1 /* Detect paths through the CFG which can never be executed in a conforming
2 program and isolate them.
4 Copyright (C) 2013-2015 Free Software Foundation, Inc.
6 This file is part of GCC.
8 GCC is free software; you can redistribute it and/or modify
9 it under the terms of the GNU General Public License as published by
10 the Free Software Foundation; either version 3, or (at your option)
13 GCC is distributed in the hope that it will be useful,
14 but WITHOUT ANY WARRANTY; without even the implied warranty of
15 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
16 GNU General Public License for more details.
18 You should have received a copy of the GNU General Public License
19 along with GCC; see the file COPYING3. If not see
20 <http://www.gnu.org/licenses/>. */
24 #include "coretypes.h"
28 #include "double-int.h"
36 #include "fold-const.h"
40 #include "hard-reg-set.h"
43 #include "dominance.h"
45 #include "basic-block.h"
46 #include "tree-ssa-alias.h"
47 #include "internal-fn.h"
48 #include "gimple-expr.h"
51 #include "gimple-iterator.h"
52 #include "gimple-walk.h"
54 #include "stringpool.h"
55 #include "tree-ssanames.h"
56 #include "gimple-ssa.h"
57 #include "tree-ssa-operands.h"
58 #include "tree-phinodes.h"
59 #include "ssa-iterators.h"
61 #include "tree-pass.h"
63 #include "diagnostic-core.h"
67 static bool cfg_altered;
69 /* Callback for walk_stmt_load_store_ops.
71 Return TRUE if OP will dereference the tree stored in DATA, FALSE
74 This routine only makes a superficial check for a dereference. Thus,
75 it must only be used if it is safe to return a false negative. */
77 check_loadstore (gimple stmt, tree op, tree, void *data)
79 if ((TREE_CODE (op) == MEM_REF || TREE_CODE (op) == TARGET_MEM_REF)
80 && operand_equal_p (TREE_OPERAND (op, 0), (tree)data, 0))
82 TREE_THIS_VOLATILE (op) = 1;
83 TREE_SIDE_EFFECTS (op) = 1;
90 /* Insert a trap after SI and remove SI and all statements after the trap. */
93 insert_trap_and_remove_trailing_statements (gimple_stmt_iterator *si_p, tree op)
95 /* We want the NULL pointer dereference to actually occur so that
96 code that wishes to catch the signal can do so.
98 If the dereference is a load, then there's nothing to do as the
99 LHS will be a throw-away SSA_NAME and the RHS is the NULL dereference.
101 If the dereference is a store and we can easily transform the RHS,
102 then simplify the RHS to enable more DCE. Note that we require the
103 statement to be a GIMPLE_ASSIGN which filters out calls on the RHS. */
104 gimple stmt = gsi_stmt (*si_p);
105 if (walk_stmt_load_store_ops (stmt, (void *)op, NULL, check_loadstore)
106 && is_gimple_assign (stmt)
107 && INTEGRAL_TYPE_P (TREE_TYPE (gimple_assign_lhs (stmt))))
109 /* We just need to turn the RHS into zero converted to the proper
111 tree type = TREE_TYPE (gimple_assign_lhs (stmt));
112 gimple_assign_set_rhs_code (stmt, INTEGER_CST);
113 gimple_assign_set_rhs1 (stmt, fold_convert (type, integer_zero_node));
118 = gimple_build_call (builtin_decl_explicit (BUILT_IN_TRAP), 0);
119 gimple_seq seq = NULL;
120 gimple_seq_add_stmt (&seq, new_stmt);
122 /* If we had a NULL pointer dereference, then we want to insert the
123 __builtin_trap after the statement, for the other cases we want
124 to insert before the statement. */
125 if (walk_stmt_load_store_ops (stmt, (void *)op,
129 gsi_insert_after (si_p, seq, GSI_NEW_STMT);
130 if (stmt_ends_bb_p (stmt))
132 split_block (gimple_bb (stmt), stmt);
137 gsi_insert_before (si_p, seq, GSI_NEW_STMT);
139 /* We must remove statements from the end of the block so that we
140 never reference a released SSA_NAME. */
141 basic_block bb = gimple_bb (gsi_stmt (*si_p));
142 for (gimple_stmt_iterator si = gsi_last_bb (bb);
143 gsi_stmt (si) != gsi_stmt (*si_p);
144 si = gsi_last_bb (bb))
146 stmt = gsi_stmt (si);
147 unlink_stmt_vdef (stmt);
148 gsi_remove (&si, true);
153 /* BB when reached via incoming edge E will exhibit undefined behaviour
154 at STMT. Isolate and optimize the path which exhibits undefined
157 Isolation is simple. Duplicate BB and redirect E to BB'.
159 Optimization is simple as well. Replace STMT in BB' with an
160 unconditional trap and remove all outgoing edges from BB'.
162 If RET_ZERO, do not trap, only return NULL.
164 DUPLICATE is a pre-existing duplicate, use it as BB' if it exists.
169 isolate_path (basic_block bb, basic_block duplicate,
170 edge e, gimple stmt, tree op, bool ret_zero)
172 gimple_stmt_iterator si, si2;
176 /* First duplicate BB if we have not done so already and remove all
177 the duplicate's outgoing edges as duplicate is going to unconditionally
178 trap. Removing the outgoing edges is both an optimization and ensures
179 we don't need to do any PHI node updates. */
182 duplicate = duplicate_block (bb, NULL, NULL);
184 for (ei = ei_start (duplicate->succs); (e2 = ei_safe_edge (ei)); )
188 /* Complete the isolation step by redirecting E to reach DUPLICATE. */
189 e2 = redirect_edge_and_branch (e, duplicate);
191 flush_pending_stmts (e2);
194 /* There may be more than one statement in DUPLICATE which exhibits
195 undefined behaviour. Ultimately we want the first such statement in
196 DUPLCIATE so that we're able to delete as much code as possible.
198 So each time we discover undefined behaviour in DUPLICATE, search for
199 the statement which triggers undefined behaviour. If found, then
200 transform the statement into a trap and delete everything after the
201 statement. If not found, then this particular instance was subsumed by
202 an earlier instance of undefined behaviour and there's nothing to do.
204 This is made more complicated by the fact that we have STMT, which is in
205 BB rather than in DUPLICATE. So we set up two iterators, one for each
206 block and walk forward looking for STMT in BB, advancing each iterator at
209 When we find STMT the second iterator should point to STMT's equivalent in
210 duplicate. If DUPLICATE ends before STMT is found in BB, then there's
213 Ignore labels and debug statements. */
214 si = gsi_start_nondebug_after_labels_bb (bb);
215 si2 = gsi_start_nondebug_after_labels_bb (duplicate);
216 while (!gsi_end_p (si) && !gsi_end_p (si2) && gsi_stmt (si) != stmt)
218 gsi_next_nondebug (&si);
219 gsi_next_nondebug (&si2);
222 /* This would be an indicator that we never found STMT in BB, which should
224 gcc_assert (!gsi_end_p (si));
226 /* If we did not run to the end of DUPLICATE, then SI points to STMT and
227 SI2 points to the duplicate of STMT in DUPLICATE. Insert a trap
228 before SI2 and remove SI2 and all trailing statements. */
229 if (!gsi_end_p (si2))
233 greturn *ret = as_a <greturn *> (gsi_stmt (si2));
234 tree zero = build_zero_cst (TREE_TYPE (gimple_return_retval (ret)));
235 gimple_return_set_retval (ret, zero);
239 insert_trap_and_remove_trailing_statements (&si2, op);
245 /* Look for PHI nodes which feed statements in the same block where
246 the value of the PHI node implies the statement is erroneous.
248 For example, a NULL PHI arg value which then feeds a pointer
251 When found isolate and optimize the path associated with the PHI
252 argument feeding the erroneous statement. */
254 find_implicit_erroneous_behaviour (void)
258 FOR_EACH_BB_FN (bb, cfun)
262 /* Out of an abundance of caution, do not isolate paths to a
263 block where the block has any abnormal outgoing edges.
265 We might be able to relax this in the future. We have to detect
266 when we have to split the block with the NULL dereference and
267 the trap we insert. We have to preserve abnormal edges out
268 of the isolated block which in turn means updating PHIs at
269 the targets of those abnormal outgoing edges. */
270 if (has_abnormal_or_eh_outgoing_edge_p (bb))
273 /* First look for a PHI which sets a pointer to NULL and which
274 is then dereferenced within BB. This is somewhat overly
275 conservative, but probably catches most of the interesting
277 for (si = gsi_start_phis (bb); !gsi_end_p (si); gsi_next (&si))
279 gphi *phi = si.phi ();
280 tree lhs = gimple_phi_result (phi);
282 /* If the result is not a pointer, then there is no need to
283 examine the arguments. */
284 if (!POINTER_TYPE_P (TREE_TYPE (lhs)))
287 /* PHI produces a pointer result. See if any of the PHI's
290 When we remove an edge, we want to reprocess the current
291 index, hence the ugly way we update I for each iteration. */
292 basic_block duplicate = NULL;
293 for (unsigned i = 0, next_i = 0;
294 i < gimple_phi_num_args (phi);
297 tree op = gimple_phi_arg_def (phi, i);
298 edge e = gimple_phi_arg_edge (phi, i);
299 imm_use_iterator iter;
304 if (TREE_CODE (op) == ADDR_EXPR)
306 tree valbase = get_base_address (TREE_OPERAND (op, 0));
307 if ((TREE_CODE (valbase) == VAR_DECL
308 && !is_global_var (valbase))
309 || TREE_CODE (valbase) == PARM_DECL)
311 FOR_EACH_IMM_USE_STMT (use_stmt, iter, lhs)
314 = dyn_cast <greturn *> (use_stmt);
318 if (gimple_return_retval (return_stmt) != lhs)
321 if (warning_at (gimple_location (use_stmt),
322 OPT_Wreturn_local_addr,
323 "function may return address "
324 "of local variable"))
325 inform (DECL_SOURCE_LOCATION(valbase),
328 if (gimple_bb (use_stmt) == bb)
330 duplicate = isolate_path (bb, duplicate, e,
331 use_stmt, lhs, true);
333 /* When we remove an incoming edge, we need to
334 reprocess the Ith element. */
342 if (!integer_zerop (op))
345 /* We've got a NULL PHI argument. Now see if the
346 PHI's result is dereferenced within BB. */
347 FOR_EACH_IMM_USE_STMT (use_stmt, iter, lhs)
349 /* We only care about uses in BB. Catching cases in
350 in other blocks would require more complex path
352 if (gimple_bb (use_stmt) != bb)
355 if (infer_nonnull_range (use_stmt, lhs,
356 flag_isolate_erroneous_paths_dereference,
357 flag_isolate_erroneous_paths_attribute))
360 duplicate = isolate_path (bb, duplicate, e,
361 use_stmt, lhs, false);
363 /* When we remove an incoming edge, we need to
364 reprocess the Ith element. */
374 /* Look for statements which exhibit erroneous behaviour. For example
375 a NULL pointer dereference.
377 When found, optimize the block containing the erroneous behaviour. */
379 find_explicit_erroneous_behaviour (void)
383 FOR_EACH_BB_FN (bb, cfun)
385 gimple_stmt_iterator si;
387 /* Out of an abundance of caution, do not isolate paths to a
388 block where the block has any abnormal outgoing edges.
390 We might be able to relax this in the future. We have to detect
391 when we have to split the block with the NULL dereference and
392 the trap we insert. We have to preserve abnormal edges out
393 of the isolated block which in turn means updating PHIs at
394 the targets of those abnormal outgoing edges. */
395 if (has_abnormal_or_eh_outgoing_edge_p (bb))
398 /* Now look at the statements in the block and see if any of
399 them explicitly dereference a NULL pointer. This happens
400 because of jump threading and constant propagation. */
401 for (si = gsi_start_bb (bb); !gsi_end_p (si); gsi_next (&si))
403 gimple stmt = gsi_stmt (si);
405 /* By passing null_pointer_node, we can use infer_nonnull_range
406 to detect explicit NULL pointer dereferences and other uses
407 where a non-NULL value is required. */
408 if (infer_nonnull_range (stmt, null_pointer_node,
409 flag_isolate_erroneous_paths_dereference,
410 flag_isolate_erroneous_paths_attribute))
412 insert_trap_and_remove_trailing_statements (&si,
415 /* And finally, remove all outgoing edges from BB. */
417 for (edge_iterator ei = ei_start (bb->succs);
418 (e = ei_safe_edge (ei)); )
421 /* Ignore any more operands on this statement and
422 continue the statement iterator (which should
423 terminate its loop immediately. */
428 /* Detect returning the address of a local variable. This only
429 becomes undefined behavior if the result is used, so we do not
430 insert a trap and only return NULL instead. */
431 if (greturn *return_stmt = dyn_cast <greturn *> (stmt))
433 tree val = gimple_return_retval (return_stmt);
434 if (val && TREE_CODE (val) == ADDR_EXPR)
436 tree valbase = get_base_address (TREE_OPERAND (val, 0));
437 if ((TREE_CODE (valbase) == VAR_DECL
438 && !is_global_var (valbase))
439 || TREE_CODE (valbase) == PARM_DECL)
441 /* We only need it for this particular case. */
442 calculate_dominance_info (CDI_POST_DOMINATORS);
444 bool always_executed = dominated_by_p
445 (CDI_POST_DOMINATORS,
446 single_succ (ENTRY_BLOCK_PTR_FOR_FN (cfun)), bb);
448 msg = N_("function returns address of local variable");
450 msg = N_("function may return address of "
453 if (warning_at (gimple_location (stmt),
454 OPT_Wreturn_local_addr, msg))
455 inform (DECL_SOURCE_LOCATION(valbase), "declared here");
456 tree zero = build_zero_cst (TREE_TYPE (val));
457 gimple_return_set_retval (return_stmt, zero);
466 /* Search the function for statements which, if executed, would cause
467 the program to fault such as a dereference of a NULL pointer.
469 Such a program can't be valid if such a statement was to execute
470 according to ISO standards.
472 We detect explicit NULL pointer dereferences as well as those implied
473 by a PHI argument having a NULL value which unconditionally flows into
474 a dereference in the same block as the PHI.
476 In the former case we replace the offending statement with an
477 unconditional trap and eliminate the outgoing edges from the statement's
478 basic block. This may expose secondary optimization opportunities.
480 In the latter case, we isolate the path(s) with the NULL PHI
481 feeding the dereference. We can then replace the offending statement
482 and eliminate the outgoing edges in the duplicate. Again, this may
483 expose secondary optimization opportunities.
485 A warning for both cases may be advisable as well.
487 Other statically detectable violations of the ISO standard could be
488 handled in a similar way, such as out-of-bounds array indexing. */
491 gimple_ssa_isolate_erroneous_paths (void)
493 initialize_original_copy_tables ();
495 /* Search all the blocks for edges which, if traversed, will
496 result in undefined behaviour. */
499 /* First handle cases where traversal of a particular edge
500 triggers undefined behaviour. These cases require creating
501 duplicate blocks and thus new SSA_NAMEs.
503 We want that process complete prior to the phase where we start
504 removing edges from the CFG. Edge removal may ultimately result in
505 removal of PHI nodes and thus releasing SSA_NAMEs back to the
508 If the two processes run in parallel we could release an SSA_NAME
509 back to the manager but we could still have dangling references
510 to the released SSA_NAME in unreachable blocks.
511 that any released names not have dangling references in the IL. */
512 find_implicit_erroneous_behaviour ();
513 find_explicit_erroneous_behaviour ();
515 free_original_copy_tables ();
517 /* We scramble the CFG and loop structures a bit, clean up
518 appropriately. We really should incrementally update the
519 loop structures, in theory it shouldn't be that hard. */
520 free_dominance_info (CDI_POST_DOMINATORS);
523 free_dominance_info (CDI_DOMINATORS);
524 loops_state_set (LOOPS_NEED_FIXUP);
525 return TODO_cleanup_cfg | TODO_update_ssa;
531 const pass_data pass_data_isolate_erroneous_paths =
533 GIMPLE_PASS, /* type */
534 "isolate-paths", /* name */
535 OPTGROUP_NONE, /* optinfo_flags */
536 TV_ISOLATE_ERRONEOUS_PATHS, /* tv_id */
537 ( PROP_cfg | PROP_ssa ), /* properties_required */
538 0, /* properties_provided */
539 0, /* properties_destroyed */
540 0, /* todo_flags_start */
541 0, /* todo_flags_finish */
544 class pass_isolate_erroneous_paths : public gimple_opt_pass
547 pass_isolate_erroneous_paths (gcc::context *ctxt)
548 : gimple_opt_pass (pass_data_isolate_erroneous_paths, ctxt)
551 /* opt_pass methods: */
552 opt_pass * clone () { return new pass_isolate_erroneous_paths (m_ctxt); }
553 virtual bool gate (function *)
555 /* If we do not have a suitable builtin function for the trap statement,
556 then do not perform the optimization. */
557 return (flag_isolate_erroneous_paths_dereference != 0
558 || flag_isolate_erroneous_paths_attribute != 0);
561 virtual unsigned int execute (function *)
563 return gimple_ssa_isolate_erroneous_paths ();
566 }; // class pass_isolate_erroneous_paths
570 make_pass_isolate_erroneous_paths (gcc::context *ctxt)
572 return new pass_isolate_erroneous_paths (ctxt);