gcc/tree-ssa-loop-unswitch.c

   1 /* Loop unswitching.
   2    Copyright (C) 2004, 2005, 2007, 2008 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 "output.h"
  28 #include "diagnostic.h"
  29 #include "tree-flow.h"
  30 #include "tree-dump.h"
  31 #include "timevar.h"
  32 #include "cfgloop.h"
  33 #include "params.h"
  34 #include "tree-pass.h"
  35 #include "tree-inline.h"
  36
  37 /* This file implements the loop unswitching, i.e. transformation of loops like
  38
  39    while (A)
  40      {
  41        if (inv)
  42          B;
  43
  44        X;
  45
  46        if (!inv)
  47          C;
  48      }
  49
  50    where inv is the loop invariant, into
  51
  52    if (inv)
  53      {
  54        while (A)
  55          {
  56            B;
  57            X;
  58          }
  59      }
  60    else
  61      {
  62        while (A)
  63          {
  64            X;
  65            C;
  66          }
  67      }
  68
  69    Inv is considered invariant iff the values it compares are both invariant;
  70    tree-ssa-loop-im.c ensures that all the suitable conditions are in this
  71    shape.  */
  72
  73 static struct loop *tree_unswitch_loop (struct loop *, basic_block, tree);
  74 static bool tree_unswitch_single_loop (struct loop *, int);
  75 static tree tree_may_unswitch_on (basic_block, struct loop *);
  76
  77 /* Main entry point.  Perform loop unswitching on all suitable loops.  */
  78
  79 unsigned int
  80 tree_ssa_unswitch_loops (void)
  81 {
  82   loop_iterator li;
  83   struct loop *loop;
  84   bool changed = false;
  85
  86   /* Go through inner loops (only original ones).  */
  87   FOR_EACH_LOOP (li, loop, LI_ONLY_INNERMOST)
  88     {
  89       if (dump_file && (dump_flags & TDF_DETAILS))
  90         fprintf (dump_file, ";; Considering loop %d\n", loop->num);
  91
  92       /* Do not unswitch in cold regions. */
  93       if (optimize_loop_for_size_p (loop))
  94         {
  95           if (dump_file && (dump_flags & TDF_DETAILS))
  96             fprintf (dump_file, ";; Not unswitching cold loops\n");
  97           continue;
  98         }
  99
 100       /* The loop should not be too large, to limit code growth. */
 101       if (tree_num_loop_insns (loop, &eni_size_weights)
 102           > (unsigned) PARAM_VALUE (PARAM_MAX_UNSWITCH_INSNS))
 103         {
 104           if (dump_file && (dump_flags & TDF_DETAILS))
 105             fprintf (dump_file, ";; Not unswitching, loop too big\n");
 106           continue;
 107         }
 108
 109       changed |= tree_unswitch_single_loop (loop, 0);
 110     }
 111
 112   if (changed)
 113     return TODO_cleanup_cfg;
 114   return 0;
 115 }
 116
 117 /* Checks whether we can unswitch LOOP on condition at end of BB -- one of its
 118    basic blocks (for what it means see comments below).  */
 119
 120 static tree
 121 tree_may_unswitch_on (basic_block bb, struct loop *loop)
 122 {
 123   gimple stmt, def;
 124   tree cond, use;
 125   basic_block def_bb;
 126   ssa_op_iter iter;
 127
 128   /* BB must end in a simple conditional jump.  */
 129   stmt = last_stmt (bb);
 130   if (!stmt || gimple_code (stmt) != GIMPLE_COND)
 131     return NULL_TREE;
 132
 133   /* Condition must be invariant.  */
 134   FOR_EACH_SSA_TREE_OPERAND (use, stmt, iter, SSA_OP_USE)
 135     {
 136       def = SSA_NAME_DEF_STMT (use);
 137       def_bb = gimple_bb (def);
 138       if (def_bb
 139           && flow_bb_inside_loop_p (loop, def_bb))
 140         return NULL_TREE;
 141     }
 142
 143   cond = build2 (gimple_cond_code (stmt), boolean_type_node,
 144                  gimple_cond_lhs (stmt), gimple_cond_rhs (stmt));
 145
 146   /* To keep the things simple, we do not directly remove the conditions,
 147      but just replace tests with 0/1.  Prevent the infinite loop where we
 148      would unswitch again on such a condition.  */
 149   if (integer_zerop (cond) || integer_nonzerop (cond))
 150     return NULL_TREE;
 151
 152   return cond;
 153 }
 154
 155 /* Simplifies COND using checks in front of the entry of the LOOP.  Just very
 156    simplish (sufficient to prevent us from duplicating loop in unswitching
 157    unnecessarily).  */
 158
 159 static tree
 160 simplify_using_entry_checks (struct loop *loop, tree cond)
 161 {
 162   edge e = loop_preheader_edge (loop);
 163   gimple stmt;
 164
 165   while (1)
 166     {
 167       stmt = last_stmt (e->src);
 168       if (stmt
 169           && gimple_code (stmt) == GIMPLE_COND
 170           && gimple_cond_code (stmt) == TREE_CODE (cond)
 171           && operand_equal_p (gimple_cond_lhs (stmt),
 172                               TREE_OPERAND (cond, 0), 0)
 173           && operand_equal_p (gimple_cond_rhs (stmt),
 174                               TREE_OPERAND (cond, 1), 0))
 175         return (e->flags & EDGE_TRUE_VALUE
 176                 ? boolean_true_node
 177                 : boolean_false_node);
 178
 179       if (!single_pred_p (e->src))
 180         return cond;
 181
 182       e = single_pred_edge (e->src);
 183       if (e->src == ENTRY_BLOCK_PTR)
 184         return cond;
 185     }
 186 }
 187
 188 /* Unswitch single LOOP.  NUM is number of unswitchings done; we do not allow
 189    it to grow too much, it is too easy to create example on that the code would
 190    grow exponentially.  */
 191
 192 static bool
 193 tree_unswitch_single_loop (struct loop *loop, int num)
 194 {
 195   basic_block *bbs;
 196   struct loop *nloop;
 197   unsigned i;
 198   tree cond = NULL_TREE;
 199   gimple stmt;
 200   bool changed = false;
 201
 202   /* Do not unswitch too much.  */
 203   if (num > PARAM_VALUE (PARAM_MAX_UNSWITCH_LEVEL))
 204     {
 205       if (dump_file && (dump_flags & TDF_DETAILS))
 206         fprintf (dump_file, ";; Not unswitching anymore, hit max level\n");
 207       return false;
 208     }
 209
 210   i = 0;
 211   bbs = get_loop_body (loop);
 212
 213   while (1)
 214     {
 215       /* Find a bb to unswitch on.  */
 216       for (; i < loop->num_nodes; i++)
 217         if ((cond = tree_may_unswitch_on (bbs[i], loop)))
 218           break;
 219
 220       if (i == loop->num_nodes)
 221         {
 222           free (bbs);
 223           return changed;
 224         }
 225
 226       cond = simplify_using_entry_checks (loop, cond);
 227       stmt = last_stmt (bbs[i]);
 228       if (integer_nonzerop (cond))
 229         {
 230           /* Remove false path.  */
 231           gimple_cond_set_condition_from_tree (stmt, boolean_true_node);
 232           changed = true;
 233         }
 234       else if (integer_zerop (cond))
 235         {
 236           /* Remove true path.  */
 237           gimple_cond_set_condition_from_tree (stmt, boolean_false_node);
 238           changed = true;
 239         }
 240       else
 241         break;
 242
 243       update_stmt (stmt);
 244       i++;
 245     }
 246
 247   if (dump_file && (dump_flags & TDF_DETAILS))
 248     fprintf (dump_file, ";; Unswitching loop\n");
 249
 250   initialize_original_copy_tables ();
 251   /* Unswitch the loop on this condition.  */
 252   nloop = tree_unswitch_loop (loop, bbs[i], cond);
 253   if (!nloop)
 254     {
 255       free_original_copy_tables ();
 256       free (bbs);
 257       return changed;
 258     }
 259
 260   /* Update the SSA form after unswitching.  */
 261   update_ssa (TODO_update_ssa);
 262   free_original_copy_tables ();
 263
 264   /* Invoke itself on modified loops.  */
 265   tree_unswitch_single_loop (nloop, num + 1);
 266   tree_unswitch_single_loop (loop, num + 1);
 267   free (bbs);
 268   return true;
 269 }
 270
 271 /* Unswitch a LOOP w.r. to given basic block UNSWITCH_ON.  We only support
 272    unswitching of innermost loops.  COND is the condition determining which
 273    loop is entered -- the new loop is entered if COND is true.  Returns NULL
 274    if impossible, new loop otherwise.  */
 275
 276 static struct loop *
 277 tree_unswitch_loop (struct loop *loop,
 278                     basic_block unswitch_on, tree cond)
 279 {
 280   unsigned prob_true;
 281   edge edge_true, edge_false;
 282
 283   /* Some sanity checking.  */
 284   gcc_assert (flow_bb_inside_loop_p (loop, unswitch_on));
 285   gcc_assert (EDGE_COUNT (unswitch_on->succs) == 2);
 286   gcc_assert (loop->inner == NULL);
 287
 288   extract_true_false_edges_from_block (unswitch_on, &edge_true, &edge_false);
 289   prob_true = edge_true->probability;
 290   return loop_version (loop, unshare_expr (cond),
 291                        NULL, prob_true, prob_true,
 292                        REG_BR_PROB_BASE - prob_true, false);
 293 }