1/* Loop unswitching.
2   Copyright (C) 2004, 2005 Free Software Foundation, Inc.
3
4This file is part of GCC.
5
6GCC is free software; you can redistribute it and/or modify it
7under the terms of the GNU General Public License as published by the
8Free Software Foundation; either version 2, or (at your option) any
9later version.
10
11GCC is distributed in the hope that it will be useful, but WITHOUT
12ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
13FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
14for more details.
15
16You should have received a copy of the GNU General Public License
17along with GCC; see the file COPYING.  If not, write to the Free
18Software Foundation, 51 Franklin Street, Fifth Floor, Boston, MA
1902110-1301, 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
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
76static struct loop *tree_unswitch_loop (struct loops *, struct loop *, basic_block,
77				   tree);
78static bool tree_unswitch_single_loop (struct loops *, struct loop *, int);
79static tree tree_may_unswitch_on (basic_block, struct loop *);
80
81/* Main entry point.  Perform loop unswitching on all suitable LOOPS.  */
82
83void
84tree_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    }
105
106  if (changed)
107    cleanup_tree_cfg_loop ();
108}
109
110/* Checks whether we can unswitch LOOP on condition at end of BB -- one of its
111   basic blocks (for what it means see comments below).  */
112
113static tree
114tree_may_unswitch_on (basic_block bb, struct loop *loop)
115{
116  tree stmt, def, cond, use;
117  basic_block def_bb;
118  ssa_op_iter iter;
119
120  /* BB must end in a simple conditional jump.  */
121  stmt = last_stmt (bb);
122  if (!stmt || TREE_CODE (stmt) != COND_EXPR)
123    return NULL_TREE;
124
125  /* Condition must be invariant.  */
126  FOR_EACH_SSA_TREE_OPERAND (use, stmt, iter, SSA_OP_USE)
127    {
128      def = SSA_NAME_DEF_STMT (use);
129      def_bb = bb_for_stmt (def);
130      if (def_bb
131	  && flow_bb_inside_loop_p (loop, def_bb))
132	return NULL_TREE;
133    }
134
135  cond = COND_EXPR_COND (stmt);
136  /* To keep the things simple, we do not directly remove the conditions,
137     but just replace tests with 0/1.  Prevent the infinite loop where we
138     would unswitch again on such a condition.  */
139  if (integer_zerop (cond) || integer_nonzerop (cond))
140    return NULL_TREE;
141
142  return cond;
143}
144
145/* Simplifies COND using checks in front of the entry of the LOOP.  Just very
146   simplish (sufficient to prevent us from duplicating loop in unswitching
147   unnecessarily).  */
148
149static tree
150simplify_using_entry_checks (struct loop *loop, tree cond)
151{
152  edge e = loop_preheader_edge (loop);
153  tree stmt;
154
155  while (1)
156    {
157      stmt = last_stmt (e->src);
158      if (stmt
159	  && TREE_CODE (stmt) == COND_EXPR
160	  && operand_equal_p (COND_EXPR_COND (stmt), cond, 0))
161	return (e->flags & EDGE_TRUE_VALUE
162		? boolean_true_node
163		: boolean_false_node);
164
165      if (!single_pred_p (e->src))
166	return cond;
167
168      e = single_pred_edge (e->src);
169      if (e->src == ENTRY_BLOCK_PTR)
170	return cond;
171    }
172}
173
174/* Unswitch single LOOP.  NUM is number of unswitchings done; we do not allow
175   it to grow too much, it is too easy to create example on that the code would
176   grow exponentially.  */
177
178static bool
179tree_unswitch_single_loop (struct loops *loops, struct loop *loop, int num)
180{
181  basic_block *bbs;
182  struct loop *nloop;
183  unsigned i;
184  tree cond = NULL_TREE, stmt;
185  bool changed = false;
186
187  /* Do not unswitch too much.  */
188  if (num > PARAM_VALUE (PARAM_MAX_UNSWITCH_LEVEL))
189    {
190      if (dump_file && (dump_flags & TDF_DETAILS))
191	fprintf (dump_file, ";; Not unswitching anymore, hit max level\n");
192      return false;
193    }
194
195  /* Only unswitch innermost loops.  */
196  if (loop->inner)
197    {
198      if (dump_file && (dump_flags & TDF_DETAILS))
199	fprintf (dump_file, ";; Not unswitching, not innermost loop\n");
200      return false;
201    }
202
203  /* The loop should not be too large, to limit code growth.  */
204  if (tree_num_loop_insns (loop)
205      > (unsigned) PARAM_VALUE (PARAM_MAX_UNSWITCH_INSNS))
206    {
207      if (dump_file && (dump_flags & TDF_DETAILS))
208	fprintf (dump_file, ";; Not unswitching, loop too big\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	  COND_EXPR_COND (stmt) = boolean_true_node;
234	  changed = true;
235	}
236      else if (integer_zerop (cond))
237	{
238	  /* Remove true path.  */
239	  COND_EXPR_COND (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 (loops, loop, bbs[i], cond);
255  if (!nloop)
256    {
257      free_original_copy_tables ();
258      return changed;
259    }
260
261  /* Update the SSA form after unswitching.  */
262  update_ssa (TODO_update_ssa);
263  free_original_copy_tables ();
264
265  /* Invoke itself on modified loops.  */
266  tree_unswitch_single_loop (loops, nloop, num + 1);
267  tree_unswitch_single_loop (loops, loop, num + 1);
268  return true;
269}
270
271/* Unswitch a LOOP w.r. to given basic block UNSWITCH_ON.  We only support
272   unswitching of innermost loops.  COND is the condition determining which
273   loop is entered -- the new loop is entered if COND is true.  Returns NULL
274   if impossible, new loop otherwise.  */
275
276static struct loop *
277tree_unswitch_loop (struct loops *loops, struct loop *loop,
278		    basic_block unswitch_on, tree cond)
279{
280  basic_block condition_bb;
281
282  /* Some sanity checking.  */
283  gcc_assert (flow_bb_inside_loop_p (loop, unswitch_on));
284  gcc_assert (EDGE_COUNT (unswitch_on->succs) == 2);
285  gcc_assert (loop->inner == NULL);
286
287  return loop_version (loops, loop, unshare_expr (cond),
288		       &condition_bb, false);
289}
290