gcc/tree-ssa-loop-ch.c

   1 /* Loop header copying on trees.
   2    Copyright (C) 2004-2015 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 "hash-set.h"
  25 #include "machmode.h"
  26 #include "vec.h"
  27 #include "double-int.h"
  28 #include "input.h"
  29 #include "alias.h"
  30 #include "symtab.h"
  31 #include "wide-int.h"
  32 #include "inchash.h"
  33 #include "tree.h"
  34 #include "fold-const.h"
  35 #include "tm_p.h"
  36 #include "predict.h"
  37 #include "hard-reg-set.h"
  38 #include "input.h"
  39 #include "function.h"
  40 #include "dominance.h"
  41 #include "cfg.h"
  42 #include "basic-block.h"
  43 #include "tree-ssa-alias.h"
  44 #include "internal-fn.h"
  45 #include "gimple-expr.h"
  46 #include "is-a.h"
  47 #include "gimple.h"
  48 #include "gimple-iterator.h"
  49 #include "gimple-ssa.h"
  50 #include "tree-cfg.h"
  51 #include "tree-into-ssa.h"
  52 #include "tree-pass.h"
  53 #include "cfgloop.h"
  54 #include "tree-inline.h"
  55 #include "flags.h"
  56 #include "tree-ssa-threadedge.h"
  57
  58 /* Duplicates headers of loops if they are small enough, so that the statements
  59    in the loop body are always executed when the loop is entered.  This
  60    increases effectiveness of code motion optimizations, and reduces the need
  61    for loop preconditioning.  */
  62
  63 /* Check whether we should duplicate HEADER of LOOP.  At most *LIMIT
  64    instructions should be duplicated, limit is decreased by the actual
  65    amount.  */
  66
  67 static bool
  68 should_duplicate_loop_header_p (basic_block header, struct loop *loop,
  69                                 int *limit)
  70 {
  71   gimple_stmt_iterator bsi;
  72   gimple last;
  73
  74   /* Do not copy one block more than once (we do not really want to do
  75      loop peeling here).  */
  76   if (header->aux)
  77     return false;
  78
  79   /* Loop header copying usually increases size of the code.  This used not to
  80      be true, since quite often it is possible to verify that the condition is
  81      satisfied in the first iteration and therefore to eliminate it.  Jump
  82      threading handles these cases now.  */
  83   if (optimize_loop_for_size_p (loop))
  84     return false;
  85
  86   gcc_assert (EDGE_COUNT (header->succs) > 0);
  87   if (single_succ_p (header))
  88     return false;
  89   if (flow_bb_inside_loop_p (loop, EDGE_SUCC (header, 0)->dest)
  90       && flow_bb_inside_loop_p (loop, EDGE_SUCC (header, 1)->dest))
  91     return false;
  92
  93   /* If this is not the original loop header, we want it to have just
  94      one predecessor in order to match the && pattern.  */
  95   if (header != loop->header && !single_pred_p (header))
  96     return false;
  97
  98   last = last_stmt (header);
  99   if (gimple_code (last) != GIMPLE_COND)
 100     return false;
 101
 102   /* Approximately copy the conditions that used to be used in jump.c --
 103      at most 20 insns and no calls.  */
 104   for (bsi = gsi_start_bb (header); !gsi_end_p (bsi); gsi_next (&bsi))
 105     {
 106       last = gsi_stmt (bsi);
 107
 108       if (gimple_code (last) == GIMPLE_LABEL)
 109         continue;
 110
 111       if (is_gimple_debug (last))
 112         continue;
 113
 114       if (is_gimple_call (last))
 115         return false;
 116
 117       *limit -= estimate_num_insns (last, &eni_size_weights);
 118       if (*limit < 0)
 119         return false;
 120     }
 121
 122   return true;
 123 }
 124
 125 /* Checks whether LOOP is a do-while style loop.  */
 126
 127 static bool
 128 do_while_loop_p (struct loop *loop)
 129 {
 130   gimple stmt = last_stmt (loop->latch);
 131
 132   /* If the latch of the loop is not empty, it is not a do-while loop.  */
 133   if (stmt
 134       && gimple_code (stmt) != GIMPLE_LABEL)
 135     return false;
 136
 137   /* If the header contains just a condition, it is not a do-while loop.  */
 138   stmt = last_and_only_stmt (loop->header);
 139   if (stmt
 140       && gimple_code (stmt) == GIMPLE_COND)
 141     return false;
 142
 143   return true;
 144 }
 145
 146 /* For all loops, copy the condition at the end of the loop body in front
 147    of the loop.  This is beneficial since it increases efficiency of
 148    code motion optimizations.  It also saves one jump on entry to the loop.  */
 149
 150 namespace {
 151
 152 const pass_data pass_data_ch =
 153 {
 154   GIMPLE_PASS, /* type */
 155   "ch", /* name */
 156   OPTGROUP_LOOP, /* optinfo_flags */
 157   TV_TREE_CH, /* tv_id */
 158   ( PROP_cfg | PROP_ssa ), /* properties_required */
 159   0, /* properties_provided */
 160   0, /* properties_destroyed */
 161   0, /* todo_flags_start */
 162   TODO_cleanup_cfg, /* todo_flags_finish */
 163 };
 164
 165 class pass_ch : public gimple_opt_pass
 166 {
 167 public:
 168   pass_ch (gcc::context *ctxt)
 169     : gimple_opt_pass (pass_data_ch, ctxt)
 170   {}
 171
 172   /* opt_pass methods: */
 173   virtual bool gate (function *) { return flag_tree_ch != 0; }
 174   virtual unsigned int execute (function *);
 175
 176 }; // class pass_ch
 177
 178 unsigned int
 179 pass_ch::execute (function *fun)
 180 {
 181   struct loop *loop;
 182   basic_block header;
 183   edge exit, entry;
 184   basic_block *bbs, *copied_bbs;
 185   unsigned n_bbs;
 186   unsigned bbs_size;
 187
 188   loop_optimizer_init (LOOPS_HAVE_PREHEADERS
 189                        | LOOPS_HAVE_SIMPLE_LATCHES);
 190   if (number_of_loops (fun) <= 1)
 191     {
 192       loop_optimizer_finalize ();
 193       return 0;
 194     }
 195
 196   bbs = XNEWVEC (basic_block, n_basic_blocks_for_fn (fun));
 197   copied_bbs = XNEWVEC (basic_block, n_basic_blocks_for_fn (fun));
 198   bbs_size = n_basic_blocks_for_fn (fun);
 199
 200   FOR_EACH_LOOP (loop, 0)
 201     {
 202       /* Copy at most 20 insns.  */
 203       int limit = 20;
 204
 205       header = loop->header;
 206
 207       /* If the loop is already a do-while style one (either because it was
 208          written as such, or because jump threading transformed it into one),
 209          we might be in fact peeling the first iteration of the loop.  This
 210          in general is not a good idea.  */
 211       if (do_while_loop_p (loop))
 212         continue;
 213
 214       /* Iterate the header copying up to limit; this takes care of the cases
 215          like while (a && b) {...}, where we want to have both of the conditions
 216          copied.  TODO -- handle while (a || b) - like cases, by not requiring
 217          the header to have just a single successor and copying up to
 218          postdominator.  */
 219
 220       exit = NULL;
 221       n_bbs = 0;
 222       while (should_duplicate_loop_header_p (header, loop, &limit))
 223         {
 224           /* Find a successor of header that is inside a loop; i.e. the new
 225              header after the condition is copied.  */
 226           if (flow_bb_inside_loop_p (loop, EDGE_SUCC (header, 0)->dest))
 227             exit = EDGE_SUCC (header, 0);
 228           else
 229             exit = EDGE_SUCC (header, 1);
 230           bbs[n_bbs++] = header;
 231           gcc_assert (bbs_size > n_bbs);
 232           header = exit->dest;
 233         }
 234
 235       if (!exit)
 236         continue;
 237
 238       if (dump_file && (dump_flags & TDF_DETAILS))
 239         fprintf (dump_file,
 240                  "Duplicating header of the loop %d up to edge %d->%d.\n",
 241                  loop->num, exit->src->index, exit->dest->index);
 242
 243       /* Ensure that the header will have just the latch as a predecessor
 244          inside the loop.  */
 245       if (!single_pred_p (exit->dest))
 246         exit = single_pred_edge (split_edge (exit));
 247
 248       entry = loop_preheader_edge (loop);
 249
 250       propagate_threaded_block_debug_into (exit->dest, entry->dest);
 251       if (!gimple_duplicate_sese_region (entry, exit, bbs, n_bbs, copied_bbs,
 252                                          true))
 253         {
 254           fprintf (dump_file, "Duplication failed.\n");
 255           continue;
 256         }
 257
 258       /* If the loop has the form "for (i = j; i < j + 10; i++)" then
 259          this copying can introduce a case where we rely on undefined
 260          signed overflow to eliminate the preheader condition, because
 261          we assume that "j < j + 10" is true.  We don't want to warn
 262          about that case for -Wstrict-overflow, because in general we
 263          don't warn about overflow involving loops.  Prevent the
 264          warning by setting the no_warning flag in the condition.  */
 265       if (warn_strict_overflow > 0)
 266         {
 267           unsigned int i;
 268
 269           for (i = 0; i < n_bbs; ++i)
 270             {
 271               gimple_stmt_iterator bsi;
 272
 273               for (bsi = gsi_start_bb (copied_bbs[i]);
 274                    !gsi_end_p (bsi);
 275                    gsi_next (&bsi))
 276                 {
 277                   gimple stmt = gsi_stmt (bsi);
 278                   if (gimple_code (stmt) == GIMPLE_COND)
 279                     gimple_set_no_warning (stmt, true);
 280                   else if (is_gimple_assign (stmt))
 281                     {
 282                       enum tree_code rhs_code = gimple_assign_rhs_code (stmt);
 283                       if (TREE_CODE_CLASS (rhs_code) == tcc_comparison)
 284                         gimple_set_no_warning (stmt, true);
 285                     }
 286                 }
 287             }
 288         }
 289
 290       /* Ensure that the latch and the preheader is simple (we know that they
 291          are not now, since there was the loop exit condition.  */
 292       split_edge (loop_preheader_edge (loop));
 293       split_edge (loop_latch_edge (loop));
 294     }
 295
 296   update_ssa (TODO_update_ssa);
 297   free (bbs);
 298   free (copied_bbs);
 299
 300   loop_optimizer_finalize ();
 301   return 0;
 302 }
 303
 304 } // anon namespace
 305
 306 gimple_opt_pass *
 307 make_pass_ch (gcc::context *ctxt)
 308 {
 309   return new pass_ch (ctxt);
 310 }