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 (LOOPS_HAVE_PREHEADERS
 136                                | LOOPS_HAVE_SIMPLE_LATCHES);
 137   if (!loops)
 138     return;
 139
 140 #ifdef ENABLE_CHECKING
 141   verify_loop_structure (loops);
 142 #endif
 143
 144   bbs = XNEWVEC (basic_block, n_basic_blocks);
 145   copied_bbs = XNEWVEC (basic_block, n_basic_blocks);
 146   bbs_size = n_basic_blocks;
 147
 148   for (i = 1; i < loops->num; i++)
 149     {
 150       /* Copy at most 20 insns.  */
 151       int limit = 20;
 152
 153       loop = loops->parray[i];
 154       if (!loop)
 155         continue;
 156       header = loop->header;
 157
 158       /* If the loop is already a do-while style one (either because it was
 159          written as such, or because jump threading transformed it into one),
 160          we might be in fact peeling the first iteration of the loop.  This
 161          in general is not a good idea.  */
 162       if (do_while_loop_p (loop))
 163         continue;
 164
 165       /* Iterate the header copying up to limit; this takes care of the cases
 166          like while (a && b) {...}, where we want to have both of the conditions
 167          copied.  TODO -- handle while (a || b) - like cases, by not requiring
 168          the header to have just a single successor and copying up to
 169          postdominator.  */
 170
 171       exit = NULL;
 172       n_bbs = 0;
 173       while (should_duplicate_loop_header_p (header, loop, &limit))
 174         {
 175           /* Find a successor of header that is inside a loop; i.e. the new
 176              header after the condition is copied.  */
 177           if (flow_bb_inside_loop_p (loop, EDGE_SUCC (header, 0)->dest))
 178             exit = EDGE_SUCC (header, 0);
 179           else
 180             exit = EDGE_SUCC (header, 1);
 181           bbs[n_bbs++] = header;
 182           gcc_assert (bbs_size > n_bbs);
 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_pred_edge (loop_split_edge_with (exit, NULL));
 198
 199       entry = loop_preheader_edge (loop);
 200
 201       if (!tree_duplicate_sese_region (entry, exit, bbs, n_bbs, copied_bbs))
 202         {
 203           fprintf (dump_file, "Duplication failed.\n");
 204           continue;
 205         }
 206
 207       /* Ensure that the latch and the preheader is simple (we know that they
 208          are not now, since there was the loop exit condition.  */
 209       loop_split_edge_with (loop_preheader_edge (loop), NULL);
 210       loop_split_edge_with (loop_latch_edge (loop), NULL);
 211     }
 212
 213   free (bbs);
 214   free (copied_bbs);
 215
 216   loop_optimizer_finalize (loops);
 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 };