gcc/tree-ssa-loop-unswitch.c

   1 /* Loop unswitching.
   2    Copyright (C) 2004, 2005 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 2, 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 COPYING.  If not, write to the Free
  18 Software Foundation, 51 Franklin Street, Fifth Floor, Boston, MA
  19 02110-1301, USA.  */
  20
  21 #include "config.h"
  22 #include "system.h"
  23 #include "coretypes.h"
  24 #include "tm.h"
  25 #include "tree.h"
  26 #include "rtl.h"
  27 #include "tm_p.h"
  28 #include "hard-reg-set.h"
  29 #include "basic-block.h"
  30 #include "output.h"
  31 #include "diagnostic.h"
  32 #include "tree-flow.h"
  33 #include "tree-dump.h"
  34 #include "timevar.h"
  35 #include "cfgloop.h"
  36 #include "domwalk.h"
  37 #include "params.h"
  38 #include "tree-pass.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_OLD | 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   tree stmt, def, cond, use;
 107   basic_block def_bb;
 108   ssa_op_iter iter;
 109
 110   /* BB must end in a simple conditional jump.  */
 111   stmt = last_stmt (bb);
 112   if (!stmt || TREE_CODE (stmt) != COND_EXPR)
 113     return NULL_TREE;
 114
 115   /* Condition must be invariant.  */
 116   FOR_EACH_SSA_TREE_OPERAND (use, stmt, iter, SSA_OP_USE)
 117     {
 118       def = SSA_NAME_DEF_STMT (use);
 119       def_bb = bb_for_stmt (def);
 120       if (def_bb
 121           && flow_bb_inside_loop_p (loop, def_bb))
 122         return NULL_TREE;
 123     }
 124
 125   cond = COND_EXPR_COND (stmt);
 126   /* To keep the things simple, we do not directly remove the conditions,
 127      but just replace tests with 0/1.  Prevent the infinite loop where we
 128      would unswitch again on such a condition.  */
 129   if (integer_zerop (cond) || integer_nonzerop (cond))
 130     return NULL_TREE;
 131
 132   return cond;
 133 }
 134
 135 /* Simplifies COND using checks in front of the entry of the LOOP.  Just very
 136    simplish (sufficient to prevent us from duplicating loop in unswitching
 137    unnecessarily).  */
 138
 139 static tree
 140 simplify_using_entry_checks (struct loop *loop, tree cond)
 141 {
 142   edge e = loop_preheader_edge (loop);
 143   tree stmt;
 144
 145   while (1)
 146     {
 147       stmt = last_stmt (e->src);
 148       if (stmt
 149           && TREE_CODE (stmt) == COND_EXPR
 150           && operand_equal_p (COND_EXPR_COND (stmt), cond, 0))
 151         return (e->flags & EDGE_TRUE_VALUE
 152                 ? boolean_true_node
 153                 : boolean_false_node);
 154
 155       if (!single_pred_p (e->src))
 156         return cond;
 157
 158       e = single_pred_edge (e->src);
 159       if (e->src == ENTRY_BLOCK_PTR)
 160         return cond;
 161     }
 162 }
 163
 164 /* Unswitch single LOOP.  NUM is number of unswitchings done; we do not allow
 165    it to grow too much, it is too easy to create example on that the code would
 166    grow exponentially.  */
 167
 168 static bool
 169 tree_unswitch_single_loop (struct loop *loop, int num)
 170 {
 171   basic_block *bbs;
 172   struct loop *nloop;
 173   unsigned i;
 174   tree cond = NULL_TREE, stmt;
 175   bool changed = false;
 176
 177   /* Do not unswitch too much.  */
 178   if (num > PARAM_VALUE (PARAM_MAX_UNSWITCH_LEVEL))
 179     {
 180       if (dump_file && (dump_flags & TDF_DETAILS))
 181         fprintf (dump_file, ";; Not unswitching anymore, hit max level\n");
 182       return false;
 183     }
 184
 185   /* Only unswitch innermost loops.  */
 186   if (loop->inner)
 187     {
 188       if (dump_file && (dump_flags & TDF_DETAILS))
 189         fprintf (dump_file, ";; Not unswitching, not innermost loop\n");
 190       return false;
 191     }
 192
 193   /* The loop should not be too large, to limit code growth.  */
 194   if (tree_num_loop_insns (loop)
 195       > (unsigned) PARAM_VALUE (PARAM_MAX_UNSWITCH_INSNS))
 196     {
 197       if (dump_file && (dump_flags & TDF_DETAILS))
 198         fprintf (dump_file, ";; Not unswitching, loop too big\n");
 199       return false;
 200     }
 201
 202   i = 0;
 203   bbs = get_loop_body (loop);
 204
 205   while (1)
 206     {
 207       /* Find a bb to unswitch on.  */
 208       for (; i < loop->num_nodes; i++)
 209         if ((cond = tree_may_unswitch_on (bbs[i], loop)))
 210           break;
 211
 212       if (i == loop->num_nodes)
 213         {
 214           free (bbs);
 215           return changed;
 216         }
 217
 218       cond = simplify_using_entry_checks (loop, cond);
 219       stmt = last_stmt (bbs[i]);
 220       if (integer_nonzerop (cond))
 221         {
 222           /* Remove false path.  */
 223           COND_EXPR_COND (stmt) = boolean_true_node;
 224           changed = true;
 225         }
 226       else if (integer_zerop (cond))
 227         {
 228           /* Remove true path.  */
 229           COND_EXPR_COND (stmt) = boolean_false_node;
 230           changed = true;
 231         }
 232       else
 233         break;
 234
 235       update_stmt (stmt);
 236       i++;
 237     }
 238
 239   if (dump_file && (dump_flags & TDF_DETAILS))
 240     fprintf (dump_file, ";; Unswitching loop\n");
 241
 242   initialize_original_copy_tables ();
 243   /* Unswitch the loop on this condition.  */
 244   nloop = tree_unswitch_loop (loop, bbs[i], cond);
 245   if (!nloop)
 246     {
 247       free_original_copy_tables ();
 248       free (bbs);
 249       return changed;
 250     }
 251
 252   /* Update the SSA form after unswitching.  */
 253   update_ssa (TODO_update_ssa);
 254   free_original_copy_tables ();
 255
 256   /* Invoke itself on modified loops.  */
 257   tree_unswitch_single_loop (nloop, num + 1);
 258   tree_unswitch_single_loop (loop, num + 1);
 259   free (bbs);
 260   return true;
 261 }
 262
 263 /* Unswitch a LOOP w.r. to given basic block UNSWITCH_ON.  We only support
 264    unswitching of innermost loops.  COND is the condition determining which
 265    loop is entered -- the new loop is entered if COND is true.  Returns NULL
 266    if impossible, new loop otherwise.  */
 267
 268 static struct loop *
 269 tree_unswitch_loop (struct loop *loop,
 270                     basic_block unswitch_on, tree cond)
 271 {
 272   basic_block condition_bb;
 273
 274   /* Some sanity checking.  */
 275   gcc_assert (flow_bb_inside_loop_p (loop, unswitch_on));
 276   gcc_assert (EDGE_COUNT (unswitch_on->succs) == 2);
 277   gcc_assert (loop->inner == NULL);
 278
 279   return loop_version (loop, unshare_expr (cond),
 280                        &condition_bb, false);
 281 }