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. */
78 gimple stmt
= gsi_stmt (*si_p
);
79 if (walk_stmt_load_store_ops (stmt
, (void *)op
, NULL
, check_loadstore
)
80 && INTEGRAL_TYPE_P (TREE_TYPE (gimple_assign_lhs (stmt
))))
82 /* We just need to turn the RHS into zero converted to the proper
84 tree type
= TREE_TYPE (gimple_assign_lhs (stmt
));
85 gimple_assign_set_rhs_code (stmt
, INTEGER_CST
);
86 gimple_assign_set_rhs1 (stmt
, fold_convert (type
, integer_zero_node
));
91 = gimple_build_call (builtin_decl_explicit (BUILT_IN_TRAP
), 0);
92 gimple_seq seq
= NULL
;
93 gimple_seq_add_stmt (&seq
, new_stmt
);
95 /* If we had a NULL pointer dereference, then we want to insert the
96 __builtin_trap after the statement, for the other cases we want
97 to insert before the statement. */
98 if (walk_stmt_load_store_ops (stmt
, (void *)op
,
101 gsi_insert_after (si_p
, seq
, GSI_NEW_STMT
);
103 gsi_insert_before (si_p
, seq
, GSI_NEW_STMT
);
105 /* We must remove statements from the end of the block so that we
106 never reference a released SSA_NAME. */
107 basic_block bb
= gimple_bb (gsi_stmt (*si_p
));
108 for (gimple_stmt_iterator si
= gsi_last_bb (bb
);
109 gsi_stmt (si
) != gsi_stmt (*si_p
);
110 si
= gsi_last_bb (bb
))
112 stmt
= gsi_stmt (si
);
113 unlink_stmt_vdef (stmt
);
114 gsi_remove (&si
, true);
119 /* BB when reached via incoming edge E will exhibit undefined behaviour
120 at STMT. Isolate and optimize the path which exhibits undefined
123 Isolation is simple. Duplicate BB and redirect E to BB'.
125 Optimization is simple as well. Replace STMT in BB' with an
126 unconditional trap and remove all outgoing edges from BB'.
128 DUPLICATE is a pre-existing duplicate, use it as BB' if it exists.
133 isolate_path (basic_block bb
, basic_block duplicate
,
134 edge e
, gimple stmt
, tree op
)
136 gimple_stmt_iterator si
, si2
;
141 /* First duplicate BB if we have not done so already and remove all
142 the duplicate's outgoing edges as duplicate is going to unconditionally
143 trap. Removing the outgoing edges is both an optimization and ensures
144 we don't need to do any PHI node updates. */
147 duplicate
= duplicate_block (bb
, NULL
, NULL
);
148 for (ei
= ei_start (duplicate
->succs
); (e2
= ei_safe_edge (ei
)); )
152 /* Complete the isolation step by redirecting E to reach DUPLICATE. */
153 e2
= redirect_edge_and_branch (e
, duplicate
);
155 flush_pending_stmts (e2
);
158 /* There may be more than one statement in DUPLICATE which exhibits
159 undefined behaviour. Ultimately we want the first such statement in
160 DUPLCIATE so that we're able to delete as much code as possible.
162 So each time we discover undefined behaviour in DUPLICATE, search for
163 the statement which triggers undefined behaviour. If found, then
164 transform the statement into a trap and delete everything after the
165 statement. If not found, then this particular instance was subsumed by
166 an earlier instance of undefined behaviour and there's nothing to do.
168 This is made more complicated by the fact that we have STMT, which is in
169 BB rather than in DUPLICATE. So we set up two iterators, one for each
170 block and walk forward looking for STMT in BB, advancing each iterator at
173 When we find STMT the second iterator should point to STMT's equivalent in
174 duplicate. If DUPLICATE ends before STMT is found in BB, then there's
177 Ignore labels and debug statements. */
178 si
= gsi_start_nondebug_after_labels_bb (bb
);
179 si2
= gsi_start_nondebug_after_labels_bb (duplicate
);
180 while (!gsi_end_p (si
) && !gsi_end_p (si2
) && gsi_stmt (si
) != stmt
)
182 gsi_next_nondebug (&si
);
183 gsi_next_nondebug (&si2
);
186 /* This would be an indicator that we never found STMT in BB, which should
188 gcc_assert (!gsi_end_p (si
));
190 /* If we did not run to the end of DUPLICATE, then SI points to STMT and
191 SI2 points to the duplicate of STMT in DUPLICATE. Insert a trap
192 before SI2 and remove SI2 and all trailing statements. */
193 if (!gsi_end_p (si2
))
194 insert_trap_and_remove_trailing_statements (&si2
, op
);
199 /* Look for PHI nodes which feed statements in the same block where
200 the value of the PHI node implies the statement is erroneous.
202 For example, a NULL PHI arg value which then feeds a pointer
205 When found isolate and optimize the path associated with the PHI
206 argument feeding the erroneous statement. */
208 find_implicit_erroneous_behaviour (void)
214 gimple_stmt_iterator si
;
216 /* First look for a PHI which sets a pointer to NULL and which
217 is then dereferenced within BB. This is somewhat overly
218 conservative, but probably catches most of the interesting
220 for (si
= gsi_start_phis (bb
); !gsi_end_p (si
); gsi_next (&si
))
222 gimple phi
= gsi_stmt (si
);
223 tree lhs
= gimple_phi_result (phi
);
225 /* If the result is not a pointer, then there is no need to
226 examine the arguments. */
227 if (!POINTER_TYPE_P (TREE_TYPE (lhs
)))
230 /* PHI produces a pointer result. See if any of the PHI's
233 When we remove an edge, we want to reprocess the current
234 index, hence the ugly way we update I for each iteration. */
235 basic_block duplicate
= NULL
;
236 for (unsigned i
= 0, next_i
= 0;
237 i
< gimple_phi_num_args (phi
);
240 tree op
= gimple_phi_arg_def (phi
, i
);
244 if (!integer_zerop (op
))
247 edge e
= gimple_phi_arg_edge (phi
, i
);
248 imm_use_iterator iter
;
251 /* We've got a NULL PHI argument. Now see if the
252 PHI's result is dereferenced within BB. */
253 FOR_EACH_IMM_USE_STMT (use_stmt
, iter
, lhs
)
255 /* We only care about uses in BB. Catching cases in
256 in other blocks would require more complex path
258 if (gimple_bb (use_stmt
) != bb
)
261 if (infer_nonnull_range (use_stmt
, lhs
))
263 duplicate
= isolate_path (bb
, duplicate
,
266 /* When we remove an incoming edge, we need to
267 reprocess the Ith element. */
277 /* Look for statements which exhibit erroneous behaviour. For example
278 a NULL pointer dereference.
280 When found, optimize the block containing the erroneous behaviour. */
282 find_explicit_erroneous_behaviour (void)
288 gimple_stmt_iterator si
;
290 /* Now look at the statements in the block and see if any of
291 them explicitly dereference a NULL pointer. This happens
292 because of jump threading and constant propagation. */
293 for (si
= gsi_start_bb (bb
); !gsi_end_p (si
); gsi_next (&si
))
295 gimple stmt
= gsi_stmt (si
);
297 /* By passing null_pointer_node, we can use infer_nonnull_range
298 to detect explicit NULL pointer dereferences and other uses
299 where a non-NULL value is required. */
300 if (infer_nonnull_range (stmt
, null_pointer_node
))
302 insert_trap_and_remove_trailing_statements (&si
,
305 /* And finally, remove all outgoing edges from BB. */
307 for (edge_iterator ei
= ei_start (bb
->succs
);
308 (e
= ei_safe_edge (ei
)); )
311 /* Ignore any more operands on this statement and
312 continue the statement iterator (which should
313 terminate its loop immediately. */
320 /* Search the function for statements which, if executed, would cause
321 the program to fault such as a dereference of a NULL pointer.
323 Such a program can't be valid if such a statement was to execute
324 according to ISO standards.
326 We detect explicit NULL pointer dereferences as well as those implied
327 by a PHI argument having a NULL value which unconditionally flows into
328 a dereference in the same block as the PHI.
330 In the former case we replace the offending statement with an
331 unconditional trap and eliminate the outgoing edges from the statement's
332 basic block. This may expose secondary optimization opportunities.
334 In the latter case, we isolate the path(s) with the NULL PHI
335 feeding the dereference. We can then replace the offending statement
336 and eliminate the outgoing edges in the duplicate. Again, this may
337 expose secondary optimization opportunities.
339 A warning for both cases may be advisable as well.
341 Other statically detectable violations of the ISO standard could be
342 handled in a similar way, such as out-of-bounds array indexing. */
345 gimple_ssa_isolate_erroneous_paths (void)
347 initialize_original_copy_tables ();
349 /* Search all the blocks for edges which, if traversed, will
350 result in undefined behaviour. */
353 /* First handle cases where traversal of a particular edge
354 triggers undefined behaviour. These cases require creating
355 duplicate blocks and thus new SSA_NAMEs.
357 We want that process complete prior to the phase where we start
358 removing edges from the CFG. Edge removal may ultimately result in
359 removal of PHI nodes and thus releasing SSA_NAMEs back to the
362 If the two processes run in parallel we could release an SSA_NAME
363 back to the manager but we could still have dangling references
364 to the released SSA_NAME in unreachable blocks.
365 that any released names not have dangling references in the IL. */
366 find_implicit_erroneous_behaviour ();
367 find_explicit_erroneous_behaviour ();
369 free_original_copy_tables ();
371 /* We scramble the CFG and loop structures a bit, clean up
372 appropriately. We really should incrementally update the
373 loop structures, in theory it shouldn't be that hard. */
376 free_dominance_info (CDI_DOMINATORS
);
377 free_dominance_info (CDI_POST_DOMINATORS
);
378 loops_state_set (LOOPS_NEED_FIXUP
);
379 return TODO_cleanup_cfg
| TODO_update_ssa
;
385 gate_isolate_erroneous_paths (void)
387 /* If we do not have a suitable builtin function for the trap statement,
388 then do not perform the optimization. */
389 return (flag_isolate_erroneous_paths
!= 0);
393 const pass_data pass_data_isolate_erroneous_paths
=
395 GIMPLE_PASS
, /* type */
396 "isolate-paths", /* name */
397 OPTGROUP_NONE
, /* optinfo_flags */
399 true, /* has_execute */
400 TV_ISOLATE_ERRONEOUS_PATHS
, /* tv_id */
401 ( PROP_cfg
| PROP_ssa
), /* properties_required */
402 0, /* properties_provided */
403 0, /* properties_destroyed */
404 0, /* todo_flags_start */
405 TODO_verify_ssa
, /* todo_flags_finish */
408 class pass_isolate_erroneous_paths
: public gimple_opt_pass
411 pass_isolate_erroneous_paths (gcc::context
*ctxt
)
412 : gimple_opt_pass (pass_data_isolate_erroneous_paths
, ctxt
)
415 /* opt_pass methods: */
416 opt_pass
* clone () { return new pass_isolate_erroneous_paths (m_ctxt
); }
417 bool gate () { return gate_isolate_erroneous_paths (); }
418 unsigned int execute () { return gimple_ssa_isolate_erroneous_paths (); }
420 }; // class pass_isolate_erroneous_paths
424 make_pass_isolate_erroneous_paths (gcc::context
*ctxt
)
426 return new pass_isolate_erroneous_paths (ctxt
);