gcc/tree-ssa-loop-unswitch.c

   1 /* Loop unswitching.
   2    Copyright (C) 2004, 2005, 2007, 2008 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 3, 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 COPYING3.  If not see
  18 <http://www.gnu.org/licenses/>.  */
  19
  20 #include "config.h"
  21 #include "system.h"
  22 #include "coretypes.h"
  23 #include "tm.h"
  24 #include "tree.h"
  25 #include "rtl.h"
  26 #include "tm_p.h"
  27 #include "hard-reg-set.h"
  28 #include "basic-block.h"
  29 #include "output.h"
  30 #include "diagnostic.h"
  31 #include "tree-flow.h"
  32 #include "tree-dump.h"
  33 #include "timevar.h"
  34 #include "cfgloop.h"
  35 #include "params.h"
  36 #include "tree-pass.h"
  37 #include "tree-inline.h"
  38
  39 /* This file implements the loop unswitching, i.e. transformation of loops like
  40
  41    while (A)
  42      {
  43        if (inv)
  44          B;
  45
  46        X;
  47
  48        if (!inv)
  49          C;
  50      }
  51
  52    where inv is the loop invariant, into
  53
  54    if (inv)
  55      {
  56        while (A)
  57          {
  58            B;
  59            X;
  60          }
  61      }
  62    else
  63      {
  64        while (A)
  65          {
  66            X;
  67            C;
  68          }
  69      }
  70
  71    Inv is considered invariant iff the values it compares are both invariant;
  72    tree-ssa-loop-im.c ensures that all the suitable conditions are in this
  73    shape.  */
  74
  75 static struct loop *tree_unswitch_loop (struct loop *, basic_block, tree);
  76 static bool tree_unswitch_single_loop (struct loop *, int);
  77 static tree tree_may_unswitch_on (basic_block, struct loop *);
  78
  79 /* Main entry point.  Perform loop unswitching on all suitable loops.  */
  80
  81 unsigned int
  82 tree_ssa_unswitch_loops (void)
  83 {
  84   loop_iterator li;
  85   struct loop *loop;
  86   bool changed = false;
  87
  88   /* Go through inner loops (only original ones).  */
  89   FOR_EACH_LOOP (li, loop, LI_ONLY_INNERMOST)
  90     {
  91       changed |= tree_unswitch_single_loop (loop, 0);
  92     }
  93
  94   if (changed)
  95     return TODO_cleanup_cfg;
  96   return 0;
  97 }
  98
  99 /* Checks whether we can unswitch LOOP on condition at end of BB -- one of its
 100    basic blocks (for what it means see comments below).  */
 101
 102 static tree
 103 tree_may_unswitch_on (basic_block bb, struct loop *loop)
 104 {
 105   gimple stmt, def;
 106   tree cond, use;
 107   basic_block def_bb;
 108   ssa_op_iter iter;
 109
 110   /* BB must end in a simple conditional jump.  */
 111   stmt = last_stmt (bb);
 112   if (!stmt || gimple_code (stmt) != GIMPLE_COND)
 113     return NULL_TREE;
 114
 115   /* Condition must be invariant.  */
 116   FOR_EACH_SSA_TREE_OPERAND (use, stmt, iter, SSA_OP_USE)
 117     {
 118       def = SSA_NAME_DEF_STMT (use);
 119       def_bb = gimple_bb (def);
 120       if (def_bb
 121           && flow_bb_inside_loop_p (loop, def_bb))
 122         return NULL_TREE;
 123     }
 124
 125   cond = build2 (gimple_cond_code (stmt), boolean_type_node,
 126                  gimple_cond_lhs (stmt), gimple_cond_rhs (stmt));
 127
 128   /* To keep the things simple, we do not directly remove the conditions,
 129      but just replace tests with 0/1.  Prevent the infinite loop where we
 130      would unswitch again on such a condition.  */
 131   if (integer_zerop (cond) || integer_nonzerop (cond))
 132     return NULL_TREE;
 133
 134   return cond;
 135 }
 136
 137 /* Simplifies COND using checks in front of the entry of the LOOP.  Just very
 138    simplish (sufficient to prevent us from duplicating loop in unswitching
 139    unnecessarily).  */
 140
 141 static tree
 142 simplify_using_entry_checks (struct loop *loop, tree cond)
 143 {
 144   edge e = loop_preheader_edge (loop);
 145   gimple stmt;
 146
 147   while (1)
 148     {
 149       stmt = last_stmt (e->src);
 150       if (stmt
 151           && gimple_code (stmt) == GIMPLE_COND
 152           && gimple_cond_code (stmt) == TREE_CODE (cond)
 153           && operand_equal_p (gimple_cond_lhs (stmt),
 154                               TREE_OPERAND (cond, 0), 0)
 155           && operand_equal_p (gimple_cond_rhs (stmt),
 156                               TREE_OPERAND (cond, 1), 0))
 157         return (e->flags & EDGE_TRUE_VALUE
 158                 ? boolean_true_node
 159                 : boolean_false_node);
 160
 161       if (!single_pred_p (e->src))
 162         return cond;
 163
 164       e = single_pred_edge (e->src);
 165       if (e->src == ENTRY_BLOCK_PTR)
 166         return cond;
 167     }
 168 }
 169
 170 /* Unswitch single LOOP.  NUM is number of unswitchings done; we do not allow
 171    it to grow too much, it is too easy to create example on that the code would
 172    grow exponentially.  */
 173
 174 static bool
 175 tree_unswitch_single_loop (struct loop *loop, int num)
 176 {
 177   basic_block *bbs;
 178   struct loop *nloop;
 179   unsigned i;
 180   tree cond = NULL_TREE;
 181   gimple stmt;
 182   bool changed = false;
 183
 184   /* Do not unswitch too much.  */
 185   if (num > PARAM_VALUE (PARAM_MAX_UNSWITCH_LEVEL))
 186     {
 187       if (dump_file && (dump_flags & TDF_DETAILS))
 188         fprintf (dump_file, ";; Not unswitching anymore, hit max level\n");
 189       return false;
 190     }
 191
 192   /* Only unswitch innermost loops.  */
 193   if (loop->inner)
 194     {
 195       if (dump_file && (dump_flags & TDF_DETAILS))
 196         fprintf (dump_file, ";; Not unswitching, not innermost loop\n");
 197       return false;
 198     }
 199
 200   /* Do not unswitch in cold regions.  */
 201   if (optimize_loop_for_size_p (loop))
 202     {
 203       if (dump_file && (dump_flags & TDF_DETAILS))
 204         fprintf (dump_file, ";; Not unswitching cold loops\n");
 205       return false;
 206     }
 207
 208   /* The loop should not be too large, to limit code growth.  */
 209   if (tree_num_loop_insns (loop, &eni_size_weights)
 210       > (unsigned) PARAM_VALUE (PARAM_MAX_UNSWITCH_INSNS))
 211     {
 212       if (dump_file && (dump_flags & TDF_DETAILS))
 213         fprintf (dump_file, ";; Not unswitching, loop too big\n");
 214       return false;
 215     }
 216
 217   i = 0;
 218   bbs = get_loop_body (loop);
 219
 220   while (1)
 221     {
 222       /* Find a bb to unswitch on.  */
 223       for (; i < loop->num_nodes; i++)
 224         if ((cond = tree_may_unswitch_on (bbs[i], loop)))
 225           break;
 226
 227       if (i == loop->num_nodes)
 228         {
 229           free (bbs);
 230           return changed;
 231         }
 232
 233       cond = simplify_using_entry_checks (loop, cond);
 234       stmt = last_stmt (bbs[i]);
 235       if (integer_nonzerop (cond))
 236         {
 237           /* Remove false path.  */
 238           gimple_cond_set_condition_from_tree (stmt, boolean_true_node);
 239           changed = true;
 240         }
 241       else if (integer_zerop (cond))
 242         {
 243           /* Remove true path.  */
 244           gimple_cond_set_condition_from_tree (stmt, boolean_false_node);
 245           changed = true;
 246         }
 247       else
 248         break;
 249
 250       update_stmt (stmt);
 251       i++;
 252     }
 253
 254   if (dump_file && (dump_flags & TDF_DETAILS))
 255     fprintf (dump_file, ";; Unswitching loop\n");
 256
 257   initialize_original_copy_tables ();
 258   /* Unswitch the loop on this condition.  */
 259   nloop = tree_unswitch_loop (loop, bbs[i], cond);
 260   if (!nloop)
 261     {
 262       free_original_copy_tables ();
 263       free (bbs);
 264       return changed;
 265     }
 266
 267   /* Update the SSA form after unswitching.  */
 268   update_ssa (TODO_update_ssa);
 269   free_original_copy_tables ();
 270
 271   /* Invoke itself on modified loops.  */
 272   tree_unswitch_single_loop (nloop, num + 1);
 273   tree_unswitch_single_loop (loop, num + 1);
 274   free (bbs);
 275   return true;
 276 }
 277
 278 /* Unswitch a LOOP w.r. to given basic block UNSWITCH_ON.  We only support
 279    unswitching of innermost loops.  COND is the condition determining which
 280    loop is entered -- the new loop is entered if COND is true.  Returns NULL
 281    if impossible, new loop otherwise.  */
 282
 283 static struct loop *
 284 tree_unswitch_loop (struct loop *loop,
 285                     basic_block unswitch_on, tree cond)
 286 {
 287   unsigned prob_true;
 288   edge edge_true, edge_false;
 289
 290   /* Some sanity checking.  */
 291   gcc_assert (flow_bb_inside_loop_p (loop, unswitch_on));
 292   gcc_assert (EDGE_COUNT (unswitch_on->succs) == 2);
 293   gcc_assert (loop->inner == NULL);
 294
 295   extract_true_false_edges_from_block (unswitch_on, &edge_true, &edge_false);
 296   prob_true = edge_true->probability;
 297   return loop_version (loop, unshare_expr (cond),
 298                        NULL, prob_true, prob_true,
 299                        REG_BR_PROB_BASE - prob_true, false);
 300 }