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"
31 #include "tree-ssanames.h"
32 #include "gimple-ssa.h"
33 #include "tree-ssa-operands.h"
34 #include "tree-phinodes.h"
35 #include "ssa-iterators.h"
37 #include "tree-pass.h"
40 static bool cfg_altered
;
42 /* Callback for walk_stmt_load_store_ops.
44 Return TRUE if OP will dereference the tree stored in DATA, FALSE
47 This routine only makes a superficial check for a dereference. Thus,
48 it must only be used if it is safe to return a false negative. */
50 check_loadstore (gimple stmt ATTRIBUTE_UNUSED
, tree op
, void *data
)
52 if ((TREE_CODE (op
) == MEM_REF
|| TREE_CODE (op
) == TARGET_MEM_REF
)
53 && operand_equal_p (TREE_OPERAND (op
, 0), (tree
)data
, 0))
55 TREE_THIS_VOLATILE (op
) = 1;
56 TREE_SIDE_EFFECTS (op
) = 1;
63 /* Insert a trap after SI and remove SI and all statements after the trap. */
66 insert_trap_and_remove_trailing_statements (gimple_stmt_iterator
*si_p
, tree op
)
68 /* We want the NULL pointer dereference to actually occur so that
69 code that wishes to catch the signal can do so.
71 If the dereference is a load, then there's nothing to do as the
72 LHS will be a throw-away SSA_NAME and the RHS is the NULL dereference.
74 If the dereference is a store and we can easily transform the RHS,
75 then simplify the RHS to enable more DCE. */
76 gimple stmt
= gsi_stmt (*si_p
);
77 if (walk_stmt_load_store_ops (stmt
, (void *)op
, NULL
, check_loadstore
)
78 && INTEGRAL_TYPE_P (TREE_TYPE (gimple_assign_lhs (stmt
))))
80 /* We just need to turn the RHS into zero converted to the proper
82 tree type
= TREE_TYPE (gimple_assign_lhs (stmt
));
83 gimple_assign_set_rhs_code (stmt
, INTEGER_CST
);
84 gimple_assign_set_rhs1 (stmt
, fold_convert (type
, integer_zero_node
));
89 = gimple_build_call (builtin_decl_explicit (BUILT_IN_TRAP
), 0);
90 gimple_seq seq
= NULL
;
91 gimple_seq_add_stmt (&seq
, new_stmt
);
93 /* If we had a NULL pointer dereference, then we want to insert the
94 __builtin_trap after the statement, for the other cases we want
95 to insert before the statement. */
96 if (walk_stmt_load_store_ops (stmt
, (void *)op
,
99 gsi_insert_after (si_p
, seq
, GSI_NEW_STMT
);
101 gsi_insert_before (si_p
, seq
, GSI_NEW_STMT
);
103 /* The iterator points to the __builtin_trap. Advance the iterator
104 and delete everything else in the block. */
106 for (; !gsi_end_p (*si_p
);)
108 stmt
= gsi_stmt (*si_p
);
109 unlink_stmt_vdef (stmt
);
110 gsi_remove (si_p
, true);
115 /* BB when reached via incoming edge E will exhibit undefined behaviour
116 at STMT. Isolate and optimize the path which exhibits undefined
119 Isolation is simple. Duplicate BB and redirect E to BB'.
121 Optimization is simple as well. Replace STMT in BB' with an
122 unconditional trap and remove all outgoing edges from BB'.
124 DUPLICATE is a pre-existing duplicate, use it as BB' if it exists.
129 isolate_path (basic_block bb
, basic_block duplicate
,
130 edge e
, gimple stmt
, tree op
)
132 gimple_stmt_iterator si
, si2
;
137 /* First duplicate BB if we have not done so already and remove all
138 the duplicate's outgoing edges as duplicate is going to unconditionally
139 trap. Removing the outgoing edges is both an optimization and ensures
140 we don't need to do any PHI node updates. */
143 duplicate
= duplicate_block (bb
, NULL
, NULL
);
144 for (ei
= ei_start (duplicate
->succs
); (e2
= ei_safe_edge (ei
)); )
148 /* Complete the isolation step by redirecting E to reach DUPLICATE. */
149 e2
= redirect_edge_and_branch (e
, duplicate
);
151 flush_pending_stmts (e2
);
154 /* There may be more than one statement in DUPLICATE which exhibits
155 undefined behaviour. Ultimately we want the first such statement in
156 DUPLCIATE so that we're able to delete as much code as possible.
158 So each time we discover undefined behaviour in DUPLICATE, search for
159 the statement which triggers undefined behaviour. If found, then
160 transform the statement into a trap and delete everything after the
161 statement. If not found, then this particular instance was subsumed by
162 an earlier instance of undefined behaviour and there's nothing to do.
164 This is made more complicated by the fact that we have STMT, which is in
165 BB rather than in DUPLICATE. So we set up two iterators, one for each
166 block and walk forward looking for STMT in BB, advancing each iterator at
169 When we find STMT the second iterator should point to STMT's equivalent in
170 duplicate. If DUPLICATE ends before STMT is found in BB, then there's
173 Ignore labels and debug statements. */
174 si
= gsi_start_nondebug_after_labels_bb (bb
);
175 si2
= gsi_start_nondebug_after_labels_bb (duplicate
);
176 while (!gsi_end_p (si
) && !gsi_end_p (si2
) && gsi_stmt (si
) != stmt
)
178 gsi_next_nondebug (&si
);
179 gsi_next_nondebug (&si2
);
182 /* This would be an indicator that we never found STMT in BB, which should
184 gcc_assert (!gsi_end_p (si
));
186 /* If we did not run to the end of DUPLICATE, then SI points to STMT and
187 SI2 points to the duplicate of STMT in DUPLICATE. Insert a trap
188 before SI2 and remove SI2 and all trailing statements. */
189 if (!gsi_end_p (si2
))
190 insert_trap_and_remove_trailing_statements (&si2
, op
);
195 /* Search the function for statements which, if executed, would cause
196 the program to fault such as a dereference of a NULL pointer.
198 Such a program can't be valid if such a statement was to execute
199 according to ISO standards.
201 We detect explicit NULL pointer dereferences as well as those implied
202 by a PHI argument having a NULL value which unconditionally flows into
203 a dereference in the same block as the PHI.
205 In the former case we replace the offending statement with an
206 unconditional trap and eliminate the outgoing edges from the statement's
207 basic block. This may expose secondary optimization opportunities.
209 In the latter case, we isolate the path(s) with the NULL PHI
210 feeding the dereference. We can then replace the offending statement
211 and eliminate the outgoing edges in the duplicate. Again, this may
212 expose secondary optimization opportunities.
214 A warning for both cases may be advisable as well.
216 Other statically detectable violations of the ISO standard could be
217 handled in a similar way, such as out-of-bounds array indexing. */
220 gimple_ssa_isolate_erroneous_paths (void)
224 initialize_original_copy_tables ();
226 /* Search all the blocks for edges which, if traversed, will
227 result in undefined behaviour. */
231 gimple_stmt_iterator si
;
233 /* First look for a PHI which sets a pointer to NULL and which
234 is then dereferenced within BB. This is somewhat overly
235 conservative, but probably catches most of the interesting
237 for (si
= gsi_start_phis (bb
); !gsi_end_p (si
); gsi_next (&si
))
239 gimple phi
= gsi_stmt (si
);
240 tree lhs
= gimple_phi_result (phi
);
242 /* If the result is not a pointer, then there is no need to
243 examine the arguments. */
244 if (!POINTER_TYPE_P (TREE_TYPE (lhs
)))
247 /* PHI produces a pointer result. See if any of the PHI's
250 When we remove an edge, we want to reprocess the current
251 index, hence the ugly way we update I for each iteration. */
252 basic_block duplicate
= NULL
;
253 for (unsigned i
= 0, next_i
= 0;
254 i
< gimple_phi_num_args (phi
);
257 tree op
= gimple_phi_arg_def (phi
, i
);
261 if (!integer_zerop (op
))
264 edge e
= gimple_phi_arg_edge (phi
, i
);
265 imm_use_iterator iter
;
268 /* We've got a NULL PHI argument. Now see if the
269 PHI's result is dereferenced within BB. */
270 FOR_EACH_IMM_USE_STMT (use_stmt
, iter
, lhs
)
272 /* We only care about uses in BB. Catching cases in
273 in other blocks would require more complex path
275 if (gimple_bb (use_stmt
) != bb
)
278 if (infer_nonnull_range (use_stmt
, lhs
))
280 duplicate
= isolate_path (bb
, duplicate
,
283 /* When we remove an incoming edge, we need to
284 reprocess the Ith element. */
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. */
321 free_original_copy_tables ();
323 /* We scramble the CFG and loop structures a bit, clean up
324 appropriately. We really should incrementally update the
325 loop structures, in theory it shouldn't be that hard. */
328 free_dominance_info (CDI_DOMINATORS
);
329 free_dominance_info (CDI_POST_DOMINATORS
);
330 loops_state_set (LOOPS_NEED_FIXUP
);
331 return TODO_cleanup_cfg
| TODO_update_ssa
;
337 gate_isolate_erroneous_paths (void)
339 /* If we do not have a suitable builtin function for the trap statement,
340 then do not perform the optimization. */
341 return (flag_isolate_erroneous_paths
!= 0);
345 const pass_data pass_data_isolate_erroneous_paths
=
347 GIMPLE_PASS
, /* type */
348 "isolate-paths", /* name */
349 OPTGROUP_NONE
, /* optinfo_flags */
351 true, /* has_execute */
352 TV_ISOLATE_ERRONEOUS_PATHS
, /* tv_id */
353 ( PROP_cfg
| PROP_ssa
), /* properties_required */
354 0, /* properties_provided */
355 0, /* properties_destroyed */
356 0, /* todo_flags_start */
357 TODO_verify_ssa
, /* todo_flags_finish */
360 class pass_isolate_erroneous_paths
: public gimple_opt_pass
363 pass_isolate_erroneous_paths (gcc::context
*ctxt
)
364 : gimple_opt_pass (pass_data_isolate_erroneous_paths
, ctxt
)
367 /* opt_pass methods: */
368 opt_pass
* clone () { return new pass_isolate_erroneous_paths (m_ctxt
); }
369 bool gate () { return gate_isolate_erroneous_paths (); }
370 unsigned int execute () { return gimple_ssa_isolate_erroneous_paths (); }
372 }; // class pass_uncprop
376 make_pass_isolate_erroneous_paths (gcc::context
*ctxt
)
378 return new pass_isolate_erroneous_paths (ctxt
);