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, 59 Temple Place - Suite 330, Boston, MA
  19 02111-1307, 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 loops *, struct loop *, basic_block,
  77                                    tree);
  78 static bool tree_unswitch_single_loop (struct loops *, struct loop *, int);
  79 static tree tree_may_unswitch_on (basic_block, struct loop *);
  80
  81 /* Main entry point.  Perform loop unswitching on all suitable LOOPS.  */
  82
  83 void
  84 tree_ssa_unswitch_loops (struct loops *loops)
  85 {
  86   int i, num;
  87   struct loop *loop;
  88   bool changed = false;
  89
  90   /* Go through inner loops (only original ones).  */
  91   num = loops->num;
  92
  93   for (i = 1; i < num; i++)
  94     {
  95       /* Removed loop?  */
  96       loop = loops->parray[i];
  97       if (!loop)
  98         continue;
  99
 100       if (loop->inner)
 101         continue;
 102
 103       changed |= tree_unswitch_single_loop (loops, loop, 0);
 104     }
 105
 106   if (changed)
 107     cleanup_tree_cfg_loop ();
 108 }
 109
 110 /* Checks whether we can unswitch LOOP on condition at end of BB -- one of its
 111    basic blocks (for what it means see comments below).  */
 112
 113 static tree
 114 tree_may_unswitch_on (basic_block bb, struct loop *loop)
 115 {
 116   tree stmt, def, cond;
 117   basic_block def_bb;
 118   use_optype uses;
 119   unsigned i;
 120
 121   /* BB must end in a simple conditional jump.  */
 122   stmt = last_stmt (bb);
 123   if (!stmt || TREE_CODE (stmt) != COND_EXPR)
 124     return NULL_TREE;
 125
 126   /* Condition must be invariant.  */
 127   uses = STMT_USE_OPS (stmt);
 128   for (i = 0; i < NUM_USES (uses); i++)
 129     {
 130       def = SSA_NAME_DEF_STMT (USE_OP (uses, i));
 131       def_bb = bb_for_stmt (def);
 132       if (def_bb
 133           && flow_bb_inside_loop_p (loop, def_bb))
 134         return NULL_TREE;
 135     }
 136
 137   cond = COND_EXPR_COND (stmt);
 138   /* To keep the things simple, we do not directly remove the conditions,
 139      but just replace tests with 0/1.  Prevent the infinite loop where we
 140      would unswitch again on such a condition.  */
 141   if (integer_zerop (cond) || integer_nonzerop (cond))
 142     return NULL_TREE;
 143
 144   return cond;
 145 }
 146
 147 /* Simplifies COND using checks in front of the entry of the LOOP.  Just very
 148    simplish (sufficient to prevent us from duplicating loop in unswitching
 149    unnecessarily).  */
 150
 151 static tree
 152 simplify_using_entry_checks (struct loop *loop, tree cond)
 153 {
 154   edge e = loop_preheader_edge (loop);
 155   tree stmt;
 156
 157   while (1)
 158     {
 159       stmt = last_stmt (e->src);
 160       if (stmt
 161           && TREE_CODE (stmt) == COND_EXPR
 162           && operand_equal_p (COND_EXPR_COND (stmt), cond, 0))
 163         return (e->flags & EDGE_TRUE_VALUE
 164                 ? boolean_true_node
 165                 : boolean_false_node);
 166
 167       if (!single_pred_p (e->src))
 168         return cond;
 169
 170       e = single_pred_edge (e->src);
 171       if (e->src == ENTRY_BLOCK_PTR)
 172         return cond;
 173     }
 174 }
 175
 176 /* Unswitch single LOOP.  NUM is number of unswitchings done; we do not allow
 177    it to grow too much, it is too easy to create example on that the code would
 178    grow exponentially.  */
 179
 180 static bool
 181 tree_unswitch_single_loop (struct loops *loops, struct loop *loop, int num)
 182 {
 183   basic_block *bbs;
 184   struct loop *nloop;
 185   unsigned i;
 186   tree cond = NULL_TREE, stmt;
 187   bool changed = false;
 188
 189   /* Do not unswitch too much.  */
 190   if (num > PARAM_VALUE (PARAM_MAX_UNSWITCH_LEVEL))
 191     {
 192       if (dump_file && (dump_flags & TDF_DETAILS))
 193         fprintf (dump_file, ";; Not unswitching anymore, hit max level\n");
 194       return false;
 195     }
 196
 197   /* Only unswitch innermost loops.  */
 198   if (loop->inner)
 199     {
 200       if (dump_file && (dump_flags & TDF_DETAILS))
 201         fprintf (dump_file, ";; Not unswitching, not innermost loop\n");
 202       return false;
 203     }
 204
 205   /* The loop should not be too large, to limit code growth.  */
 206   if (tree_num_loop_insns (loop)
 207       > (unsigned) PARAM_VALUE (PARAM_MAX_UNSWITCH_INSNS))
 208     {
 209       if (dump_file && (dump_flags & TDF_DETAILS))
 210         fprintf (dump_file, ";; Not unswitching, loop too big\n");
 211       return false;
 212     }
 213
 214   i = 0;
 215   bbs = get_loop_body (loop);
 216
 217   while (1)
 218     {
 219       /* Find a bb to unswitch on.  */
 220       for (; i < loop->num_nodes; i++)
 221         if ((cond = tree_may_unswitch_on (bbs[i], loop)))
 222           break;
 223
 224       if (i == loop->num_nodes)
 225         {
 226           free (bbs);
 227           return changed;
 228         }
 229
 230       cond = simplify_using_entry_checks (loop, cond);
 231       stmt = last_stmt (bbs[i]);
 232       if (integer_nonzerop (cond))
 233         {
 234           /* Remove false path.  */
 235           COND_EXPR_COND (stmt) = boolean_true_node;
 236           changed = true;
 237         }
 238       else if (integer_zerop (cond))
 239         {
 240           /* Remove true path.  */
 241           COND_EXPR_COND (stmt) = boolean_false_node;
 242           changed = true;
 243         }
 244       else
 245         break;
 246
 247       update_stmt (stmt);
 248       i++;
 249     }
 250
 251   if (dump_file && (dump_flags & TDF_DETAILS))
 252     fprintf (dump_file, ";; Unswitching loop\n");
 253
 254   /* Unswitch the loop on this condition.  */
 255   nloop = tree_unswitch_loop (loops, loop, bbs[i], cond);
 256   if (!nloop)
 257     return changed;
 258
 259   /* Update the SSA form after unswitching.  */
 260   update_ssa (TODO_update_ssa);
 261
 262   /* Invoke itself on modified loops.  */
 263   tree_unswitch_single_loop (loops, nloop, num + 1);
 264   tree_unswitch_single_loop (loops, loop, num + 1);
 265   return true;
 266 }
 267
 268 /* Unswitch a LOOP w.r. to given basic block UNSWITCH_ON.  We only support
 269    unswitching of innermost loops.  COND is the condition determining which
 270    loop is entered -- the new loop is entered if COND is true.  Returns NULL
 271    if impossible, new loop otherwise.  */
 272
 273 static struct loop *
 274 tree_unswitch_loop (struct loops *loops, struct loop *loop,
 275                     basic_block unswitch_on, tree cond)
 276 {
 277   basic_block condition_bb;
 278
 279   /* Some sanity checking.  */
 280   gcc_assert (flow_bb_inside_loop_p (loop, unswitch_on));
 281   gcc_assert (EDGE_COUNT (unswitch_on->succs) == 2);
 282   gcc_assert (loop->inner == NULL);
 283
 284   return loop_version (loops, loop, unshare_expr (cond),
 285                        &condition_bb);
 286 }