gcc/tree-ssa-loop-ch.c

   1 /* Loop header copying on trees.
   2    Copyright (C) 2004, 2005, 2006, 2007 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 "rtl.h"
  26 #include "tm_p.h"
  27 #include "hard-reg-set.h"
  28 #include "basic-block.h"
  29 #include "output.h"
  30 #include "diagnostic.h"
  31 #include "tree-flow.h"
  32 #include "tree-dump.h"
  33 #include "tree-pass.h"
  34 #include "timevar.h"
  35 #include "cfgloop.h"
  36 #include "tree-inline.h"
  37 #include "flags.h"
  38 #include "tree-inline.h"
  39
  40 /* Duplicates headers of loops if they are small enough, so that the statements
  41    in the loop body are always executed when the loop is entered.  This
  42    increases effectiveness of code motion optimizations, and reduces the need
  43    for loop preconditioning.  */
  44
  45 /* Check whether we should duplicate HEADER of LOOP.  At most *LIMIT
  46    instructions should be duplicated, limit is decreased by the actual
  47    amount.  */
  48
  49 static bool
  50 should_duplicate_loop_header_p (basic_block header, struct loop *loop,
  51                                 int *limit)
  52 {
  53   block_stmt_iterator bsi;
  54   tree last;
  55
  56   /* Do not copy one block more than once (we do not really want to do
  57      loop peeling here).  */
  58   if (header->aux)
  59     return false;
  60
  61   gcc_assert (EDGE_COUNT (header->succs) > 0);
  62   if (single_succ_p (header))
  63     return false;
  64   if (flow_bb_inside_loop_p (loop, EDGE_SUCC (header, 0)->dest)
  65       && flow_bb_inside_loop_p (loop, EDGE_SUCC (header, 1)->dest))
  66     return false;
  67
  68   /* If this is not the original loop header, we want it to have just
  69      one predecessor in order to match the && pattern.  */
  70   if (header != loop->header && !single_pred_p (header))
  71     return false;
  72
  73   last = last_stmt (header);
  74   if (TREE_CODE (last) != COND_EXPR)
  75     return false;
  76
  77   /* Approximately copy the conditions that used to be used in jump.c --
  78      at most 20 insns and no calls.  */
  79   for (bsi = bsi_start (header); !bsi_end_p (bsi); bsi_next (&bsi))
  80     {
  81       last = bsi_stmt (bsi);
  82
  83       if (TREE_CODE (last) == LABEL_EXPR)
  84         continue;
  85
  86       if (get_call_expr_in (last))
  87         return false;
  88
  89       *limit -= estimate_num_insns (last);
  90       if (*limit < 0)
  91         return false;
  92     }
  93
  94   return true;
  95 }
  96
  97 /* Checks whether LOOP is a do-while style loop.  */
  98
  99 static bool
 100 do_while_loop_p (struct loop *loop)
 101 {
 102   tree stmt = last_stmt (loop->latch);
 103
 104   /* If the latch of the loop is not empty, it is not a do-while loop.  */
 105   if (stmt
 106       && TREE_CODE (stmt) != LABEL_EXPR)
 107     return false;
 108
 109   /* If the header contains just a condition, it is not a do-while loop.  */
 110   stmt = last_and_only_stmt (loop->header);
 111   if (stmt
 112       && TREE_CODE (stmt) == COND_EXPR)
 113     return false;
 114
 115   return true;
 116 }
 117
 118 /* For all loops, copy the condition at the end of the loop body in front
 119    of the loop.  This is beneficial since it increases efficiency of
 120    code motion optimizations.  It also saves one jump on entry to the loop.  */
 121
 122 static unsigned int
 123 copy_loop_headers (void)
 124 {
 125   struct loops *loops;
 126   unsigned i;
 127   struct loop *loop;
 128   basic_block header;
 129   edge exit, entry;
 130   basic_block *bbs, *copied_bbs;
 131   unsigned n_bbs;
 132   unsigned bbs_size;
 133
 134   loops = loop_optimizer_init (LOOPS_HAVE_PREHEADERS
 135                                | LOOPS_HAVE_SIMPLE_LATCHES);
 136   if (!loops)
 137     return 0;
 138
 139 #ifdef ENABLE_CHECKING
 140   verify_loop_structure (loops);
 141 #endif
 142
 143   bbs = XNEWVEC (basic_block, n_basic_blocks);
 144   copied_bbs = XNEWVEC (basic_block, n_basic_blocks);
 145   bbs_size = n_basic_blocks;
 146
 147   for (i = 1; i < loops->num; i++)
 148     {
 149       /* Copy at most 20 insns.  */
 150       int limit = 20;
 151
 152       loop = loops->parray[i];
 153       if (!loop)
 154         continue;
 155       header = loop->header;
 156
 157       /* If the loop is already a do-while style one (either because it was
 158          written as such, or because jump threading transformed it into one),
 159          we might be in fact peeling the first iteration of the loop.  This
 160          in general is not a good idea.  */
 161       if (do_while_loop_p (loop))
 162         continue;
 163
 164       /* Iterate the header copying up to limit; this takes care of the cases
 165          like while (a && b) {...}, where we want to have both of the conditions
 166          copied.  TODO -- handle while (a || b) - like cases, by not requiring
 167          the header to have just a single successor and copying up to
 168          postdominator.  */
 169
 170       exit = NULL;
 171       n_bbs = 0;
 172       while (should_duplicate_loop_header_p (header, loop, &limit))
 173         {
 174           /* Find a successor of header that is inside a loop; i.e. the new
 175              header after the condition is copied.  */
 176           if (flow_bb_inside_loop_p (loop, EDGE_SUCC (header, 0)->dest))
 177             exit = EDGE_SUCC (header, 0);
 178           else
 179             exit = EDGE_SUCC (header, 1);
 180           bbs[n_bbs++] = header;
 181           gcc_assert (bbs_size > n_bbs);
 182           header = exit->dest;
 183         }
 184
 185       if (!exit)
 186         continue;
 187
 188       if (dump_file && (dump_flags & TDF_DETAILS))
 189         fprintf (dump_file,
 190                  "Duplicating header of the loop %d up to edge %d->%d.\n",
 191                  loop->num, exit->src->index, exit->dest->index);
 192
 193       /* Ensure that the header will have just the latch as a predecessor
 194          inside the loop.  */
 195       if (!single_pred_p (exit->dest))
 196         exit = single_pred_edge (loop_split_edge_with (exit, NULL));
 197
 198       entry = loop_preheader_edge (loop);
 199
 200       if (!tree_duplicate_sese_region (entry, exit, bbs, n_bbs, copied_bbs))
 201         {
 202           fprintf (dump_file, "Duplication failed.\n");
 203           continue;
 204         }
 205
 206       /* If the loop has the form "for (i = j; i < j + 10; i++)" then
 207          this copying can introduce a case where we rely on undefined
 208          signed overflow to eliminate the preheader condition, because
 209          we assume that "j < j + 10" is true.  We don't want to warn
 210          about that case for -Wstrict-overflow, because in general we
 211          don't warn about overflow involving loops.  Prevent the
 212          warning by setting TREE_NO_WARNING.  */
 213       if (warn_strict_overflow > 0)
 214         {
 215           unsigned int i;
 216
 217           for (i = 0; i < n_bbs; ++i)
 218             {
 219               tree last;
 220
 221               last = last_stmt (copied_bbs[i]);
 222               if (TREE_CODE (last) == COND_EXPR)
 223                 TREE_NO_WARNING (last) = 1;
 224             }
 225         }
 226
 227       /* Ensure that the latch and the preheader is simple (we know that they
 228          are not now, since there was the loop exit condition.  */
 229       loop_split_edge_with (loop_preheader_edge (loop), NULL);
 230       loop_split_edge_with (loop_latch_edge (loop), NULL);
 231     }
 232
 233   free (bbs);
 234   free (copied_bbs);
 235
 236   loop_optimizer_finalize (loops);
 237   return 0;
 238 }
 239
 240 static bool
 241 gate_ch (void)
 242 {
 243   return flag_tree_ch != 0;
 244 }
 245
 246 struct tree_opt_pass pass_ch =
 247 {
 248   "ch",                                 /* name */
 249   gate_ch,                              /* gate */
 250   copy_loop_headers,                    /* execute */
 251   NULL,                                 /* sub */
 252   NULL,                                 /* next */
 253   0,                                    /* static_pass_number */
 254   TV_TREE_CH,                           /* tv_id */
 255   PROP_cfg | PROP_ssa,                  /* properties_required */
 256   0,                                    /* properties_provided */
 257   0,                                    /* properties_destroyed */
 258   0,                                    /* todo_flags_start */
 259   TODO_cleanup_cfg | TODO_dump_func
 260   | TODO_verify_ssa,                    /* todo_flags_finish */
 261   0                                     /* letter */
 262 };