1 /* Detect paths through the CFG which can never be executed in a conforming
2 program and isolate them.
5 Free Software Foundation, Inc.
7 This file is part of GCC.
9 GCC is free software; you can redistribute it and/or modify
10 it under the terms of the GNU General Public License as published by
11 the Free Software Foundation; either version 3, or (at your option)
14 GCC is distributed in the hope that it will be useful,
15 but WITHOUT ANY WARRANTY; without even the implied warranty of
16 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
17 GNU General Public License for more details.
19 You should have received a copy of the GNU General Public License
20 along with GCC; see the file COPYING3. If not see
21 <http://www.gnu.org/licenses/>. */
25 #include "coretypes.h"
28 #include "basic-block.h"
30 #include "gimple-iterator.h"
31 #include "gimple-walk.h"
33 #include "tree-ssanames.h"
34 #include "gimple-ssa.h"
35 #include "tree-ssa-operands.h"
36 #include "tree-phinodes.h"
37 #include "ssa-iterators.h"
39 #include "tree-pass.h"
42 static bool cfg_altered
;
44 /* Callback for walk_stmt_load_store_ops.
46 Return TRUE if OP will dereference the tree stored in DATA, FALSE
49 This routine only makes a superficial check for a dereference. Thus,
50 it must only be used if it is safe to return a false negative. */
52 check_loadstore (gimple stmt ATTRIBUTE_UNUSED
, tree op
, void *data
)
54 if ((TREE_CODE (op
) == MEM_REF
|| TREE_CODE (op
) == TARGET_MEM_REF
)
55 && operand_equal_p (TREE_OPERAND (op
, 0), (tree
)data
, 0))
57 TREE_THIS_VOLATILE (op
) = 1;
58 TREE_SIDE_EFFECTS (op
) = 1;
65 /* Insert a trap after SI and remove SI and all statements after the trap. */
68 insert_trap_and_remove_trailing_statements (gimple_stmt_iterator
*si_p
, tree op
)
70 /* We want the NULL pointer dereference to actually occur so that
71 code that wishes to catch the signal can do so.
73 If the dereference is a load, then there's nothing to do as the
74 LHS will be a throw-away SSA_NAME and the RHS is the NULL dereference.
76 If the dereference is a store and we can easily transform the RHS,
77 then simplify the RHS to enable more DCE. Note that we require the
78 statement to be a GIMPLE_ASSIGN which filters out calls on the RHS. */
79 gimple stmt
= gsi_stmt (*si_p
);
80 if (walk_stmt_load_store_ops (stmt
, (void *)op
, NULL
, check_loadstore
)
81 && is_gimple_assign (stmt
)
82 && INTEGRAL_TYPE_P (TREE_TYPE (gimple_assign_lhs (stmt
))))
84 /* We just need to turn the RHS into zero converted to the proper
86 tree type
= TREE_TYPE (gimple_assign_lhs (stmt
));
87 gimple_assign_set_rhs_code (stmt
, INTEGER_CST
);
88 gimple_assign_set_rhs1 (stmt
, fold_convert (type
, integer_zero_node
));
93 = gimple_build_call (builtin_decl_explicit (BUILT_IN_TRAP
), 0);
94 gimple_seq seq
= NULL
;
95 gimple_seq_add_stmt (&seq
, new_stmt
);
97 /* If we had a NULL pointer dereference, then we want to insert the
98 __builtin_trap after the statement, for the other cases we want
99 to insert before the statement. */
100 if (walk_stmt_load_store_ops (stmt
, (void *)op
,
103 gsi_insert_after (si_p
, seq
, GSI_NEW_STMT
);
105 gsi_insert_before (si_p
, seq
, GSI_NEW_STMT
);
107 /* We must remove statements from the end of the block so that we
108 never reference a released SSA_NAME. */
109 basic_block bb
= gimple_bb (gsi_stmt (*si_p
));
110 for (gimple_stmt_iterator si
= gsi_last_bb (bb
);
111 gsi_stmt (si
) != gsi_stmt (*si_p
);
112 si
= gsi_last_bb (bb
))
114 stmt
= gsi_stmt (si
);
115 unlink_stmt_vdef (stmt
);
116 gsi_remove (&si
, true);
121 /* BB when reached via incoming edge E will exhibit undefined behaviour
122 at STMT. Isolate and optimize the path which exhibits undefined
125 Isolation is simple. Duplicate BB and redirect E to BB'.
127 Optimization is simple as well. Replace STMT in BB' with an
128 unconditional trap and remove all outgoing edges from BB'.
130 DUPLICATE is a pre-existing duplicate, use it as BB' if it exists.
135 isolate_path (basic_block bb
, basic_block duplicate
,
136 edge e
, gimple stmt
, tree op
)
138 gimple_stmt_iterator si
, si2
;
143 /* First duplicate BB if we have not done so already and remove all
144 the duplicate's outgoing edges as duplicate is going to unconditionally
145 trap. Removing the outgoing edges is both an optimization and ensures
146 we don't need to do any PHI node updates. */
149 duplicate
= duplicate_block (bb
, NULL
, NULL
);
150 for (ei
= ei_start (duplicate
->succs
); (e2
= ei_safe_edge (ei
)); )
154 /* Complete the isolation step by redirecting E to reach DUPLICATE. */
155 e2
= redirect_edge_and_branch (e
, duplicate
);
157 flush_pending_stmts (e2
);
160 /* There may be more than one statement in DUPLICATE which exhibits
161 undefined behaviour. Ultimately we want the first such statement in
162 DUPLCIATE so that we're able to delete as much code as possible.
164 So each time we discover undefined behaviour in DUPLICATE, search for
165 the statement which triggers undefined behaviour. If found, then
166 transform the statement into a trap and delete everything after the
167 statement. If not found, then this particular instance was subsumed by
168 an earlier instance of undefined behaviour and there's nothing to do.
170 This is made more complicated by the fact that we have STMT, which is in
171 BB rather than in DUPLICATE. So we set up two iterators, one for each
172 block and walk forward looking for STMT in BB, advancing each iterator at
175 When we find STMT the second iterator should point to STMT's equivalent in
176 duplicate. If DUPLICATE ends before STMT is found in BB, then there's
179 Ignore labels and debug statements. */
180 si
= gsi_start_nondebug_after_labels_bb (bb
);
181 si2
= gsi_start_nondebug_after_labels_bb (duplicate
);
182 while (!gsi_end_p (si
) && !gsi_end_p (si2
) && gsi_stmt (si
) != stmt
)
184 gsi_next_nondebug (&si
);
185 gsi_next_nondebug (&si2
);
188 /* This would be an indicator that we never found STMT in BB, which should
190 gcc_assert (!gsi_end_p (si
));
192 /* If we did not run to the end of DUPLICATE, then SI points to STMT and
193 SI2 points to the duplicate of STMT in DUPLICATE. Insert a trap
194 before SI2 and remove SI2 and all trailing statements. */
195 if (!gsi_end_p (si2
))
196 insert_trap_and_remove_trailing_statements (&si2
, op
);
201 /* Look for PHI nodes which feed statements in the same block where
202 the value of the PHI node implies the statement is erroneous.
204 For example, a NULL PHI arg value which then feeds a pointer
207 When found isolate and optimize the path associated with the PHI
208 argument feeding the erroneous statement. */
210 find_implicit_erroneous_behaviour (void)
216 gimple_stmt_iterator si
;
218 /* First look for a PHI which sets a pointer to NULL and which
219 is then dereferenced within BB. This is somewhat overly
220 conservative, but probably catches most of the interesting
222 for (si
= gsi_start_phis (bb
); !gsi_end_p (si
); gsi_next (&si
))
224 gimple phi
= gsi_stmt (si
);
225 tree lhs
= gimple_phi_result (phi
);
227 /* If the result is not a pointer, then there is no need to
228 examine the arguments. */
229 if (!POINTER_TYPE_P (TREE_TYPE (lhs
)))
232 /* PHI produces a pointer result. See if any of the PHI's
235 When we remove an edge, we want to reprocess the current
236 index, hence the ugly way we update I for each iteration. */
237 basic_block duplicate
= NULL
;
238 for (unsigned i
= 0, next_i
= 0;
239 i
< gimple_phi_num_args (phi
);
242 tree op
= gimple_phi_arg_def (phi
, i
);
246 if (!integer_zerop (op
))
249 edge e
= gimple_phi_arg_edge (phi
, i
);
250 imm_use_iterator iter
;
253 /* We've got a NULL PHI argument. Now see if the
254 PHI's result is dereferenced within BB. */
255 FOR_EACH_IMM_USE_STMT (use_stmt
, iter
, lhs
)
257 /* We only care about uses in BB. Catching cases in
258 in other blocks would require more complex path
260 if (gimple_bb (use_stmt
) != bb
)
263 if (infer_nonnull_range (use_stmt
, lhs
))
265 duplicate
= isolate_path (bb
, duplicate
,
268 /* When we remove an incoming edge, we need to
269 reprocess the Ith element. */
279 /* Look for statements which exhibit erroneous behaviour. For example
280 a NULL pointer dereference.
282 When found, optimize the block containing the erroneous behaviour. */
284 find_explicit_erroneous_behaviour (void)
290 gimple_stmt_iterator si
;
292 /* Now look at the statements in the block and see if any of
293 them explicitly dereference a NULL pointer. This happens
294 because of jump threading and constant propagation. */
295 for (si
= gsi_start_bb (bb
); !gsi_end_p (si
); gsi_next (&si
))
297 gimple stmt
= gsi_stmt (si
);
299 /* By passing null_pointer_node, we can use infer_nonnull_range
300 to detect explicit NULL pointer dereferences and other uses
301 where a non-NULL value is required. */
302 if (infer_nonnull_range (stmt
, null_pointer_node
))
304 insert_trap_and_remove_trailing_statements (&si
,
307 /* And finally, remove all outgoing edges from BB. */
309 for (edge_iterator ei
= ei_start (bb
->succs
);
310 (e
= ei_safe_edge (ei
)); )
313 /* Ignore any more operands on this statement and
314 continue the statement iterator (which should
315 terminate its loop immediately. */
322 /* Search the function for statements which, if executed, would cause
323 the program to fault such as a dereference of a NULL pointer.
325 Such a program can't be valid if such a statement was to execute
326 according to ISO standards.
328 We detect explicit NULL pointer dereferences as well as those implied
329 by a PHI argument having a NULL value which unconditionally flows into
330 a dereference in the same block as the PHI.
332 In the former case we replace the offending statement with an
333 unconditional trap and eliminate the outgoing edges from the statement's
334 basic block. This may expose secondary optimization opportunities.
336 In the latter case, we isolate the path(s) with the NULL PHI
337 feeding the dereference. We can then replace the offending statement
338 and eliminate the outgoing edges in the duplicate. Again, this may
339 expose secondary optimization opportunities.
341 A warning for both cases may be advisable as well.
343 Other statically detectable violations of the ISO standard could be
344 handled in a similar way, such as out-of-bounds array indexing. */
347 gimple_ssa_isolate_erroneous_paths (void)
349 initialize_original_copy_tables ();
351 /* Search all the blocks for edges which, if traversed, will
352 result in undefined behaviour. */
355 /* First handle cases where traversal of a particular edge
356 triggers undefined behaviour. These cases require creating
357 duplicate blocks and thus new SSA_NAMEs.
359 We want that process complete prior to the phase where we start
360 removing edges from the CFG. Edge removal may ultimately result in
361 removal of PHI nodes and thus releasing SSA_NAMEs back to the
364 If the two processes run in parallel we could release an SSA_NAME
365 back to the manager but we could still have dangling references
366 to the released SSA_NAME in unreachable blocks.
367 that any released names not have dangling references in the IL. */
368 find_implicit_erroneous_behaviour ();
369 find_explicit_erroneous_behaviour ();
371 free_original_copy_tables ();
373 /* We scramble the CFG and loop structures a bit, clean up
374 appropriately. We really should incrementally update the
375 loop structures, in theory it shouldn't be that hard. */
378 free_dominance_info (CDI_DOMINATORS
);
379 free_dominance_info (CDI_POST_DOMINATORS
);
380 loops_state_set (LOOPS_NEED_FIXUP
);
381 return TODO_cleanup_cfg
| TODO_update_ssa
;
387 gate_isolate_erroneous_paths (void)
389 /* If we do not have a suitable builtin function for the trap statement,
390 then do not perform the optimization. */
391 return (flag_isolate_erroneous_paths
!= 0);
395 const pass_data pass_data_isolate_erroneous_paths
=
397 GIMPLE_PASS
, /* type */
398 "isolate-paths", /* name */
399 OPTGROUP_NONE
, /* optinfo_flags */
401 true, /* has_execute */
402 TV_ISOLATE_ERRONEOUS_PATHS
, /* tv_id */
403 ( PROP_cfg
| PROP_ssa
), /* properties_required */
404 0, /* properties_provided */
405 0, /* properties_destroyed */
406 0, /* todo_flags_start */
407 TODO_verify_ssa
, /* todo_flags_finish */
410 class pass_isolate_erroneous_paths
: public gimple_opt_pass
413 pass_isolate_erroneous_paths (gcc::context
*ctxt
)
414 : gimple_opt_pass (pass_data_isolate_erroneous_paths
, ctxt
)
417 /* opt_pass methods: */
418 opt_pass
* clone () { return new pass_isolate_erroneous_paths (m_ctxt
); }
419 bool gate () { return gate_isolate_erroneous_paths (); }
420 unsigned int execute () { return gimple_ssa_isolate_erroneous_paths (); }
422 }; // class pass_isolate_erroneous_paths
426 make_pass_isolate_erroneous_paths (gcc::context
*ctxt
)
428 return new pass_isolate_erroneous_paths (ctxt
);