Daily bump.
[official-gcc.git] / gcc / gimple-ssa-warn-alloca.c
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1 /* Warn on problematic uses of alloca and variable length arrays.
2 Copyright (C) 2016-2018 Free Software Foundation, Inc.
3 Contributed by Aldy Hernandez <aldyh@redhat.com>.
5 This file is part of GCC.
7 GCC is free software; you can redistribute it and/or modify it under
8 the terms of the GNU General Public License as published by the Free
9 Software Foundation; either version 3, or (at your option) any later
10 version.
12 GCC is distributed in the hope that it will be useful, but WITHOUT ANY
13 WARRANTY; without even the implied warranty of MERCHANTABILITY or
14 FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
15 for more details.
17 You should have received a copy of the GNU General Public License
18 along with GCC; see the file COPYING3. If not see
19 <http://www.gnu.org/licenses/>. */
21 #include "config.h"
22 #include "system.h"
23 #include "coretypes.h"
24 #include "backend.h"
25 #include "tree.h"
26 #include "gimple.h"
27 #include "tree-pass.h"
28 #include "ssa.h"
29 #include "gimple-pretty-print.h"
30 #include "diagnostic-core.h"
31 #include "fold-const.h"
32 #include "gimple-iterator.h"
33 #include "tree-ssa.h"
34 #include "params.h"
35 #include "tree-cfg.h"
36 #include "calls.h"
37 #include "cfgloop.h"
38 #include "intl.h"
40 const pass_data pass_data_walloca = {
41 GIMPLE_PASS,
42 "walloca",
43 OPTGROUP_NONE,
44 TV_NONE,
45 PROP_cfg, // properties_required
46 0, // properties_provided
47 0, // properties_destroyed
48 0, // properties_start
49 0, // properties_finish
52 class pass_walloca : public gimple_opt_pass
54 public:
55 pass_walloca (gcc::context *ctxt)
56 : gimple_opt_pass(pass_data_walloca, ctxt), first_time_p (false)
58 opt_pass *clone () { return new pass_walloca (m_ctxt); }
59 void set_pass_param (unsigned int n, bool param)
61 gcc_assert (n == 0);
62 first_time_p = param;
64 virtual bool gate (function *);
65 virtual unsigned int execute (function *);
67 private:
68 // Set to TRUE the first time we run this pass on a function.
69 bool first_time_p;
72 bool
73 pass_walloca::gate (function *fun ATTRIBUTE_UNUSED)
75 // The first time this pass is called, it is called before
76 // optimizations have been run and range information is unavailable,
77 // so we can only perform strict alloca checking.
78 if (first_time_p)
79 return warn_alloca != 0;
81 return ((unsigned HOST_WIDE_INT) warn_alloca_limit > 0
82 || (unsigned HOST_WIDE_INT) warn_vla_limit > 0);
85 // Possible problematic uses of alloca.
86 enum alloca_type {
87 // Alloca argument is within known bounds that are appropriate.
88 ALLOCA_OK,
90 // Alloca argument is KNOWN to have a value that is too large.
91 ALLOCA_BOUND_DEFINITELY_LARGE,
93 // Alloca argument may be too large.
94 ALLOCA_BOUND_MAYBE_LARGE,
96 // Alloca argument is bounded but of an indeterminate size.
97 ALLOCA_BOUND_UNKNOWN,
99 // Alloca argument was casted from a signed integer.
100 ALLOCA_CAST_FROM_SIGNED,
102 // Alloca appears in a loop.
103 ALLOCA_IN_LOOP,
105 // Alloca argument is 0.
106 ALLOCA_ARG_IS_ZERO,
108 // Alloca call is unbounded. That is, there is no controlling
109 // predicate for its argument.
110 ALLOCA_UNBOUNDED
113 // Type of an alloca call with its corresponding limit, if applicable.
114 struct alloca_type_and_limit {
115 enum alloca_type type;
116 // For ALLOCA_BOUND_MAYBE_LARGE and ALLOCA_BOUND_DEFINITELY_LARGE
117 // types, this field indicates the assumed limit if known or
118 // integer_zero_node if unknown. For any other alloca types, this
119 // field is undefined.
120 wide_int limit;
121 alloca_type_and_limit ();
122 alloca_type_and_limit (enum alloca_type type,
123 wide_int i) : type(type), limit(i) { }
124 alloca_type_and_limit (enum alloca_type type) : type(type) { }
127 // NOTE: When we get better range info, this entire function becomes
128 // irrelevant, as it should be possible to get range info for an SSA
129 // name at any point in the program.
131 // We have a few heuristics up our sleeve to determine if a call to
132 // alloca() is within bounds. Try them out and return the type of
133 // alloca call with its assumed limit (if applicable).
135 // Given a known argument (ARG) to alloca() and an EDGE (E)
136 // calculating said argument, verify that the last statement in the BB
137 // in E->SRC is a gate comparing ARG to an acceptable bound for
138 // alloca(). See examples below.
140 // If set, ARG_CASTED is the possible unsigned argument to which ARG
141 // was casted to. This is to handle cases where the controlling
142 // predicate is looking at a casted value, not the argument itself.
143 // arg_casted = (size_t) arg;
144 // if (arg_casted < N)
145 // goto bb3;
146 // else
147 // goto bb5;
149 // MAX_SIZE is WARN_ALLOCA= adjusted for VLAs. It is the maximum size
150 // in bytes we allow for arg.
152 static struct alloca_type_and_limit
153 alloca_call_type_by_arg (tree arg, tree arg_casted, edge e, unsigned max_size)
155 basic_block bb = e->src;
156 gimple_stmt_iterator gsi = gsi_last_bb (bb);
157 gimple *last = gsi_stmt (gsi);
158 if (!last || gimple_code (last) != GIMPLE_COND)
159 return alloca_type_and_limit (ALLOCA_UNBOUNDED);
161 enum tree_code cond_code = gimple_cond_code (last);
162 if (e->flags & EDGE_TRUE_VALUE)
164 else if (e->flags & EDGE_FALSE_VALUE)
165 cond_code = invert_tree_comparison (cond_code, false);
166 else
167 return alloca_type_and_limit (ALLOCA_UNBOUNDED);
169 // Check for:
170 // if (ARG .COND. N)
171 // goto <bb 3>;
172 // else
173 // goto <bb 4>;
174 // <bb 3>:
175 // alloca(ARG);
176 if ((cond_code == LE_EXPR
177 || cond_code == LT_EXPR
178 || cond_code == GT_EXPR
179 || cond_code == GE_EXPR)
180 && (gimple_cond_lhs (last) == arg
181 || gimple_cond_lhs (last) == arg_casted))
183 if (TREE_CODE (gimple_cond_rhs (last)) == INTEGER_CST)
185 tree rhs = gimple_cond_rhs (last);
186 int tst = wi::cmpu (wi::to_widest (rhs), max_size);
187 if ((cond_code == LT_EXPR && tst == -1)
188 || (cond_code == LE_EXPR && (tst == -1 || tst == 0)))
189 return alloca_type_and_limit (ALLOCA_OK);
190 else
192 // Let's not get too specific as to how large the limit
193 // may be. Someone's clearly an idiot when things
194 // degrade into "if (N > Y) alloca(N)".
195 if (cond_code == GT_EXPR || cond_code == GE_EXPR)
196 rhs = integer_zero_node;
197 return alloca_type_and_limit (ALLOCA_BOUND_MAYBE_LARGE,
198 wi::to_wide (rhs));
201 else
202 return alloca_type_and_limit (ALLOCA_BOUND_UNKNOWN);
205 // Similarly, but check for a comparison with an unknown LIMIT.
206 // if (LIMIT .COND. ARG)
207 // alloca(arg);
209 // Where LIMIT has a bound of unknown range.
211 // Note: All conditions of the form (ARG .COND. XXXX) where covered
212 // by the previous check above, so we only need to look for (LIMIT
213 // .COND. ARG) here.
214 tree limit = gimple_cond_lhs (last);
215 if ((gimple_cond_rhs (last) == arg
216 || gimple_cond_rhs (last) == arg_casted)
217 && TREE_CODE (limit) == SSA_NAME)
219 wide_int min, max;
220 value_range_type range_type = get_range_info (limit, &min, &max);
222 if (range_type == VR_UNDEFINED || range_type == VR_VARYING)
223 return alloca_type_and_limit (ALLOCA_BOUND_UNKNOWN);
225 // ?? It looks like the above `if' is unnecessary, as we never
226 // get any VR_RANGE or VR_ANTI_RANGE here. If we had a range
227 // for LIMIT, I suppose we would have taken care of it in
228 // alloca_call_type(), or handled above where we handle (ARG .COND. N).
230 // If this ever triggers, we should probably figure out why and
231 // handle it, though it is likely to be just an ALLOCA_UNBOUNDED.
232 return alloca_type_and_limit (ALLOCA_UNBOUNDED);
235 return alloca_type_and_limit (ALLOCA_UNBOUNDED);
238 // Return TRUE if SSA's definition is a cast from a signed type.
239 // If so, set *INVALID_CASTED_TYPE to the signed type.
241 static bool
242 cast_from_signed_p (tree ssa, tree *invalid_casted_type)
244 gimple *def = SSA_NAME_DEF_STMT (ssa);
245 if (def
246 && !gimple_nop_p (def)
247 && gimple_assign_cast_p (def)
248 && !TYPE_UNSIGNED (TREE_TYPE (gimple_assign_rhs1 (def))))
250 *invalid_casted_type = TREE_TYPE (gimple_assign_rhs1 (def));
251 return true;
253 return false;
256 // Return TRUE if X has a maximum range of MAX, basically covering the
257 // entire domain, in which case it's no range at all.
259 static bool
260 is_max (tree x, wide_int max)
262 return wi::max_value (TREE_TYPE (x)) == max;
265 // Analyze the alloca call in STMT and return the alloca type with its
266 // corresponding limit (if applicable). IS_VLA is set if the alloca
267 // call was created by the gimplifier for a VLA.
269 // If the alloca call may be too large because of a cast from a signed
270 // type to an unsigned type, set *INVALID_CASTED_TYPE to the
271 // problematic signed type.
273 static struct alloca_type_and_limit
274 alloca_call_type (gimple *stmt, bool is_vla, tree *invalid_casted_type)
276 gcc_assert (gimple_alloca_call_p (stmt));
277 bool tentative_cast_from_signed = false;
278 tree len = gimple_call_arg (stmt, 0);
279 tree len_casted = NULL;
280 wide_int min, max;
281 edge_iterator ei;
282 edge e;
284 gcc_assert (!is_vla || (unsigned HOST_WIDE_INT) warn_vla_limit > 0);
285 gcc_assert (is_vla || (unsigned HOST_WIDE_INT) warn_alloca_limit > 0);
287 // Adjust warn_alloca_max_size for VLAs, by taking the underlying
288 // type into account.
289 unsigned HOST_WIDE_INT max_size;
290 if (is_vla)
291 max_size = (unsigned HOST_WIDE_INT) warn_vla_limit;
292 else
293 max_size = (unsigned HOST_WIDE_INT) warn_alloca_limit;
295 // Check for the obviously bounded case.
296 if (TREE_CODE (len) == INTEGER_CST)
298 if (tree_to_uhwi (len) > max_size)
299 return alloca_type_and_limit (ALLOCA_BOUND_DEFINITELY_LARGE,
300 wi::to_wide (len));
301 if (integer_zerop (len))
302 return alloca_type_and_limit (ALLOCA_ARG_IS_ZERO);
304 return alloca_type_and_limit (ALLOCA_OK);
307 // Check the range info if available.
308 if (TREE_CODE (len) == SSA_NAME)
310 value_range_type range_type = get_range_info (len, &min, &max);
311 if (range_type == VR_RANGE)
313 if (wi::leu_p (max, max_size))
314 return alloca_type_and_limit (ALLOCA_OK);
315 else
317 // A cast may have created a range we don't care
318 // about. For instance, a cast from 16-bit to
319 // 32-bit creates a range of 0..65535, even if there
320 // is not really a determinable range in the
321 // underlying code. In this case, look through the
322 // cast at the original argument, and fall through
323 // to look at other alternatives.
325 // We only look at through the cast when its from
326 // unsigned to unsigned, otherwise we may risk
327 // looking at SIGNED_INT < N, which is clearly not
328 // what we want. In this case, we'd be interested
329 // in a VR_RANGE of [0..N].
331 // Note: None of this is perfect, and should all go
332 // away with better range information. But it gets
333 // most of the cases.
334 gimple *def = SSA_NAME_DEF_STMT (len);
335 if (gimple_assign_cast_p (def))
337 tree rhs1 = gimple_assign_rhs1 (def);
338 tree rhs1type = TREE_TYPE (rhs1);
340 // Bail if the argument type is not valid.
341 if (!INTEGRAL_TYPE_P (rhs1type))
342 return alloca_type_and_limit (ALLOCA_OK);
344 if (TYPE_UNSIGNED (rhs1type))
346 len_casted = rhs1;
347 range_type = get_range_info (len_casted, &min, &max);
350 // An unknown range or a range of the entire domain is
351 // really no range at all.
352 if (range_type == VR_VARYING
353 || (!len_casted && is_max (len, max))
354 || (len_casted && is_max (len_casted, max)))
356 // Fall through.
358 else if (range_type == VR_ANTI_RANGE)
359 return alloca_type_and_limit (ALLOCA_UNBOUNDED);
360 else if (range_type != VR_VARYING)
361 return alloca_type_and_limit (ALLOCA_BOUND_MAYBE_LARGE, max);
364 else if (range_type == VR_ANTI_RANGE)
366 // There may be some wrapping around going on. Catch it
367 // with this heuristic. Hopefully, this VR_ANTI_RANGE
368 // nonsense will go away, and we won't have to catch the
369 // sign conversion problems with this crap.
371 // This is here to catch things like:
372 // void foo(signed int n) {
373 // if (n < 100)
374 // alloca(n);
375 // ...
376 // }
377 if (cast_from_signed_p (len, invalid_casted_type))
379 // Unfortunately this also triggers:
381 // __SIZE_TYPE__ n = (__SIZE_TYPE__)blah;
382 // if (n < 100)
383 // alloca(n);
385 // ...which is clearly bounded. So, double check that
386 // the paths leading up to the size definitely don't
387 // have a bound.
388 tentative_cast_from_signed = true;
391 // No easily determined range and try other things.
394 // If we couldn't find anything, try a few heuristics for things we
395 // can easily determine. Check these misc cases but only accept
396 // them if all predecessors have a known bound.
397 struct alloca_type_and_limit ret = alloca_type_and_limit (ALLOCA_OK);
398 FOR_EACH_EDGE (e, ei, gimple_bb (stmt)->preds)
400 gcc_assert (!len_casted || TYPE_UNSIGNED (TREE_TYPE (len_casted)));
401 ret = alloca_call_type_by_arg (len, len_casted, e, max_size);
402 if (ret.type != ALLOCA_OK)
403 break;
406 if (ret.type != ALLOCA_OK && tentative_cast_from_signed)
407 ret = alloca_type_and_limit (ALLOCA_CAST_FROM_SIGNED);
409 // If we have a declared maximum size, we can take it into account.
410 if (ret.type != ALLOCA_OK
411 && gimple_call_builtin_p (stmt, BUILT_IN_ALLOCA_WITH_ALIGN_AND_MAX))
413 tree arg = gimple_call_arg (stmt, 2);
414 if (compare_tree_int (arg, max_size) <= 0)
415 ret = alloca_type_and_limit (ALLOCA_OK);
416 else
417 ret = alloca_type_and_limit (ALLOCA_BOUND_MAYBE_LARGE,
418 wi::to_wide (arg));
421 return ret;
424 // Return TRUE if STMT is in a loop, otherwise return FALSE.
426 static bool
427 in_loop_p (gimple *stmt)
429 basic_block bb = gimple_bb (stmt);
430 return
431 bb->loop_father && bb->loop_father->header != ENTRY_BLOCK_PTR_FOR_FN (cfun);
434 unsigned int
435 pass_walloca::execute (function *fun)
437 basic_block bb;
438 FOR_EACH_BB_FN (bb, fun)
440 for (gimple_stmt_iterator si = gsi_start_bb (bb); !gsi_end_p (si);
441 gsi_next (&si))
443 gimple *stmt = gsi_stmt (si);
444 location_t loc = gimple_location (stmt);
446 if (!gimple_alloca_call_p (stmt))
447 continue;
449 const bool is_vla
450 = gimple_call_alloca_for_var_p (as_a <gcall *> (stmt));
452 // Strict mode whining for VLAs is handled by the front-end,
453 // so we can safely ignore this case. Also, ignore VLAs if
454 // the user doesn't care about them.
455 if (is_vla
456 && (warn_vla > 0 || !warn_vla_limit))
457 continue;
459 if (!is_vla && (warn_alloca || !warn_alloca_limit))
461 if (warn_alloca)
462 warning_at (loc, OPT_Walloca, G_("use of %<alloca%>"));
463 continue;
466 tree invalid_casted_type = NULL;
467 struct alloca_type_and_limit t
468 = alloca_call_type (stmt, is_vla, &invalid_casted_type);
470 // Even if we think the alloca call is OK, make sure it's not in a
471 // loop, except for a VLA, since VLAs are guaranteed to be cleaned
472 // up when they go out of scope, including in a loop.
473 if (t.type == ALLOCA_OK && !is_vla && in_loop_p (stmt))
474 t = alloca_type_and_limit (ALLOCA_IN_LOOP);
476 enum opt_code wcode
477 = is_vla ? OPT_Wvla_larger_than_ : OPT_Walloca_larger_than_;
478 char buff[WIDE_INT_MAX_PRECISION / 4 + 4];
479 switch (t.type)
481 case ALLOCA_OK:
482 break;
483 case ALLOCA_BOUND_MAYBE_LARGE:
484 if (warning_at (loc, wcode,
485 is_vla ? G_("argument to variable-length array "
486 "may be too large")
487 : G_("argument to %<alloca%> may be too large"))
488 && t.limit != 0)
490 print_decu (t.limit, buff);
491 inform (loc, G_("limit is %u bytes, but argument "
492 "may be as large as %s"),
493 is_vla ? warn_vla_limit : warn_alloca_limit, buff);
495 break;
496 case ALLOCA_BOUND_DEFINITELY_LARGE:
497 if (warning_at (loc, wcode,
498 is_vla ? G_("argument to variable-length array "
499 "is too large")
500 : G_("argument to %<alloca%> is too large"))
501 && t.limit != 0)
503 print_decu (t.limit, buff);
504 inform (loc, G_("limit is %u bytes, but argument is %s"),
505 is_vla ? warn_vla_limit : warn_alloca_limit, buff);
507 break;
508 case ALLOCA_BOUND_UNKNOWN:
509 warning_at (loc, wcode,
510 is_vla ? G_("variable-length array bound is unknown")
511 : G_("%<alloca%> bound is unknown"));
512 break;
513 case ALLOCA_UNBOUNDED:
514 warning_at (loc, wcode,
515 is_vla ? G_("unbounded use of variable-length array")
516 : G_("unbounded use of %<alloca%>"));
517 break;
518 case ALLOCA_IN_LOOP:
519 gcc_assert (!is_vla);
520 warning_at (loc, wcode, G_("use of %<alloca%> within a loop"));
521 break;
522 case ALLOCA_CAST_FROM_SIGNED:
523 gcc_assert (invalid_casted_type != NULL_TREE);
524 warning_at (loc, wcode,
525 is_vla ? G_("argument to variable-length array "
526 "may be too large due to "
527 "conversion from %qT to %qT")
528 : G_("argument to %<alloca%> may be too large "
529 "due to conversion from %qT to %qT"),
530 invalid_casted_type, size_type_node);
531 break;
532 case ALLOCA_ARG_IS_ZERO:
533 warning_at (loc, wcode,
534 is_vla ? G_("argument to variable-length array "
535 "is zero")
536 : G_("argument to %<alloca%> is zero"));
537 break;
538 default:
539 gcc_unreachable ();
543 return 0;
546 gimple_opt_pass *
547 make_pass_walloca (gcc::context *ctxt)
549 return new pass_walloca (ctxt);