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 unsigned int
 124 copy_loop_headers (void)
 125 {
 126   unsigned i;
 127   struct loop *loop;
 128   basic_block header;
 129   edge exit, entry;
 130   basic_block *bbs, *copied_bbs;
 131   unsigned n_bbs;
 132   unsigned bbs_size;
 133
 134   loop_optimizer_init (LOOPS_HAVE_PREHEADERS
 135                        | LOOPS_HAVE_SIMPLE_LATCHES);
 136   if (!current_loops)
 137     return 0;
 138
 139 #ifdef ENABLE_CHECKING
 140   verify_loop_structure ();
 141 #endif
 142
 143   bbs = XNEWVEC (basic_block, n_basic_blocks);
 144   copied_bbs = XNEWVEC (basic_block, n_basic_blocks);
 145   bbs_size = n_basic_blocks;
 146
 147   for (i = 1; i < current_loops->num; i++)
 148     {
 149       /* Copy at most 20 insns.  */
 150       int limit = 20;
 151
 152       loop = current_loops->parray[i];
 153       if (!loop)
 154         continue;
 155       header = loop->header;
 156
 157       /* If the loop is already a do-while style one (either because it was
 158          written as such, or because jump threading transformed it into one),
 159          we might be in fact peeling the first iteration of the loop.  This
 160          in general is not a good idea.  */
 161       if (do_while_loop_p (loop))
 162         continue;
 163
 164       /* Iterate the header copying up to limit; this takes care of the cases
 165          like while (a && b) {...}, where we want to have both of the conditions
 166          copied.  TODO -- handle while (a || b) - like cases, by not requiring
 167          the header to have just a single successor and copying up to
 168          postdominator.  */
 169
 170       exit = NULL;
 171       n_bbs = 0;
 172       while (should_duplicate_loop_header_p (header, loop, &limit))
 173         {
 174           /* Find a successor of header that is inside a loop; i.e. the new
 175              header after the condition is copied.  */
 176           if (flow_bb_inside_loop_p (loop, EDGE_SUCC (header, 0)->dest))
 177             exit = EDGE_SUCC (header, 0);
 178           else
 179             exit = EDGE_SUCC (header, 1);
 180           bbs[n_bbs++] = header;
 181           gcc_assert (bbs_size > n_bbs);
 182           header = exit->dest;
 183         }
 184
 185       if (!exit)
 186         continue;
 187
 188       if (dump_file && (dump_flags & TDF_DETAILS))
 189         fprintf (dump_file,
 190                  "Duplicating header of the loop %d up to edge %d->%d.\n",
 191                  loop->num, exit->src->index, exit->dest->index);
 192
 193       /* Ensure that the header will have just the latch as a predecessor
 194          inside the loop.  */
 195       if (!single_pred_p (exit->dest))
 196         exit = single_pred_edge (split_edge (exit));
 197
 198       entry = loop_preheader_edge (loop);
 199
 200       if (!tree_duplicate_sese_region (entry, exit, bbs, n_bbs, copied_bbs))
 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       split_edge (loop_preheader_edge (loop));
 209       split_edge (loop_latch_edge (loop));
 210     }
 211
 212   free (bbs);
 213   free (copied_bbs);
 214
 215   loop_optimizer_finalize ();
 216   return 0;
 217 }
 218
 219 static bool
 220 gate_ch (void)
 221 {
 222   return flag_tree_ch != 0;
 223 }
 224
 225 struct tree_opt_pass pass_ch =
 226 {
 227   "ch",                                 /* name */
 228   gate_ch,                              /* gate */
 229   copy_loop_headers,                    /* execute */
 230   NULL,                                 /* sub */
 231   NULL,                                 /* next */
 232   0,                                    /* static_pass_number */
 233   TV_TREE_CH,                           /* tv_id */
 234   PROP_cfg | PROP_ssa,                  /* properties_required */
 235   0,                                    /* properties_provided */
 236   0,                                    /* properties_destroyed */
 237   0,                                    /* todo_flags_start */
 238   TODO_cleanup_cfg | TODO_dump_func
 239   | TODO_verify_ssa,                    /* todo_flags_finish */
 240   0                                     /* letter */
 241 };