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 #include "tree-inline.h"
  40
  41 /* This file implements the loop unswitching, i.e. transformation of loops like
  42
  43    while (A)
  44      {
  45        if (inv)
  46          B;
  47
  48        X;
  49
  50        if (!inv)
  51          C;
  52      }
  53
  54    where inv is the loop invariant, into
  55
  56    if (inv)
  57      {
  58        while (A)
  59          {
  60            B;
  61            X;
  62          }
  63      }
  64    else
  65      {
  66        while (A)
  67          {
  68            X;
  69            C;
  70          }
  71      }
  72
  73    Inv is considered invariant iff the values it compares are both invariant;
  74    tree-ssa-loop-im.c ensures that all the suitable conditions are in this
  75    shape.  */
  76
  77 static struct loop *tree_unswitch_loop (struct loop *, basic_block, tree);
  78 static bool tree_unswitch_single_loop (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 unsigned int
  84 tree_ssa_unswitch_loops (void)
  85 {
  86   loop_iterator li;
  87   struct loop *loop;
  88   bool changed = false;
  89
  90   /* Go through inner loops (only original ones).  */
  91   FOR_EACH_LOOP (li, loop, LI_ONLY_INNERMOST)
  92     {
  93       changed |= tree_unswitch_single_loop (loop, 0);
  94     }
  95
  96   if (changed)
  97     return TODO_cleanup_cfg;
  98   return 0;
  99 }
 100
 101 /* Checks whether we can unswitch LOOP on condition at end of BB -- one of its
 102    basic blocks (for what it means see comments below).  */
 103
 104 static tree
 105 tree_may_unswitch_on (basic_block bb, struct loop *loop)
 106 {
 107   tree stmt, def, cond, use;
 108   basic_block def_bb;
 109   ssa_op_iter iter;
 110
 111   /* BB must end in a simple conditional jump.  */
 112   stmt = last_stmt (bb);
 113   if (!stmt || TREE_CODE (stmt) != COND_EXPR)
 114     return NULL_TREE;
 115
 116   /* Condition must be invariant.  */
 117   FOR_EACH_SSA_TREE_OPERAND (use, stmt, iter, SSA_OP_USE)
 118     {
 119       def = SSA_NAME_DEF_STMT (use);
 120       def_bb = bb_for_stmt (def);
 121       if (def_bb
 122           && flow_bb_inside_loop_p (loop, def_bb))
 123         return NULL_TREE;
 124     }
 125
 126   cond = COND_EXPR_COND (stmt);
 127   /* To keep the things simple, we do not directly remove the conditions,
 128      but just replace tests with 0/1.  Prevent the infinite loop where we
 129      would unswitch again on such a condition.  */
 130   if (integer_zerop (cond) || integer_nonzerop (cond))
 131     return NULL_TREE;
 132
 133   return cond;
 134 }
 135
 136 /* Simplifies COND using checks in front of the entry of the LOOP.  Just very
 137    simplish (sufficient to prevent us from duplicating loop in unswitching
 138    unnecessarily).  */
 139
 140 static tree
 141 simplify_using_entry_checks (struct loop *loop, tree cond)
 142 {
 143   edge e = loop_preheader_edge (loop);
 144   tree stmt;
 145
 146   while (1)
 147     {
 148       stmt = last_stmt (e->src);
 149       if (stmt
 150           && TREE_CODE (stmt) == COND_EXPR
 151           && operand_equal_p (COND_EXPR_COND (stmt), cond, 0))
 152         return (e->flags & EDGE_TRUE_VALUE
 153                 ? boolean_true_node
 154                 : boolean_false_node);
 155
 156       if (!single_pred_p (e->src))
 157         return cond;
 158
 159       e = single_pred_edge (e->src);
 160       if (e->src == ENTRY_BLOCK_PTR)
 161         return cond;
 162     }
 163 }
 164
 165 /* Unswitch single LOOP.  NUM is number of unswitchings done; we do not allow
 166    it to grow too much, it is too easy to create example on that the code would
 167    grow exponentially.  */
 168
 169 static bool
 170 tree_unswitch_single_loop (struct loop *loop, int num)
 171 {
 172   basic_block *bbs;
 173   struct loop *nloop;
 174   unsigned i;
 175   tree cond = NULL_TREE, stmt;
 176   bool changed = false;
 177
 178   /* Do not unswitch too much.  */
 179   if (num > PARAM_VALUE (PARAM_MAX_UNSWITCH_LEVEL))
 180     {
 181       if (dump_file && (dump_flags & TDF_DETAILS))
 182         fprintf (dump_file, ";; Not unswitching anymore, hit max level\n");
 183       return false;
 184     }
 185
 186   /* Only unswitch innermost loops.  */
 187   if (loop->inner)
 188     {
 189       if (dump_file && (dump_flags & TDF_DETAILS))
 190         fprintf (dump_file, ";; Not unswitching, not innermost loop\n");
 191       return false;
 192     }
 193
 194   /* The loop should not be too large, to limit code growth.  */
 195   if (tree_num_loop_insns (loop, &eni_size_weights)
 196       > (unsigned) PARAM_VALUE (PARAM_MAX_UNSWITCH_INSNS))
 197     {
 198       if (dump_file && (dump_flags & TDF_DETAILS))
 199         fprintf (dump_file, ";; Not unswitching, loop too big\n");
 200       return false;
 201     }
 202
 203   i = 0;
 204   bbs = get_loop_body (loop);
 205
 206   while (1)
 207     {
 208       /* Find a bb to unswitch on.  */
 209       for (; i < loop->num_nodes; i++)
 210         if ((cond = tree_may_unswitch_on (bbs[i], loop)))
 211           break;
 212
 213       if (i == loop->num_nodes)
 214         {
 215           free (bbs);
 216           return changed;
 217         }
 218
 219       cond = simplify_using_entry_checks (loop, cond);
 220       stmt = last_stmt (bbs[i]);
 221       if (integer_nonzerop (cond))
 222         {
 223           /* Remove false path.  */
 224           COND_EXPR_COND (stmt) = boolean_true_node;
 225           changed = true;
 226         }
 227       else if (integer_zerop (cond))
 228         {
 229           /* Remove true path.  */
 230           COND_EXPR_COND (stmt) = boolean_false_node;
 231           changed = true;
 232         }
 233       else
 234         break;
 235
 236       update_stmt (stmt);
 237       i++;
 238     }
 239
 240   if (dump_file && (dump_flags & TDF_DETAILS))
 241     fprintf (dump_file, ";; Unswitching loop\n");
 242
 243   initialize_original_copy_tables ();
 244   /* Unswitch the loop on this condition.  */
 245   nloop = tree_unswitch_loop (loop, bbs[i], cond);
 246   if (!nloop)
 247     {
 248       free_original_copy_tables ();
 249       free (bbs);
 250       return changed;
 251     }
 252
 253   /* Update the SSA form after unswitching.  */
 254   update_ssa (TODO_update_ssa);
 255   free_original_copy_tables ();
 256
 257   /* Invoke itself on modified loops.  */
 258   tree_unswitch_single_loop (nloop, num + 1);
 259   tree_unswitch_single_loop (loop, num + 1);
 260   free (bbs);
 261   return true;
 262 }
 263
 264 /* Unswitch a LOOP w.r. to given basic block UNSWITCH_ON.  We only support
 265    unswitching of innermost loops.  COND is the condition determining which
 266    loop is entered -- the new loop is entered if COND is true.  Returns NULL
 267    if impossible, new loop otherwise.  */
 268
 269 static struct loop *
 270 tree_unswitch_loop (struct loop *loop,
 271                     basic_block unswitch_on, tree cond)
 272 {
 273   unsigned prob_true;
 274   edge edge_true, edge_false;
 275
 276   /* Some sanity checking.  */
 277   gcc_assert (flow_bb_inside_loop_p (loop, unswitch_on));
 278   gcc_assert (EDGE_COUNT (unswitch_on->succs) == 2);
 279   gcc_assert (loop->inner == NULL);
 280
 281   extract_true_false_edges_from_block (unswitch_on, &edge_true, &edge_false);
 282   prob_true = edge_true->probability;
 283   return loop_version (loop, unshare_expr (cond),
 284                        NULL, prob_true, prob_true,
 285                        REG_BR_PROB_BASE - prob_true, false);
 286 }