gcc/tree-ssa-loop-ch.c

   1 /* Loop header copying on trees.
   2    Copyright (C) 2004, 2005, 2006, 2007, 2008, 2010
   3    Free Software Foundation, Inc.
   4
   5 This file is part of GCC.
   6
   7 GCC is free software; you can redistribute it and/or modify it
   8 under the terms of the GNU General Public License as published by the
   9 Free Software Foundation; either version 3, or (at your option) any
  10 later version.
  11
  12 GCC is distributed in the hope that it will be useful, but WITHOUT
  13 ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  14 FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  15 for more details.
  16
  17 You should have received a copy of the GNU General Public License
  18 along with GCC; see the file COPYING3.  If not see
  19 <http://www.gnu.org/licenses/>.  */
  20
  21 #include "config.h"
  22 #include "system.h"
  23 #include "coretypes.h"
  24 #include "tm.h"
  25 #include "tree.h"
  26 #include "tm_p.h"
  27 #include "basic-block.h"
  28 #include "tree-flow.h"
  29 #include "tree-pass.h"
  30 #include "cfgloop.h"
  31 #include "tree-inline.h"
  32 #include "flags.h"
  33 #include "tree-inline.h"
  34
  35 /* Duplicates headers of loops if they are small enough, so that the statements
  36    in the loop body are always executed when the loop is entered.  This
  37    increases effectiveness of code motion optimizations, and reduces the need
  38    for loop preconditioning.  */
  39
  40 /* Check whether we should duplicate HEADER of LOOP.  At most *LIMIT
  41    instructions should be duplicated, limit is decreased by the actual
  42    amount.  */
  43
  44 static bool
  45 should_duplicate_loop_header_p (basic_block header, struct loop *loop,
  46                                 int *limit)
  47 {
  48   gimple_stmt_iterator bsi;
  49   gimple last;
  50
  51   /* Do not copy one block more than once (we do not really want to do
  52      loop peeling here).  */
  53   if (header->aux)
  54     return false;
  55
  56   /* Loop header copying usually increases size of the code.  This used not to
  57      be true, since quite often it is possible to verify that the condition is
  58      satisfied in the first iteration and therefore to eliminate it.  Jump
  59      threading handles these cases now.  */
  60   if (optimize_loop_for_size_p (loop))
  61     return false;
  62
  63   gcc_assert (EDGE_COUNT (header->succs) > 0);
  64   if (single_succ_p (header))
  65     return false;
  66   if (flow_bb_inside_loop_p (loop, EDGE_SUCC (header, 0)->dest)
  67       && flow_bb_inside_loop_p (loop, EDGE_SUCC (header, 1)->dest))
  68     return false;
  69
  70   /* If this is not the original loop header, we want it to have just
  71      one predecessor in order to match the && pattern.  */
  72   if (header != loop->header && !single_pred_p (header))
  73     return false;
  74
  75   last = last_stmt (header);
  76   if (gimple_code (last) != GIMPLE_COND)
  77     return false;
  78
  79   /* Approximately copy the conditions that used to be used in jump.c --
  80      at most 20 insns and no calls.  */
  81   for (bsi = gsi_start_bb (header); !gsi_end_p (bsi); gsi_next (&bsi))
  82     {
  83       last = gsi_stmt (bsi);
  84
  85       if (gimple_code (last) == GIMPLE_LABEL)
  86         continue;
  87
  88       if (is_gimple_debug (last))
  89         continue;
  90
  91       if (is_gimple_call (last))
  92         return false;
  93
  94       *limit -= estimate_num_insns (last, &eni_size_weights);
  95       if (*limit < 0)
  96         return false;
  97     }
  98
  99   return true;
 100 }
 101
 102 /* Checks whether LOOP is a do-while style loop.  */
 103
 104 bool
 105 do_while_loop_p (struct loop *loop)
 106 {
 107   gimple stmt = last_stmt (loop->latch);
 108
 109   /* If the latch of the loop is not empty, it is not a do-while loop.  */
 110   if (stmt
 111       && gimple_code (stmt) != GIMPLE_LABEL)
 112     return false;
 113
 114   /* If the header contains just a condition, it is not a do-while loop.  */
 115   stmt = last_and_only_stmt (loop->header);
 116   if (stmt
 117       && gimple_code (stmt) == GIMPLE_COND)
 118     return false;
 119
 120   return true;
 121 }
 122
 123 /* For all loops, copy the condition at the end of the loop body in front
 124    of the loop.  This is beneficial since it increases efficiency of
 125    code motion optimizations.  It also saves one jump on entry to the loop.  */
 126
 127 static unsigned int
 128 copy_loop_headers (void)
 129 {
 130   loop_iterator li;
 131   struct loop *loop;
 132   basic_block header;
 133   edge exit, entry;
 134   basic_block *bbs, *copied_bbs;
 135   unsigned n_bbs;
 136   unsigned bbs_size;
 137
 138   loop_optimizer_init (LOOPS_HAVE_PREHEADERS
 139                        | LOOPS_HAVE_SIMPLE_LATCHES);
 140   if (number_of_loops () <= 1)
 141     {
 142       loop_optimizer_finalize ();
 143       return 0;
 144     }
 145
 146 #ifdef ENABLE_CHECKING
 147   verify_loop_structure ();
 148 #endif
 149
 150   bbs = XNEWVEC (basic_block, n_basic_blocks);
 151   copied_bbs = XNEWVEC (basic_block, n_basic_blocks);
 152   bbs_size = n_basic_blocks;
 153
 154   FOR_EACH_LOOP (li, loop, 0)
 155     {
 156       /* Copy at most 20 insns.  */
 157       int limit = 20;
 158
 159       header = loop->header;
 160
 161       /* If the loop is already a do-while style one (either because it was
 162          written as such, or because jump threading transformed it into one),
 163          we might be in fact peeling the first iteration of the loop.  This
 164          in general is not a good idea.  */
 165       if (do_while_loop_p (loop))
 166         continue;
 167
 168       /* Iterate the header copying up to limit; this takes care of the cases
 169          like while (a && b) {...}, where we want to have both of the conditions
 170          copied.  TODO -- handle while (a || b) - like cases, by not requiring
 171          the header to have just a single successor and copying up to
 172          postdominator.  */
 173
 174       exit = NULL;
 175       n_bbs = 0;
 176       while (should_duplicate_loop_header_p (header, loop, &limit))
 177         {
 178           /* Find a successor of header that is inside a loop; i.e. the new
 179              header after the condition is copied.  */
 180           if (flow_bb_inside_loop_p (loop, EDGE_SUCC (header, 0)->dest))
 181             exit = EDGE_SUCC (header, 0);
 182           else
 183             exit = EDGE_SUCC (header, 1);
 184           bbs[n_bbs++] = header;
 185           gcc_assert (bbs_size > n_bbs);
 186           header = exit->dest;
 187         }
 188
 189       if (!exit)
 190         continue;
 191
 192       if (dump_file && (dump_flags & TDF_DETAILS))
 193         fprintf (dump_file,
 194                  "Duplicating header of the loop %d up to edge %d->%d.\n",
 195                  loop->num, exit->src->index, exit->dest->index);
 196
 197       /* Ensure that the header will have just the latch as a predecessor
 198          inside the loop.  */
 199       if (!single_pred_p (exit->dest))
 200         exit = single_pred_edge (split_edge (exit));
 201
 202       entry = loop_preheader_edge (loop);
 203
 204       if (!gimple_duplicate_sese_region (entry, exit, bbs, n_bbs, copied_bbs))
 205         {
 206           fprintf (dump_file, "Duplication failed.\n");
 207           continue;
 208         }
 209
 210       /* If the loop has the form "for (i = j; i < j + 10; i++)" then
 211          this copying can introduce a case where we rely on undefined
 212          signed overflow to eliminate the preheader condition, because
 213          we assume that "j < j + 10" is true.  We don't want to warn
 214          about that case for -Wstrict-overflow, because in general we
 215          don't warn about overflow involving loops.  Prevent the
 216          warning by setting the no_warning flag in the condition.  */
 217       if (warn_strict_overflow > 0)
 218         {
 219           unsigned int i;
 220
 221           for (i = 0; i < n_bbs; ++i)
 222             {
 223               gimple_stmt_iterator bsi;
 224
 225               for (bsi = gsi_start_bb (copied_bbs[i]);
 226                    !gsi_end_p (bsi);
 227                    gsi_next (&bsi))
 228                 {
 229                   gimple stmt = gsi_stmt (bsi);
 230                   if (gimple_code (stmt) == GIMPLE_COND)
 231                     gimple_set_no_warning (stmt, true);
 232                   else if (is_gimple_assign (stmt))
 233                     {
 234                       enum tree_code rhs_code = gimple_assign_rhs_code (stmt);
 235                       if (TREE_CODE_CLASS (rhs_code) == tcc_comparison)
 236                         gimple_set_no_warning (stmt, true);
 237                     }
 238                 }
 239             }
 240         }
 241
 242       /* Ensure that the latch and the preheader is simple (we know that they
 243          are not now, since there was the loop exit condition.  */
 244       split_edge (loop_preheader_edge (loop));
 245       split_edge (loop_latch_edge (loop));
 246     }
 247
 248   free (bbs);
 249   free (copied_bbs);
 250
 251   loop_optimizer_finalize ();
 252   return 0;
 253 }
 254
 255 static bool
 256 gate_ch (void)
 257 {
 258   return flag_tree_ch != 0;
 259 }
 260
 261 struct gimple_opt_pass pass_ch =
 262 {
 263  {
 264   GIMPLE_PASS,
 265   "ch",                                 /* name */
 266   gate_ch,                              /* gate */
 267   copy_loop_headers,                    /* execute */
 268   NULL,                                 /* sub */
 269   NULL,                                 /* next */
 270   0,                                    /* static_pass_number */
 271   TV_TREE_CH,                           /* tv_id */
 272   PROP_cfg | PROP_ssa,                  /* properties_required */
 273   0,                                    /* properties_provided */
 274   0,                                    /* properties_destroyed */
 275   0,                                    /* todo_flags_start */
 276   TODO_cleanup_cfg
 277     | TODO_verify_ssa
 278     | TODO_verify_flow                  /* todo_flags_finish */
 279  }
 280 };