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