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 #ifdef ENABLE_CHECKING
 105       verify_dominators (CDI_DOMINATORS);
 106       verify_loop_structure (loops);
 107 #endif
 108     }
 109
 110   if (changed)
 111     cleanup_tree_cfg_loop ();
 112 }
 113
 114 /* Checks whether we can unswitch LOOP on condition at end of BB -- one of its
 115    basic blocks (for what it means see comments below).  */
 116
 117 static tree
 118 tree_may_unswitch_on (basic_block bb, struct loop *loop)
 119 {
 120   tree stmt, def, cond;
 121   basic_block def_bb;
 122   use_optype uses;
 123   unsigned i;
 124
 125   /* BB must end in a simple conditional jump.  */
 126   stmt = last_stmt (bb);
 127   if (!stmt || TREE_CODE (stmt) != COND_EXPR)
 128     return NULL_TREE;
 129
 130   /* Condition must be invariant.  */
 131   get_stmt_operands (stmt);
 132   uses = STMT_USE_OPS (stmt);
 133   for (i = 0; i < NUM_USES (uses); i++)
 134     {
 135       def = SSA_NAME_DEF_STMT (USE_OP (uses, i));
 136       def_bb = bb_for_stmt (def);
 137       if (def_bb
 138           && flow_bb_inside_loop_p (loop, def_bb))
 139         return NULL_TREE;
 140     }
 141
 142   cond = COND_EXPR_COND (stmt);
 143   /* To keep the things simple, we do not directly remove the conditions,
 144      but just replace tests with 0/1.  Prevent the infinite loop where we
 145      would unswitch again on such a condition.  */
 146   if (integer_zerop (cond) || integer_nonzerop (cond))
 147     return NULL_TREE;
 148
 149   return cond;
 150 }
 151
 152 /* Simplifies COND using checks in front of the entry of the LOOP.  Just very
 153    simplish (sufficient to prevent us from duplicating loop in unswitching
 154    unnecessarily).  */
 155
 156 static tree
 157 simplify_using_entry_checks (struct loop *loop, tree cond)
 158 {
 159   edge e = loop_preheader_edge (loop);
 160   tree stmt;
 161
 162   while (1)
 163     {
 164       stmt = last_stmt (e->src);
 165       if (stmt
 166           && TREE_CODE (stmt) == COND_EXPR
 167           && operand_equal_p (COND_EXPR_COND (stmt), cond, 0))
 168         return (e->flags & EDGE_TRUE_VALUE
 169                 ? boolean_true_node
 170                 : boolean_false_node);
 171
 172       if (!single_pred_p (e->src))
 173         return cond;
 174
 175       e = single_pred_edge (e->src);
 176       if (e->src == ENTRY_BLOCK_PTR)
 177         return cond;
 178     }
 179 }
 180
 181 /* Unswitch single LOOP.  NUM is number of unswitchings done; we do not allow
 182    it to grow too much, it is too easy to create example on that the code would
 183    grow exponentially.  */
 184
 185 static bool
 186 tree_unswitch_single_loop (struct loops *loops, struct loop *loop, int num)
 187 {
 188   basic_block *bbs;
 189   struct loop *nloop;
 190   unsigned i;
 191   tree cond = NULL_TREE, stmt;
 192   bool changed = false;
 193
 194   /* Do not unswitch too much.  */
 195   if (num > PARAM_VALUE (PARAM_MAX_UNSWITCH_LEVEL))
 196     {
 197       if (dump_file && (dump_flags & TDF_DETAILS))
 198         fprintf (dump_file, ";; Not unswitching anymore, hit max level\n");
 199       return false;
 200     }
 201
 202   /* Only unswitch innermost loops.  */
 203   if (loop->inner)
 204     {
 205       if (dump_file && (dump_flags & TDF_DETAILS))
 206         fprintf (dump_file, ";; Not unswitching, not innermost loop\n");
 207       return false;
 208     }
 209
 210   /* The loop should not be too large, to limit code growth.  */
 211   if (tree_num_loop_insns (loop)
 212       > (unsigned) PARAM_VALUE (PARAM_MAX_UNSWITCH_INSNS))
 213     {
 214       if (dump_file && (dump_flags & TDF_DETAILS))
 215         fprintf (dump_file, ";; Not unswitching, loop too big\n");
 216       return false;
 217     }
 218
 219   i = 0;
 220   bbs = get_loop_body (loop);
 221
 222   while (1)
 223     {
 224       /* Find a bb to unswitch on.  */
 225       for (; i < loop->num_nodes; i++)
 226         if ((cond = tree_may_unswitch_on (bbs[i], loop)))
 227           break;
 228
 229       if (i == loop->num_nodes)
 230         {
 231           free (bbs);
 232           return changed;
 233         }
 234
 235       cond = simplify_using_entry_checks (loop, cond);
 236       stmt = last_stmt (bbs[i]);
 237       if (integer_nonzerop (cond))
 238         {
 239           /* Remove false path.  */
 240           COND_EXPR_COND (stmt) = boolean_true_node;
 241           changed = true;
 242         }
 243       else if (integer_zerop (cond))
 244         {
 245           /* Remove true path.  */
 246           COND_EXPR_COND (stmt) = boolean_false_node;
 247           changed = true;
 248         }
 249       else
 250         break;
 251
 252       modify_stmt (stmt);
 253       i++;
 254     }
 255
 256   if (dump_file && (dump_flags & TDF_DETAILS))
 257     fprintf (dump_file, ";; Unswitching loop\n");
 258
 259   /* Unswitch the loop on this condition.  */
 260   nloop = tree_unswitch_loop (loops, loop, bbs[i], cond);
 261   if (!nloop)
 262     return changed;
 263
 264   /* Invoke itself on modified loops.  */
 265   tree_unswitch_single_loop (loops, nloop, num + 1);
 266   tree_unswitch_single_loop (loops, loop, num + 1);
 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 loops *loops, struct loop *loop,
 277                     basic_block unswitch_on, tree cond)
 278 {
 279   basic_block condition_bb;
 280
 281   /* Some sanity checking.  */
 282   gcc_assert (flow_bb_inside_loop_p (loop, unswitch_on));
 283   gcc_assert (EDGE_COUNT (unswitch_on->succs) == 2);
 284   gcc_assert (loop->inner == NULL);
 285
 286   return tree_ssa_loop_version (loops, loop, unshare_expr (cond),
 287                                 &condition_bb);
 288 }