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 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, use;
 117   basic_block def_bb;
 118   ssa_op_iter iter;
 119
 120   /* BB must end in a simple conditional jump.  */
 121   stmt = last_stmt (bb);
 122   if (!stmt || TREE_CODE (stmt) != COND_EXPR)
 123     return NULL_TREE;
 124
 125   /* Condition must be invariant.  */
 126   FOR_EACH_SSA_TREE_OPERAND (use, stmt, iter, SSA_OP_USE)
 127     {
 128       def = SSA_NAME_DEF_STMT (use);
 129       def_bb = bb_for_stmt (def);
 130       if (def_bb
 131           && flow_bb_inside_loop_p (loop, def_bb))
 132         return NULL_TREE;
 133     }
 134
 135   cond = COND_EXPR_COND (stmt);
 136   /* To keep the things simple, we do not directly remove the conditions,
 137      but just replace tests with 0/1.  Prevent the infinite loop where we
 138      would unswitch again on such a condition.  */
 139   if (integer_zerop (cond) || integer_nonzerop (cond))
 140     return NULL_TREE;
 141
 142   return cond;
 143 }
 144
 145 /* Simplifies COND using checks in front of the entry of the LOOP.  Just very
 146    simplish (sufficient to prevent us from duplicating loop in unswitching
 147    unnecessarily).  */
 148
 149 static tree
 150 simplify_using_entry_checks (struct loop *loop, tree cond)
 151 {
 152   edge e = loop_preheader_edge (loop);
 153   tree stmt;
 154
 155   while (1)
 156     {
 157       stmt = last_stmt (e->src);
 158       if (stmt
 159           && TREE_CODE (stmt) == COND_EXPR
 160           && operand_equal_p (COND_EXPR_COND (stmt), cond, 0))
 161         return (e->flags & EDGE_TRUE_VALUE
 162                 ? boolean_true_node
 163                 : boolean_false_node);
 164
 165       if (!single_pred_p (e->src))
 166         return cond;
 167
 168       e = single_pred_edge (e->src);
 169       if (e->src == ENTRY_BLOCK_PTR)
 170         return cond;
 171     }
 172 }
 173
 174 /* Unswitch single LOOP.  NUM is number of unswitchings done; we do not allow
 175    it to grow too much, it is too easy to create example on that the code would
 176    grow exponentially.  */
 177
 178 static bool
 179 tree_unswitch_single_loop (struct loops *loops, struct loop *loop, int num)
 180 {
 181   basic_block *bbs;
 182   struct loop *nloop;
 183   unsigned i;
 184   tree cond = NULL_TREE, stmt;
 185   bool changed = false;
 186
 187   /* Do not unswitch too much.  */
 188   if (num > PARAM_VALUE (PARAM_MAX_UNSWITCH_LEVEL))
 189     {
 190       if (dump_file && (dump_flags & TDF_DETAILS))
 191         fprintf (dump_file, ";; Not unswitching anymore, hit max level\n");
 192       return false;
 193     }
 194
 195   /* Only unswitch innermost loops.  */
 196   if (loop->inner)
 197     {
 198       if (dump_file && (dump_flags & TDF_DETAILS))
 199         fprintf (dump_file, ";; Not unswitching, not innermost loop\n");
 200       return false;
 201     }
 202
 203   /* The loop should not be too large, to limit code growth.  */
 204   if (tree_num_loop_insns (loop)
 205       > (unsigned) PARAM_VALUE (PARAM_MAX_UNSWITCH_INSNS))
 206     {
 207       if (dump_file && (dump_flags & TDF_DETAILS))
 208         fprintf (dump_file, ";; Not unswitching, loop too big\n");
 209       return false;
 210     }
 211
 212   i = 0;
 213   bbs = get_loop_body (loop);
 214
 215   while (1)
 216     {
 217       /* Find a bb to unswitch on.  */
 218       for (; i < loop->num_nodes; i++)
 219         if ((cond = tree_may_unswitch_on (bbs[i], loop)))
 220           break;
 221
 222       if (i == loop->num_nodes)
 223         {
 224           free (bbs);
 225           return changed;
 226         }
 227
 228       cond = simplify_using_entry_checks (loop, cond);
 229       stmt = last_stmt (bbs[i]);
 230       if (integer_nonzerop (cond))
 231         {
 232           /* Remove false path.  */
 233           COND_EXPR_COND (stmt) = boolean_true_node;
 234           changed = true;
 235         }
 236       else if (integer_zerop (cond))
 237         {
 238           /* Remove true path.  */
 239           COND_EXPR_COND (stmt) = boolean_false_node;
 240           changed = true;
 241         }
 242       else
 243         break;
 244
 245       update_stmt (stmt);
 246       i++;
 247     }
 248
 249   if (dump_file && (dump_flags & TDF_DETAILS))
 250     fprintf (dump_file, ";; Unswitching loop\n");
 251
 252   initialize_original_copy_tables ();
 253   /* Unswitch the loop on this condition.  */
 254   nloop = tree_unswitch_loop (loops, loop, bbs[i], cond);
 255   if (!nloop)
 256     {
 257       free_original_copy_tables ();
 258       free (bbs);
 259       return changed;
 260     }
 261
 262   /* Update the SSA form after unswitching.  */
 263   update_ssa (TODO_update_ssa);
 264   free_original_copy_tables ();
 265
 266   /* Invoke itself on modified loops.  */
 267   tree_unswitch_single_loop (loops, nloop, num + 1);
 268   tree_unswitch_single_loop (loops, loop, num + 1);
 269   free (bbs);
 270   return true;
 271 }
 272
 273 /* Unswitch a LOOP w.r. to given basic block UNSWITCH_ON.  We only support
 274    unswitching of innermost loops.  COND is the condition determining which
 275    loop is entered -- the new loop is entered if COND is true.  Returns NULL
 276    if impossible, new loop otherwise.  */
 277
 278 static struct loop *
 279 tree_unswitch_loop (struct loops *loops, struct loop *loop,
 280                     basic_block unswitch_on, tree cond)
 281 {
 282   basic_block condition_bb;
 283
 284   /* Some sanity checking.  */
 285   gcc_assert (flow_bb_inside_loop_p (loop, unswitch_on));
 286   gcc_assert (EDGE_COUNT (unswitch_on->succs) == 2);
 287   gcc_assert (loop->inner == NULL);
 288
 289   return loop_version (loops, loop, unshare_expr (cond),
 290                        &condition_bb, false);
 291 }