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       if (dump_file && (dump_flags & TDF_DETAILS))
  92         fprintf (dump_file, ";; Considering loop %d\n", loop->num);
  93
  94       /* Do not unswitch in cold regions. */
  95       if (optimize_loop_for_size_p (loop))
  96         {
  97           if (dump_file && (dump_flags & TDF_DETAILS))
  98             fprintf (dump_file, ";; Not unswitching cold loops\n");
  99           continue;
 100         }
 101
 102       /* The loop should not be too large, to limit code growth. */
 103       if (tree_num_loop_insns (loop, &eni_size_weights)
 104           > (unsigned) PARAM_VALUE (PARAM_MAX_UNSWITCH_INSNS))
 105         {
 106           if (dump_file && (dump_flags & TDF_DETAILS))
 107             fprintf (dump_file, ";; Not unswitching, loop too big\n");
 108           continue;
 109         }
 110
 111       changed |= tree_unswitch_single_loop (loop, 0);
 112     }
 113
 114   if (changed)
 115     return TODO_cleanup_cfg;
 116   return 0;
 117 }
 118
 119 /* Checks whether we can unswitch LOOP on condition at end of BB -- one of its
 120    basic blocks (for what it means see comments below).  */
 121
 122 static tree
 123 tree_may_unswitch_on (basic_block bb, struct loop *loop)
 124 {
 125   gimple stmt, def;
 126   tree cond, use;
 127   basic_block def_bb;
 128   ssa_op_iter iter;
 129
 130   /* BB must end in a simple conditional jump.  */
 131   stmt = last_stmt (bb);
 132   if (!stmt || gimple_code (stmt) != GIMPLE_COND)
 133     return NULL_TREE;
 134
 135   /* Condition must be invariant.  */
 136   FOR_EACH_SSA_TREE_OPERAND (use, stmt, iter, SSA_OP_USE)
 137     {
 138       def = SSA_NAME_DEF_STMT (use);
 139       def_bb = gimple_bb (def);
 140       if (def_bb
 141           && flow_bb_inside_loop_p (loop, def_bb))
 142         return NULL_TREE;
 143     }
 144
 145   cond = build2 (gimple_cond_code (stmt), boolean_type_node,
 146                  gimple_cond_lhs (stmt), gimple_cond_rhs (stmt));
 147
 148   /* To keep the things simple, we do not directly remove the conditions,
 149      but just replace tests with 0/1.  Prevent the infinite loop where we
 150      would unswitch again on such a condition.  */
 151   if (integer_zerop (cond) || integer_nonzerop (cond))
 152     return NULL_TREE;
 153
 154   return cond;
 155 }
 156
 157 /* Simplifies COND using checks in front of the entry of the LOOP.  Just very
 158    simplish (sufficient to prevent us from duplicating loop in unswitching
 159    unnecessarily).  */
 160
 161 static tree
 162 simplify_using_entry_checks (struct loop *loop, tree cond)
 163 {
 164   edge e = loop_preheader_edge (loop);
 165   gimple stmt;
 166
 167   while (1)
 168     {
 169       stmt = last_stmt (e->src);
 170       if (stmt
 171           && gimple_code (stmt) == GIMPLE_COND
 172           && gimple_cond_code (stmt) == TREE_CODE (cond)
 173           && operand_equal_p (gimple_cond_lhs (stmt),
 174                               TREE_OPERAND (cond, 0), 0)
 175           && operand_equal_p (gimple_cond_rhs (stmt),
 176                               TREE_OPERAND (cond, 1), 0))
 177         return (e->flags & EDGE_TRUE_VALUE
 178                 ? boolean_true_node
 179                 : boolean_false_node);
 180
 181       if (!single_pred_p (e->src))
 182         return cond;
 183
 184       e = single_pred_edge (e->src);
 185       if (e->src == ENTRY_BLOCK_PTR)
 186         return cond;
 187     }
 188 }
 189
 190 /* Unswitch single LOOP.  NUM is number of unswitchings done; we do not allow
 191    it to grow too much, it is too easy to create example on that the code would
 192    grow exponentially.  */
 193
 194 static bool
 195 tree_unswitch_single_loop (struct loop *loop, int num)
 196 {
 197   basic_block *bbs;
 198   struct loop *nloop;
 199   unsigned i;
 200   tree cond = NULL_TREE;
 201   gimple stmt;
 202   bool changed = false;
 203
 204   /* Do not unswitch too much.  */
 205   if (num > PARAM_VALUE (PARAM_MAX_UNSWITCH_LEVEL))
 206     {
 207       if (dump_file && (dump_flags & TDF_DETAILS))
 208         fprintf (dump_file, ";; Not unswitching anymore, hit max level\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           gimple_cond_set_condition_from_tree (stmt, boolean_true_node);
 234           changed = true;
 235         }
 236       else if (integer_zerop (cond))
 237         {
 238           /* Remove true path.  */
 239           gimple_cond_set_condition_from_tree (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 (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 (nloop, num + 1);
 268   tree_unswitch_single_loop (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 loop *loop,
 280                     basic_block unswitch_on, tree cond)
 281 {
 282   unsigned prob_true;
 283   edge edge_true, edge_false;
 284
 285   /* Some sanity checking.  */
 286   gcc_assert (flow_bb_inside_loop_p (loop, unswitch_on));
 287   gcc_assert (EDGE_COUNT (unswitch_on->succs) == 2);
 288   gcc_assert (loop->inner == NULL);
 289
 290   extract_true_false_edges_from_block (unswitch_on, &edge_true, &edge_false);
 291   prob_true = edge_true->probability;
 292   return loop_version (loop, unshare_expr (cond),
 293                        NULL, prob_true, prob_true,
 294                        REG_BR_PROB_BASE - prob_true, false);
 295 }