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;
 131   basic_block *bbs;
 132   unsigned n_bbs;
 133
 134   loops = loop_optimizer_init (dump_file);
 135   if (!loops)
 136     return;
 137   rewrite_into_loop_closed_ssa (NULL);
 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
 149   for (i = 1; i < loops->num; i++)
 150     {
 151       /* Copy at most 20 insns.  */
 152       int limit = 20;
 153
 154       loop = loops->parray[i];
 155       if (!loop)
 156         continue;
 157       header = loop->header;
 158
 159       /* If the loop is already a do-while style one (either because it was
 160          written as such, or because jump threading transformed it into one),
 161          we might be in fact peeling the first iteration of the loop.  This
 162          in general is not a good idea.  */
 163       if (do_while_loop_p (loop))
 164         continue;
 165
 166       /* Iterate the header copying up to limit; this takes care of the cases
 167          like while (a && b) {...}, where we want to have both of the conditions
 168          copied.  TODO -- handle while (a || b) - like cases, by not requiring
 169          the header to have just a single successor and copying up to
 170          postdominator.  */
 171
 172       exit = NULL;
 173       n_bbs = 0;
 174       while (should_duplicate_loop_header_p (header, loop, &limit))
 175         {
 176           /* Find a successor of header that is inside a loop; i.e. the new
 177              header after the condition is copied.  */
 178           if (flow_bb_inside_loop_p (loop, EDGE_SUCC (header, 0)->dest))
 179             exit = EDGE_SUCC (header, 0);
 180           else
 181             exit = EDGE_SUCC (header, 1);
 182           bbs[n_bbs++] = header;
 183           header = exit->dest;
 184         }
 185
 186       if (!exit)
 187         continue;
 188
 189       if (dump_file && (dump_flags & TDF_DETAILS))
 190         fprintf (dump_file,
 191                  "Duplicating header of the loop %d up to edge %d->%d.\n",
 192                  loop->num, exit->src->index, exit->dest->index);
 193
 194       /* Ensure that the header will have just the latch as a predecessor
 195          inside the loop.  */
 196       if (!single_pred_p (exit->dest))
 197         exit = single_succ_edge (loop_split_edge_with (exit, NULL));
 198
 199       if (!tree_duplicate_sese_region (loop_preheader_edge (loop), exit,
 200                                        bbs, n_bbs, NULL))
 201         {
 202           fprintf (dump_file, "Duplication failed.\n");
 203           continue;
 204         }
 205
 206       /* Ensure that the latch and the preheader is simple (we know that they
 207          are not now, since there was the loop exit condition.  */
 208       loop_split_edge_with (loop_preheader_edge (loop), NULL);
 209       loop_split_edge_with (loop_latch_edge (loop), NULL);
 210     }
 211
 212   free (bbs);
 213
 214 #ifdef ENABLE_CHECKING
 215   verify_loop_closed_ssa ();
 216 #endif
 217
 218   loop_optimizer_finalize (loops, NULL);
 219 }
 220
 221 static bool
 222 gate_ch (void)
 223 {
 224   return flag_tree_ch != 0;
 225 }
 226
 227 struct tree_opt_pass pass_ch =
 228 {
 229   "ch",                                 /* name */
 230   gate_ch,                              /* gate */
 231   copy_loop_headers,                    /* execute */
 232   NULL,                                 /* sub */
 233   NULL,                                 /* next */
 234   0,                                    /* static_pass_number */
 235   TV_TREE_CH,                           /* tv_id */
 236   PROP_cfg | PROP_ssa,                  /* properties_required */
 237   0,                                    /* properties_provided */
 238   0,                                    /* properties_destroyed */
 239   0,                                    /* todo_flags_start */
 240   TODO_cleanup_cfg | TODO_dump_func
 241   | TODO_verify_ssa,                    /* todo_flags_finish */
 242   0                                     /* letter */
 243 };