gcc/tree-ssa-loop-unswitch.c

   1 /* Loop unswitching.
   2    Copyright (C) 2004, 2005, 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 "timevar.h"
  34 #include "cfgloop.h"
  35 #include "domwalk.h"
  36 #include "params.h"
  37 #include "tree-pass.h"
  38 #include "tree-inline.h"
  39
  40 /* This file implements the loop unswitching, i.e. transformation of loops like
  41
  42    while (A)
  43      {
  44        if (inv)
  45          B;
  46
  47        X;
  48
  49        if (!inv)
  50          C;
  51      }
  52
  53    where inv is the loop invariant, into
  54
  55    if (inv)
  56      {
  57        while (A)
  58          {
  59            B;
  60            X;
  61          }
  62      }
  63    else
  64      {
  65        while (A)
  66          {
  67            X;
  68            C;
  69          }
  70      }
  71
  72    Inv is considered invariant iff the values it compares are both invariant;
  73    tree-ssa-loop-im.c ensures that all the suitable conditions are in this
  74    shape.  */
  75
  76 static struct loop *tree_unswitch_loop (struct loop *, basic_block, tree);
  77 static bool tree_unswitch_single_loop (struct loop *, int);
  78 static tree tree_may_unswitch_on (basic_block, struct loop *);
  79
  80 /* Main entry point.  Perform loop unswitching on all suitable loops.  */
  81
  82 unsigned int
  83 tree_ssa_unswitch_loops (void)
  84 {
  85   loop_iterator li;
  86   struct loop *loop;
  87   bool changed = false;
  88
  89   /* Go through inner loops (only original ones).  */
  90   FOR_EACH_LOOP (li, loop, LI_ONLY_INNERMOST)
  91     {
  92       changed |= tree_unswitch_single_loop (loop, 0);
  93     }
  94
  95   if (changed)
  96     return TODO_cleanup_cfg;
  97   return 0;
  98 }
  99
 100 /* Checks whether we can unswitch LOOP on condition at end of BB -- one of its
 101    basic blocks (for what it means see comments below).  */
 102
 103 static tree
 104 tree_may_unswitch_on (basic_block bb, struct loop *loop)
 105 {
 106   gimple stmt, def;
 107   tree cond, use;
 108   basic_block def_bb;
 109   ssa_op_iter iter;
 110
 111   /* BB must end in a simple conditional jump.  */
 112   stmt = last_stmt (bb);
 113   if (!stmt || gimple_code (stmt) != GIMPLE_COND)
 114     return NULL_TREE;
 115
 116   /* Condition must be invariant.  */
 117   FOR_EACH_SSA_TREE_OPERAND (use, stmt, iter, SSA_OP_USE)
 118     {
 119       def = SSA_NAME_DEF_STMT (use);
 120       def_bb = gimple_bb (def);
 121       if (def_bb
 122           && flow_bb_inside_loop_p (loop, def_bb))
 123         return NULL_TREE;
 124     }
 125
 126   cond = build2 (gimple_cond_code (stmt), boolean_type_node,
 127                  gimple_cond_lhs (stmt), gimple_cond_rhs (stmt));
 128
 129   /* To keep the things simple, we do not directly remove the conditions,
 130      but just replace tests with 0/1.  Prevent the infinite loop where we
 131      would unswitch again on such a condition.  */
 132   if (integer_zerop (cond) || integer_nonzerop (cond))
 133     return NULL_TREE;
 134
 135   return cond;
 136 }
 137
 138 /* Simplifies COND using checks in front of the entry of the LOOP.  Just very
 139    simplish (sufficient to prevent us from duplicating loop in unswitching
 140    unnecessarily).  */
 141
 142 static tree
 143 simplify_using_entry_checks (struct loop *loop, tree cond)
 144 {
 145   edge e = loop_preheader_edge (loop);
 146   gimple stmt;
 147
 148   while (1)
 149     {
 150       stmt = last_stmt (e->src);
 151       if (stmt
 152           && gimple_code (stmt) == GIMPLE_COND
 153           && gimple_cond_code (stmt) == TREE_CODE (cond)
 154           && operand_equal_p (gimple_cond_lhs (stmt),
 155                               TREE_OPERAND (cond, 0), 0)
 156           && operand_equal_p (gimple_cond_rhs (stmt),
 157                               TREE_OPERAND (cond, 1), 0))
 158         return (e->flags & EDGE_TRUE_VALUE
 159                 ? boolean_true_node
 160                 : boolean_false_node);
 161
 162       if (!single_pred_p (e->src))
 163         return cond;
 164
 165       e = single_pred_edge (e->src);
 166       if (e->src == ENTRY_BLOCK_PTR)
 167         return cond;
 168     }
 169 }
 170
 171 /* Unswitch single LOOP.  NUM is number of unswitchings done; we do not allow
 172    it to grow too much, it is too easy to create example on that the code would
 173    grow exponentially.  */
 174
 175 static bool
 176 tree_unswitch_single_loop (struct loop *loop, int num)
 177 {
 178   basic_block *bbs;
 179   struct loop *nloop;
 180   unsigned i;
 181   tree cond = NULL_TREE;
 182   gimple stmt;
 183   bool changed = false;
 184
 185   /* Do not unswitch too much.  */
 186   if (num > PARAM_VALUE (PARAM_MAX_UNSWITCH_LEVEL))
 187     {
 188       if (dump_file && (dump_flags & TDF_DETAILS))
 189         fprintf (dump_file, ";; Not unswitching anymore, hit max level\n");
 190       return false;
 191     }
 192
 193   /* Only unswitch innermost loops.  */
 194   if (loop->inner)
 195     {
 196       if (dump_file && (dump_flags & TDF_DETAILS))
 197         fprintf (dump_file, ";; Not unswitching, not innermost loop\n");
 198       return false;
 199     }
 200
 201   /* Do not unswitch in cold regions.  */
 202   if (optimize_loop_for_size_p (loop))
 203     {
 204       if (dump_file && (dump_flags & TDF_DETAILS))
 205         fprintf (dump_file, ";; Not unswitching cold loops\n");
 206       return false;
 207     }
 208
 209   /* The loop should not be too large, to limit code growth.  */
 210   if (tree_num_loop_insns (loop, &eni_size_weights)
 211       > (unsigned) PARAM_VALUE (PARAM_MAX_UNSWITCH_INSNS))
 212     {
 213       if (dump_file && (dump_flags & TDF_DETAILS))
 214         fprintf (dump_file, ";; Not unswitching, loop too big\n");
 215       return false;
 216     }
 217
 218   i = 0;
 219   bbs = get_loop_body (loop);
 220
 221   while (1)
 222     {
 223       /* Find a bb to unswitch on.  */
 224       for (; i < loop->num_nodes; i++)
 225         if ((cond = tree_may_unswitch_on (bbs[i], loop)))
 226           break;
 227
 228       if (i == loop->num_nodes)
 229         {
 230           free (bbs);
 231           return changed;
 232         }
 233
 234       cond = simplify_using_entry_checks (loop, cond);
 235       stmt = last_stmt (bbs[i]);
 236       if (integer_nonzerop (cond))
 237         {
 238           /* Remove false path.  */
 239           gimple_cond_set_condition_from_tree (stmt, boolean_true_node);
 240           changed = true;
 241         }
 242       else if (integer_zerop (cond))
 243         {
 244           /* Remove true path.  */
 245           gimple_cond_set_condition_from_tree (stmt, boolean_false_node);
 246           changed = true;
 247         }
 248       else
 249         break;
 250
 251       update_stmt (stmt);
 252       i++;
 253     }
 254
 255   if (dump_file && (dump_flags & TDF_DETAILS))
 256     fprintf (dump_file, ";; Unswitching loop\n");
 257
 258   initialize_original_copy_tables ();
 259   /* Unswitch the loop on this condition.  */
 260   nloop = tree_unswitch_loop (loop, bbs[i], cond);
 261   if (!nloop)
 262     {
 263       free_original_copy_tables ();
 264       free (bbs);
 265       return changed;
 266     }
 267
 268   /* Update the SSA form after unswitching.  */
 269   update_ssa (TODO_update_ssa);
 270   free_original_copy_tables ();
 271
 272   /* Invoke itself on modified loops.  */
 273   tree_unswitch_single_loop (nloop, num + 1);
 274   tree_unswitch_single_loop (loop, num + 1);
 275   free (bbs);
 276   return true;
 277 }
 278
 279 /* Unswitch a LOOP w.r. to given basic block UNSWITCH_ON.  We only support
 280    unswitching of innermost loops.  COND is the condition determining which
 281    loop is entered -- the new loop is entered if COND is true.  Returns NULL
 282    if impossible, new loop otherwise.  */
 283
 284 static struct loop *
 285 tree_unswitch_loop (struct loop *loop,
 286                     basic_block unswitch_on, tree cond)
 287 {
 288   unsigned prob_true;
 289   edge edge_true, edge_false;
 290
 291   /* Some sanity checking.  */
 292   gcc_assert (flow_bb_inside_loop_p (loop, unswitch_on));
 293   gcc_assert (EDGE_COUNT (unswitch_on->succs) == 2);
 294   gcc_assert (loop->inner == NULL);
 295
 296   extract_true_false_edges_from_block (unswitch_on, &edge_true, &edge_false);
 297   prob_true = edge_true->probability;
 298   return loop_version (loop, unshare_expr (cond),
 299                        NULL, prob_true, prob_true,
 300                        REG_BR_PROB_BASE - prob_true, false);
 301 }