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_verify_ssa
 148     | TODO_verify_flow ), /* todo_flags_finish */
 149 };
 150
 151 class pass_ch : public gimple_opt_pass
 152 {
 153 public:
 154   pass_ch (gcc::context *ctxt)
 155     : gimple_opt_pass (pass_data_ch, ctxt)
 156   {}
 157
 158   /* opt_pass methods: */
 159   virtual bool gate (function *) { return flag_tree_ch != 0; }
 160   virtual unsigned int execute (function *);
 161
 162 }; // class pass_ch
 163
 164 unsigned int
 165 pass_ch::execute (function *fun)
 166 {
 167   struct loop *loop;
 168   basic_block header;
 169   edge exit, entry;
 170   basic_block *bbs, *copied_bbs;
 171   unsigned n_bbs;
 172   unsigned bbs_size;
 173
 174   loop_optimizer_init (LOOPS_HAVE_PREHEADERS
 175                        | LOOPS_HAVE_SIMPLE_LATCHES);
 176   if (number_of_loops (fun) <= 1)
 177     {
 178       loop_optimizer_finalize ();
 179       return 0;
 180     }
 181
 182   bbs = XNEWVEC (basic_block, n_basic_blocks_for_fn (fun));
 183   copied_bbs = XNEWVEC (basic_block, n_basic_blocks_for_fn (fun));
 184   bbs_size = n_basic_blocks_for_fn (fun);
 185
 186   FOR_EACH_LOOP (loop, 0)
 187     {
 188       /* Copy at most 20 insns.  */
 189       int limit = 20;
 190
 191       header = loop->header;
 192
 193       /* If the loop is already a do-while style one (either because it was
 194          written as such, or because jump threading transformed it into one),
 195          we might be in fact peeling the first iteration of the loop.  This
 196          in general is not a good idea.  */
 197       if (do_while_loop_p (loop))
 198         continue;
 199
 200       /* Iterate the header copying up to limit; this takes care of the cases
 201          like while (a && b) {...}, where we want to have both of the conditions
 202          copied.  TODO -- handle while (a || b) - like cases, by not requiring
 203          the header to have just a single successor and copying up to
 204          postdominator.  */
 205
 206       exit = NULL;
 207       n_bbs = 0;
 208       while (should_duplicate_loop_header_p (header, loop, &limit))
 209         {
 210           /* Find a successor of header that is inside a loop; i.e. the new
 211              header after the condition is copied.  */
 212           if (flow_bb_inside_loop_p (loop, EDGE_SUCC (header, 0)->dest))
 213             exit = EDGE_SUCC (header, 0);
 214           else
 215             exit = EDGE_SUCC (header, 1);
 216           bbs[n_bbs++] = header;
 217           gcc_assert (bbs_size > n_bbs);
 218           header = exit->dest;
 219         }
 220
 221       if (!exit)
 222         continue;
 223
 224       if (dump_file && (dump_flags & TDF_DETAILS))
 225         fprintf (dump_file,
 226                  "Duplicating header of the loop %d up to edge %d->%d.\n",
 227                  loop->num, exit->src->index, exit->dest->index);
 228
 229       /* Ensure that the header will have just the latch as a predecessor
 230          inside the loop.  */
 231       if (!single_pred_p (exit->dest))
 232         exit = single_pred_edge (split_edge (exit));
 233
 234       entry = loop_preheader_edge (loop);
 235
 236       propagate_threaded_block_debug_into (exit->dest, entry->dest);
 237       if (!gimple_duplicate_sese_region (entry, exit, bbs, n_bbs, copied_bbs,
 238                                          true))
 239         {
 240           fprintf (dump_file, "Duplication failed.\n");
 241           continue;
 242         }
 243
 244       /* If the loop has the form "for (i = j; i < j + 10; i++)" then
 245          this copying can introduce a case where we rely on undefined
 246          signed overflow to eliminate the preheader condition, because
 247          we assume that "j < j + 10" is true.  We don't want to warn
 248          about that case for -Wstrict-overflow, because in general we
 249          don't warn about overflow involving loops.  Prevent the
 250          warning by setting the no_warning flag in the condition.  */
 251       if (warn_strict_overflow > 0)
 252         {
 253           unsigned int i;
 254
 255           for (i = 0; i < n_bbs; ++i)
 256             {
 257               gimple_stmt_iterator bsi;
 258
 259               for (bsi = gsi_start_bb (copied_bbs[i]);
 260                    !gsi_end_p (bsi);
 261                    gsi_next (&bsi))
 262                 {
 263                   gimple stmt = gsi_stmt (bsi);
 264                   if (gimple_code (stmt) == GIMPLE_COND)
 265                     gimple_set_no_warning (stmt, true);
 266                   else if (is_gimple_assign (stmt))
 267                     {
 268                       enum tree_code rhs_code = gimple_assign_rhs_code (stmt);
 269                       if (TREE_CODE_CLASS (rhs_code) == tcc_comparison)
 270                         gimple_set_no_warning (stmt, true);
 271                     }
 272                 }
 273             }
 274         }
 275
 276       /* Ensure that the latch and the preheader is simple (we know that they
 277          are not now, since there was the loop exit condition.  */
 278       split_edge (loop_preheader_edge (loop));
 279       split_edge (loop_latch_edge (loop));
 280     }
 281
 282   update_ssa (TODO_update_ssa);
 283   free (bbs);
 284   free (copied_bbs);
 285
 286   loop_optimizer_finalize ();
 287   return 0;
 288 }
 289
 290 } // anon namespace
 291
 292 gimple_opt_pass *
 293 make_pass_ch (gcc::context *ctxt)
 294 {
 295   return new pass_ch (ctxt);
 296 }