gcc/tree-ssa-loop-ch.c

   1 /* Loop header copying on trees.
   2    Copyright (C) 2004-2014 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 3, 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 COPYING3.  If not see
  18 <http://www.gnu.org/licenses/>.  */
  19
  20 #include "config.h"
  21 #include "system.h"
  22 #include "coretypes.h"
  23 #include "tm.h"
  24 #include "tree.h"
  25 #include "tm_p.h"
  26 #include "basic-block.h"
  27 #include "tree-ssa-alias.h"
  28 #include "internal-fn.h"
  29 #include "gimple-expr.h"
  30 #include "is-a.h"
  31 #include "gimple.h"
  32 #include "gimple-iterator.h"
  33 #include "gimple-ssa.h"
  34 #include "tree-cfg.h"
  35 #include "tree-into-ssa.h"
  36 #include "tree-pass.h"
  37 #include "cfgloop.h"
  38 #include "tree-inline.h"
  39 #include "flags.h"
  40 #include "tree-ssa-threadedge.h"
  41
  42 /* Duplicates headers of loops if they are small enough, so that the statements
  43    in the loop body are always executed when the loop is entered.  This
  44    increases effectiveness of code motion optimizations, and reduces the need
  45    for loop preconditioning.  */
  46
  47 /* Check whether we should duplicate HEADER of LOOP.  At most *LIMIT
  48    instructions should be duplicated, limit is decreased by the actual
  49    amount.  */
  50
  51 static bool
  52 should_duplicate_loop_header_p (basic_block header, struct loop *loop,
  53                                 int *limit)
  54 {
  55   gimple_stmt_iterator bsi;
  56   gimple last;
  57
  58   /* Do not copy one block more than once (we do not really want to do
  59      loop peeling here).  */
  60   if (header->aux)
  61     return false;
  62
  63   /* Loop header copying usually increases size of the code.  This used not to
  64      be true, since quite often it is possible to verify that the condition is
  65      satisfied in the first iteration and therefore to eliminate it.  Jump
  66      threading handles these cases now.  */
  67   if (optimize_loop_for_size_p (loop))
  68     return false;
  69
  70   gcc_assert (EDGE_COUNT (header->succs) > 0);
  71   if (single_succ_p (header))
  72     return false;
  73   if (flow_bb_inside_loop_p (loop, EDGE_SUCC (header, 0)->dest)
  74       && flow_bb_inside_loop_p (loop, EDGE_SUCC (header, 1)->dest))
  75     return false;
  76
  77   /* If this is not the original loop header, we want it to have just
  78      one predecessor in order to match the && pattern.  */
  79   if (header != loop->header && !single_pred_p (header))
  80     return false;
  81
  82   last = last_stmt (header);
  83   if (gimple_code (last) != GIMPLE_COND)
  84     return false;
  85
  86   /* Approximately copy the conditions that used to be used in jump.c --
  87      at most 20 insns and no calls.  */
  88   for (bsi = gsi_start_bb (header); !gsi_end_p (bsi); gsi_next (&bsi))
  89     {
  90       last = gsi_stmt (bsi);
  91
  92       if (gimple_code (last) == GIMPLE_LABEL)
  93         continue;
  94
  95       if (is_gimple_debug (last))
  96         continue;
  97
  98       if (is_gimple_call (last))
  99         return false;
 100
 101       *limit -= estimate_num_insns (last, &eni_size_weights);
 102       if (*limit < 0)
 103         return false;
 104     }
 105
 106   return true;
 107 }
 108
 109 /* Checks whether LOOP is a do-while style loop.  */
 110
 111 static bool
 112 do_while_loop_p (struct loop *loop)
 113 {
 114   gimple stmt = last_stmt (loop->latch);
 115
 116   /* If the latch of the loop is not empty, it is not a do-while loop.  */
 117   if (stmt
 118       && gimple_code (stmt) != GIMPLE_LABEL)
 119     return false;
 120
 121   /* If the header contains just a condition, it is not a do-while loop.  */
 122   stmt = last_and_only_stmt (loop->header);
 123   if (stmt
 124       && gimple_code (stmt) == GIMPLE_COND)
 125     return false;
 126
 127   return true;
 128 }
 129
 130 /* For all loops, copy the condition at the end of the loop body in front
 131    of the loop.  This is beneficial since it increases efficiency of
 132    code motion optimizations.  It also saves one jump on entry to the loop.  */
 133
 134 namespace {
 135
 136 const pass_data pass_data_ch =
 137 {
 138   GIMPLE_PASS, /* type */
 139   "ch", /* name */
 140   OPTGROUP_LOOP, /* optinfo_flags */
 141   true, /* has_execute */
 142   TV_TREE_CH, /* tv_id */
 143   ( PROP_cfg | PROP_ssa ), /* properties_required */
 144   0, /* properties_provided */
 145   0, /* properties_destroyed */
 146   0, /* todo_flags_start */
 147   TODO_cleanup_cfg, /* todo_flags_finish */
 148 };
 149
 150 class pass_ch : public gimple_opt_pass
 151 {
 152 public:
 153   pass_ch (gcc::context *ctxt)
 154     : gimple_opt_pass (pass_data_ch, ctxt)
 155   {}
 156
 157   /* opt_pass methods: */
 158   virtual bool gate (function *) { return flag_tree_ch != 0; }
 159   virtual unsigned int execute (function *);
 160
 161 }; // class pass_ch
 162
 163 unsigned int
 164 pass_ch::execute (function *fun)
 165 {
 166   struct loop *loop;
 167   basic_block header;
 168   edge exit, entry;
 169   basic_block *bbs, *copied_bbs;
 170   unsigned n_bbs;
 171   unsigned bbs_size;
 172
 173   loop_optimizer_init (LOOPS_HAVE_PREHEADERS
 174                        | LOOPS_HAVE_SIMPLE_LATCHES);
 175   if (number_of_loops (fun) <= 1)
 176     {
 177       loop_optimizer_finalize ();
 178       return 0;
 179     }
 180
 181   bbs = XNEWVEC (basic_block, n_basic_blocks_for_fn (fun));
 182   copied_bbs = XNEWVEC (basic_block, n_basic_blocks_for_fn (fun));
 183   bbs_size = n_basic_blocks_for_fn (fun);
 184
 185   FOR_EACH_LOOP (loop, 0)
 186     {
 187       /* Copy at most 20 insns.  */
 188       int limit = 20;
 189
 190       header = loop->header;
 191
 192       /* If the loop is already a do-while style one (either because it was
 193          written as such, or because jump threading transformed it into one),
 194          we might be in fact peeling the first iteration of the loop.  This
 195          in general is not a good idea.  */
 196       if (do_while_loop_p (loop))
 197         continue;
 198
 199       /* Iterate the header copying up to limit; this takes care of the cases
 200          like while (a && b) {...}, where we want to have both of the conditions
 201          copied.  TODO -- handle while (a || b) - like cases, by not requiring
 202          the header to have just a single successor and copying up to
 203          postdominator.  */
 204
 205       exit = NULL;
 206       n_bbs = 0;
 207       while (should_duplicate_loop_header_p (header, loop, &limit))
 208         {
 209           /* Find a successor of header that is inside a loop; i.e. the new
 210              header after the condition is copied.  */
 211           if (flow_bb_inside_loop_p (loop, EDGE_SUCC (header, 0)->dest))
 212             exit = EDGE_SUCC (header, 0);
 213           else
 214             exit = EDGE_SUCC (header, 1);
 215           bbs[n_bbs++] = header;
 216           gcc_assert (bbs_size > n_bbs);
 217           header = exit->dest;
 218         }
 219
 220       if (!exit)
 221         continue;
 222
 223       if (dump_file && (dump_flags & TDF_DETAILS))
 224         fprintf (dump_file,
 225                  "Duplicating header of the loop %d up to edge %d->%d.\n",
 226                  loop->num, exit->src->index, exit->dest->index);
 227
 228       /* Ensure that the header will have just the latch as a predecessor
 229          inside the loop.  */
 230       if (!single_pred_p (exit->dest))
 231         exit = single_pred_edge (split_edge (exit));
 232
 233       entry = loop_preheader_edge (loop);
 234
 235       propagate_threaded_block_debug_into (exit->dest, entry->dest);
 236       if (!gimple_duplicate_sese_region (entry, exit, bbs, n_bbs, copied_bbs,
 237                                          true))
 238         {
 239           fprintf (dump_file, "Duplication failed.\n");
 240           continue;
 241         }
 242
 243       /* If the loop has the form "for (i = j; i < j + 10; i++)" then
 244          this copying can introduce a case where we rely on undefined
 245          signed overflow to eliminate the preheader condition, because
 246          we assume that "j < j + 10" is true.  We don't want to warn
 247          about that case for -Wstrict-overflow, because in general we
 248          don't warn about overflow involving loops.  Prevent the
 249          warning by setting the no_warning flag in the condition.  */
 250       if (warn_strict_overflow > 0)
 251         {
 252           unsigned int i;
 253
 254           for (i = 0; i < n_bbs; ++i)
 255             {
 256               gimple_stmt_iterator bsi;
 257
 258               for (bsi = gsi_start_bb (copied_bbs[i]);
 259                    !gsi_end_p (bsi);
 260                    gsi_next (&bsi))
 261                 {
 262                   gimple stmt = gsi_stmt (bsi);
 263                   if (gimple_code (stmt) == GIMPLE_COND)
 264                     gimple_set_no_warning (stmt, true);
 265                   else if (is_gimple_assign (stmt))
 266                     {
 267                       enum tree_code rhs_code = gimple_assign_rhs_code (stmt);
 268                       if (TREE_CODE_CLASS (rhs_code) == tcc_comparison)
 269                         gimple_set_no_warning (stmt, true);
 270                     }
 271                 }
 272             }
 273         }
 274
 275       /* Ensure that the latch and the preheader is simple (we know that they
 276          are not now, since there was the loop exit condition.  */
 277       split_edge (loop_preheader_edge (loop));
 278       split_edge (loop_latch_edge (loop));
 279     }
 280
 281   update_ssa (TODO_update_ssa);
 282   free (bbs);
 283   free (copied_bbs);
 284
 285   loop_optimizer_finalize ();
 286   return 0;
 287 }
 288
 289 } // anon namespace
 290
 291 gimple_opt_pass *
 292 make_pass_ch (gcc::context *ctxt)
 293 {
 294   return new pass_ch (ctxt);
 295 }