gcc/tree-ssa-loop-unswitch.c

   1 /* Loop unswitching.
   2    Copyright (C) 2004 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 0
 111   /* The necessary infrastructure is not in yet.  */
 112   if (changed)
 113     cleanup_tree_cfg_loop ();
 114 #endif
 115 }
 116
 117 /* Checks whether we can unswitch LOOP on condition at end of BB -- one of its
 118    basic blocks (for what it means see comments below).  */
 119
 120 static tree
 121 tree_may_unswitch_on (basic_block bb, struct loop *loop)
 122 {
 123   tree stmt, def, cond;
 124   basic_block def_bb;
 125   use_optype uses;
 126   unsigned i;
 127
 128   /* BB must end in a simple conditional jump.  */
 129   stmt = last_stmt (bb);
 130   if (!stmt || TREE_CODE (stmt) != COND_EXPR)
 131     return NULL_TREE;
 132
 133   /* Condition must be invariant.  */
 134   get_stmt_operands (stmt);
 135   uses = STMT_USE_OPS (stmt);
 136   for (i = 0; i < NUM_USES (uses); i++)
 137     {
 138       def = SSA_NAME_DEF_STMT (USE_OP (uses, i));
 139       def_bb = bb_for_stmt (def);
 140       if (def_bb
 141           && flow_bb_inside_loop_p (loop, def_bb))
 142         return NULL_TREE;
 143     }
 144
 145   cond = COND_EXPR_COND (stmt);
 146   /* To keep the things simple, we do not directly remove the conditions,
 147      but just replace tests with 0/1.  Prevent the infinite loop where we
 148      would unswitch again on such a condition.  */
 149   if (integer_zerop (cond) || integer_nonzerop (cond))
 150     return NULL_TREE;
 151
 152   return cond;
 153 }
 154
 155 /* Simplifies COND using checks in front of the entry of the LOOP.  Just very
 156    simplish (sufficient to prevent us from duplicating loop in unswitching
 157    unnecessarily).  */
 158
 159 static tree
 160 simplify_using_entry_checks (struct loop *loop, tree cond)
 161 {
 162   edge e = loop_preheader_edge (loop);
 163   tree stmt;
 164
 165   while (1)
 166     {
 167       stmt = last_stmt (e->src);
 168       if (stmt
 169           && TREE_CODE (stmt) == COND_EXPR
 170           && operand_equal_p (COND_EXPR_COND (stmt), cond, 0))
 171         return (e->flags & EDGE_TRUE_VALUE
 172                 ? boolean_true_node
 173                 : boolean_false_node);
 174
 175       if (EDGE_COUNT (e->src->preds) > 1)
 176         return cond;
 177
 178       e = EDGE_PRED (e->src, 0);
 179       if (e->src == ENTRY_BLOCK_PTR)
 180         return cond;
 181     }
 182 }
 183
 184 /* Unswitch single LOOP.  NUM is number of unswitchings done; we do not allow
 185    it to grow too much, it is too easy to create example on that the code would
 186    grow exponentially.  */
 187
 188 static bool
 189 tree_unswitch_single_loop (struct loops *loops, struct loop *loop, int num)
 190 {
 191   basic_block *bbs;
 192   struct loop *nloop;
 193   unsigned i;
 194   tree cond = NULL_TREE, stmt;
 195   bool changed = false;
 196
 197   /* Do not unswitch too much.  */
 198   if (num > PARAM_VALUE (PARAM_MAX_UNSWITCH_LEVEL))
 199     {
 200       if (dump_file && (dump_flags & TDF_DETAILS))
 201         fprintf (dump_file, ";; Not unswitching anymore, hit max level\n");
 202       return false;
 203     }
 204
 205   /* Only unswitch innermost loops.  */
 206   if (loop->inner)
 207     {
 208       if (dump_file && (dump_flags & TDF_DETAILS))
 209         fprintf (dump_file, ";; Not unswitching, not innermost loop\n");
 210       return false;
 211     }
 212
 213   /* The loop should not be too large, to limit code growth.  */
 214   if (tree_num_loop_insns (loop)
 215       > (unsigned) PARAM_VALUE (PARAM_MAX_UNSWITCH_INSNS))
 216     {
 217       if (dump_file && (dump_flags & TDF_DETAILS))
 218         fprintf (dump_file, ";; Not unswitching, loop too big\n");
 219       return false;
 220     }
 221
 222   i = 0;
 223   bbs = get_loop_body (loop);
 224
 225   while (1)
 226     {
 227       /* Find a bb to unswitch on.  */
 228       for (; i < loop->num_nodes; i++)
 229         if ((cond = tree_may_unswitch_on (bbs[i], loop)))
 230           break;
 231
 232       if (i == loop->num_nodes)
 233         {
 234           free (bbs);
 235           return changed;
 236         }
 237
 238       cond = simplify_using_entry_checks (loop, cond);
 239       stmt = last_stmt (bbs[i]);
 240       if (integer_nonzerop (cond))
 241         {
 242           /* Remove false path.  */
 243           COND_EXPR_COND (stmt) = boolean_true_node;
 244           changed = true;
 245         }
 246       else if (integer_zerop (cond))
 247         {
 248           /* Remove true path.  */
 249           COND_EXPR_COND (stmt) = boolean_false_node;
 250           changed = true;
 251         }
 252       else
 253         break;
 254
 255       modify_stmt (stmt);
 256       i++;
 257     }
 258
 259   if (dump_file && (dump_flags & TDF_DETAILS))
 260     fprintf (dump_file, ";; Unswitching loop\n");
 261
 262   /* Unswitch the loop on this condition.  */
 263   nloop = tree_unswitch_loop (loops, loop, bbs[i], cond);
 264   if (!nloop)
 265     return changed;
 266
 267   /* Invoke itself on modified loops.  */
 268   tree_unswitch_single_loop (loops, nloop, num + 1);
 269   tree_unswitch_single_loop (loops, loop, num + 1);
 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 tree_ssa_loop_version (loops, loop, unshare_expr (cond),
 290                                 &condition_bb);
 291 }