_blocks);
 148   copied_bbs = xmalloc (sizeof (basic_block) * n_basic_blocks);
 149   bbs_size = n_basic_blocks;
 150
 151   for (i = 1; i < loops->num; i++)
 152     {
 153       /* Copy at most 20 insns.  */
 154       int limit = 20;
 155
 156       loop = loops->parray[i];
 157       if (!loop)
 158         continue;
 159       header = loop->header;
 160
 161       /* If the loop is already a do-while style one (either because it was
 162          written as such, or because jump threading transformed it into one),
 163          we might be in fact peeling the first iteration of the loop.  This
 164          in general is not a good idea.  */
 165       if (do_while_loop_p (loop))
 166         continue;
 167
 168       /* Iterate the header copying up to limit; this takes care of the cases
 169          like while (a && b) {...}, where we want to have both of the conditions
 170          copied.  TODO -- handle while (a || b) - like cases, by not requiring
 171          the header to have just a single successor and copying up to
 172          postdominator.  */
 173
 174       exit = NULL;
 175       n_bbs = 0;
 176       while (should_duplicate_loop_header_p (header, loop, &limit))
 177         {
 178           /* Find a successor of header that is inside a loop; i.e. the new
 179              header after the condition is copied.  */
 180           if (flow_bb_inside_loop_p (loop, EDGE_SUCC (header, 0)->dest))
 181             exit = EDGE_SUCC (header, 0);
 182           else
 183             exit = EDGE_SUCC (header, 1);
 184           bbs[n_bbs++] = header;
 185           gcc_assert (bbs_size > n_bbs);
 186           header = exit->dest;
 187         }
 188
 189       if (!exit)
 190         continue;
 191
 192       if (dump_file && (dump_flags & TDF_DETAILS))
 193         fprintf (dump_file,
 194                  "Duplicating header of the loop %d up to edge %d->%d.\n",
 195                  loop->num, exit->src->index, exit->dest->index);
 196
 197       /* Ensure that the header will have just the latch as a predecessor
 198          inside the loop.  */
 199       if (!single_pred_p (exit->dest))
 200         exit = single_succ_edge (loop_split_edge_with (exit, NULL));
 201
 202       entry = loop_preheader_edge (loop);
 203
 204       if (!tree_duplicate_sese_region (entry, exit, bbs, n_bbs, copied_bbs))
 205         {
 206           fprintf (dump_file, "Duplication failed.\n");
 207           continue;
 208         }
 209
 210       /* Ensure that the latch and the preheader is simple (we know that they
 211          are not now, since there was the loop exit condition.  */
 212       loop_split_edge_with (loop_preheader_edge (loop), NULL);
 213       loop_split_edge_with (loop_latch_edge (loop), NULL);
 214     }
 215
 216   free (bbs);
 217   free (copied_bbs);
 218
 219   loop_optimizer_finalize (loops, NULL);
 220 }
 221
 222 static bool
 223 gate_ch (void)
 224 {
 225   return flag_tree_ch != 0;
 226 }
 227
 228 struct tree_opt_pass pass_ch =
 229 {
 230   "ch",                                 /* name */
 231   gate_ch,                              /* gate */
 232   copy_loop_headers,                    /* execute */
 233   NULL,                                 /* sub */
 234   NULL,                                 /* next */
 235   0,                                    /* static_pass_number */
 236   TV_TREE_CH,                           /* tv_id */
 237   PROP_cfg | PROP_ssa,                  /* properties_required */
 238   0,                                    /* properties_provided */
 239   0,                                    /* properties_destroyed */
 240   0,                                    /* todo_flags_start */
 241   TODO_cleanup_cfg | TODO_dump_func
 242   | TODO_verify_ssa,                    /* todo_flags_finish */
 243   0                                     /* letter */
 244 };