gcc/gimple-ssa-isolate-paths.c

   1 /* Detect paths through the CFG which can never be executed in a conforming
   2    program and isolate them.
   3
   4    Copyright (C) 2013
   5    Free Software Foundation, Inc.
   6
   7 This file is part of GCC.
   8
   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)
  12 any later version.
  13
  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.
  18
  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/>.  */
  22
  23 #include "config.h"
  24 #include "system.h"
  25 #include "coretypes.h"
  26 #include "tree.h"
  27 #include "flags.h"
  28 #include "basic-block.h"
  29 #include "tree-ssa-alias.h"
  30 #include "internal-fn.h"
  31 #include "gimple-expr.h"
  32 #include "is-a.h"
  33 #include "gimple.h"
  34 #include "gimple-iterator.h"
  35 #include "gimple-walk.h"
  36 #include "tree-ssa.h"
  37 #include "stringpool.h"
  38 #include "tree-ssanames.h"
  39 #include "gimple-ssa.h"
  40 #include "tree-ssa-operands.h"
  41 #include "tree-phinodes.h"
  42 #include "ssa-iterators.h"
  43 #include "cfgloop.h"
  44 #include "tree-pass.h"
  45 #include "tree-cfg.h"
  46
  47
  48 static bool cfg_altered;
  49
  50 /* Callback for walk_stmt_load_store_ops.
  51
  52    Return TRUE if OP will dereference the tree stored in DATA, FALSE
  53    otherwise.
  54
  55    This routine only makes a superficial check for a dereference.  Thus,
  56    it must only be used if it is safe to return a false negative.  */
  57 static bool
  58 check_loadstore (gimple stmt ATTRIBUTE_UNUSED, tree op, void *data)
  59 {
  60   if ((TREE_CODE (op) == MEM_REF || TREE_CODE (op) == TARGET_MEM_REF)
  61       && operand_equal_p (TREE_OPERAND (op, 0), (tree)data, 0))
  62     {
  63       TREE_THIS_VOLATILE (op) = 1;
  64       TREE_SIDE_EFFECTS (op) = 1;
  65       update_stmt (stmt);
  66       return true;
  67     }
  68   return false;
  69 }
  70
  71 /* Insert a trap after SI and remove SI and all statements after the trap.  */
  72
  73 static void
  74 insert_trap_and_remove_trailing_statements (gimple_stmt_iterator *si_p, tree op)
  75 {
  76   /* We want the NULL pointer dereference to actually occur so that
  77      code that wishes to catch the signal can do so.
  78
  79      If the dereference is a load, then there's nothing to do as the
  80      LHS will be a throw-away SSA_NAME and the RHS is the NULL dereference.
  81
  82      If the dereference is a store and we can easily transform the RHS,
  83      then simplify the RHS to enable more DCE.   Note that we require the
  84      statement to be a GIMPLE_ASSIGN which filters out calls on the RHS.  */
  85   gimple stmt = gsi_stmt (*si_p);
  86   if (walk_stmt_load_store_ops (stmt, (void *)op, NULL, check_loadstore)
  87       && is_gimple_assign (stmt)
  88       && INTEGRAL_TYPE_P (TREE_TYPE (gimple_assign_lhs (stmt))))
  89     {
  90       /* We just need to turn the RHS into zero converted to the proper
  91          type.  */
  92       tree type = TREE_TYPE (gimple_assign_lhs (stmt));
  93       gimple_assign_set_rhs_code (stmt, INTEGER_CST);
  94       gimple_assign_set_rhs1 (stmt, fold_convert (type, integer_zero_node));
  95       update_stmt (stmt);
  96     }
  97
  98   gimple new_stmt
  99     = gimple_build_call (builtin_decl_explicit (BUILT_IN_TRAP), 0);
 100   gimple_seq seq = NULL;
 101   gimple_seq_add_stmt (&seq, new_stmt);
 102
 103   /* If we had a NULL pointer dereference, then we want to insert the
 104      __builtin_trap after the statement, for the other cases we want
 105      to insert before the statement.  */
 106   if (walk_stmt_load_store_ops (stmt, (void *)op,
 107                                 check_loadstore,
 108                                 check_loadstore))
 109     gsi_insert_after (si_p, seq, GSI_NEW_STMT);
 110   else
 111     gsi_insert_before (si_p, seq, GSI_NEW_STMT);
 112
 113   /* We must remove statements from the end of the block so that we
 114      never reference a released SSA_NAME.  */
 115   basic_block bb = gimple_bb (gsi_stmt (*si_p));
 116   for (gimple_stmt_iterator si = gsi_last_bb (bb);
 117        gsi_stmt (si) != gsi_stmt (*si_p);
 118        si = gsi_last_bb (bb))
 119     {
 120       stmt = gsi_stmt (si);
 121       unlink_stmt_vdef (stmt);
 122       gsi_remove (&si, true);
 123       release_defs (stmt);
 124     }
 125 }
 126
 127 /* BB when reached via incoming edge E will exhibit undefined behaviour
 128    at STMT.  Isolate and optimize the path which exhibits undefined
 129    behaviour.
 130
 131    Isolation is simple.  Duplicate BB and redirect E to BB'.
 132
 133    Optimization is simple as well.  Replace STMT in BB' with an
 134    unconditional trap and remove all outgoing edges from BB'.
 135
 136    DUPLICATE is a pre-existing duplicate, use it as BB' if it exists.
 137
 138    Return BB'.  */
 139
 140 basic_block
 141 isolate_path (basic_block bb, basic_block duplicate,
 142               edge e, gimple stmt, tree op)
 143 {
 144   gimple_stmt_iterator si, si2;
 145   edge_iterator ei;
 146   edge e2;
 147
 148   /* First duplicate BB if we have not done so already and remove all
 149      the duplicate's outgoing edges as duplicate is going to unconditionally
 150      trap.  Removing the outgoing edges is both an optimization and ensures
 151      we don't need to do any PHI node updates.  */
 152   if (!duplicate)
 153     {
 154       duplicate = duplicate_block (bb, NULL, NULL);
 155       for (ei = ei_start (duplicate->succs); (e2 = ei_safe_edge (ei)); )
 156         remove_edge (e2);
 157     }
 158
 159   /* Complete the isolation step by redirecting E to reach DUPLICATE.  */
 160   e2 = redirect_edge_and_branch (e, duplicate);
 161   if (e2)
 162     flush_pending_stmts (e2);
 163
 164
 165   /* There may be more than one statement in DUPLICATE which exhibits
 166      undefined behaviour.  Ultimately we want the first such statement in
 167      DUPLCIATE so that we're able to delete as much code as possible.
 168
 169      So each time we discover undefined behaviour in DUPLICATE, search for
 170      the statement which triggers undefined behaviour.  If found, then
 171      transform the statement into a trap and delete everything after the
 172      statement.  If not found, then this particular instance was subsumed by
 173      an earlier instance of undefined behaviour and there's nothing to do.
 174
 175      This is made more complicated by the fact that we have STMT, which is in
 176      BB rather than in DUPLICATE.  So we set up two iterators, one for each
 177      block and walk forward looking for STMT in BB, advancing each iterator at
 178      each step.
 179
 180      When we find STMT the second iterator should point to STMT's equivalent in
 181      duplicate.  If DUPLICATE ends before STMT is found in BB, then there's
 182      nothing to do.
 183
 184      Ignore labels and debug statements.  */
 185   si = gsi_start_nondebug_after_labels_bb (bb);
 186   si2 = gsi_start_nondebug_after_labels_bb (duplicate);
 187   while (!gsi_end_p (si) && !gsi_end_p (si2) && gsi_stmt (si) != stmt)
 188     {
 189       gsi_next_nondebug (&si);
 190       gsi_next_nondebug (&si2);
 191     }
 192
 193   /* This would be an indicator that we never found STMT in BB, which should
 194      never happen.  */
 195   gcc_assert (!gsi_end_p (si));
 196
 197   /* If we did not run to the end of DUPLICATE, then SI points to STMT and
 198      SI2 points to the duplicate of STMT in DUPLICATE.  Insert a trap
 199      before SI2 and remove SI2 and all trailing statements.  */
 200   if (!gsi_end_p (si2))
 201     insert_trap_and_remove_trailing_statements (&si2, op);
 202
 203   return duplicate;
 204 }
 205
 206 /* Look for PHI nodes which feed statements in the same block where
 207    the value of the PHI node implies the statement is erroneous.
 208
 209    For example, a NULL PHI arg value which then feeds a pointer
 210    dereference.
 211
 212    When found isolate and optimize the path associated with the PHI
 213    argument feeding the erroneous statement.  */
 214 static void
 215 find_implicit_erroneous_behaviour (void)
 216 {
 217   basic_block bb;
 218
 219   FOR_EACH_BB_FN (bb, cfun)
 220     {
 221       gimple_stmt_iterator si;
 222
 223       /* Out of an abundance of caution, do not isolate paths to a
 224          block where the block has any abnormal outgoing edges.
 225
 226          We might be able to relax this in the future.  We have to detect
 227          when we have to split the block with the NULL dereference and
 228          the trap we insert.  We have to preserve abnormal edges out
 229          of the isolated block which in turn means updating PHIs at
 230          the targets of those abnormal outgoing edges.  */
 231       if (has_abnormal_or_eh_outgoing_edge_p (bb))
 232         continue;
 233
 234       /* First look for a PHI which sets a pointer to NULL and which
 235          is then dereferenced within BB.  This is somewhat overly
 236          conservative, but probably catches most of the interesting
 237          cases.   */
 238       for (si = gsi_start_phis (bb); !gsi_end_p (si); gsi_next (&si))
 239         {
 240           gimple phi = gsi_stmt (si);
 241           tree lhs = gimple_phi_result (phi);
 242
 243           /* If the result is not a pointer, then there is no need to
 244              examine the arguments.  */
 245           if (!POINTER_TYPE_P (TREE_TYPE (lhs)))
 246             continue;
 247
 248           /* PHI produces a pointer result.  See if any of the PHI's
 249              arguments are NULL.
 250
 251              When we remove an edge, we want to reprocess the current
 252              index, hence the ugly way we update I for each iteration.  */
 253           basic_block duplicate = NULL;
 254           for (unsigned i = 0, next_i = 0;
 255                i < gimple_phi_num_args (phi);
 256                i = next_i)
 257             {
 258               tree op = gimple_phi_arg_def (phi, i);
 259
 260               next_i = i + 1;
 261
 262               if (!integer_zerop (op))
 263                 continue;
 264
 265               edge e = gimple_phi_arg_edge (phi, i);
 266               imm_use_iterator iter;
 267               gimple use_stmt;
 268
 269               /* We've got a NULL PHI argument.  Now see if the
 270                  PHI's result is dereferenced within BB.  */
 271               FOR_EACH_IMM_USE_STMT (use_stmt, iter, lhs)
 272                 {
 273                   /* We only care about uses in BB.  Catching cases in
 274                      in other blocks would require more complex path
 275                      isolation code.   */
 276                   if (gimple_bb (use_stmt) != bb)
 277                     continue;
 278
 279                   if (infer_nonnull_range (use_stmt, lhs,
 280                                            flag_isolate_erroneous_paths_dereference,
 281                                            flag_isolate_erroneous_paths_attribute))
 282
 283                     {
 284                       duplicate = isolate_path (bb, duplicate,
 285                                                 e, use_stmt, lhs);
 286
 287                       /* When we remove an incoming edge, we need to
 288                          reprocess the Ith element.  */
 289                       next_i = i;
 290                       cfg_altered = true;
 291                     }
 292                 }
 293             }
 294         }
 295     }
 296 }
 297
 298 /* Look for statements which exhibit erroneous behaviour.  For example
 299    a NULL pointer dereference.
 300
 301    When found, optimize the block containing the erroneous behaviour.  */
 302 static void
 303 find_explicit_erroneous_behaviour (void)
 304 {
 305   basic_block bb;
 306
 307   FOR_EACH_BB_FN (bb, cfun)
 308     {
 309       gimple_stmt_iterator si;
 310
 311       /* Out of an abundance of caution, do not isolate paths to a
 312          block where the block has any abnormal outgoing edges.
 313
 314          We might be able to relax this in the future.  We have to detect
 315          when we have to split the block with the NULL dereference and
 316          the trap we insert.  We have to preserve abnormal edges out
 317          of the isolated block which in turn means updating PHIs at
 318          the targets of those abnormal outgoing edges.  */
 319       if (has_abnormal_or_eh_outgoing_edge_p (bb))
 320         continue;
 321
 322       /* Now look at the statements in the block and see if any of
 323          them explicitly dereference a NULL pointer.  This happens
 324          because of jump threading and constant propagation.  */
 325       for (si = gsi_start_bb (bb); !gsi_end_p (si); gsi_next (&si))
 326         {
 327           gimple stmt = gsi_stmt (si);
 328
 329           /* By passing null_pointer_node, we can use infer_nonnull_range
 330              to detect explicit NULL pointer dereferences and other uses
 331              where a non-NULL value is required.  */
 332           if (infer_nonnull_range (stmt, null_pointer_node,
 333                                    flag_isolate_erroneous_paths_dereference,
 334                                    flag_isolate_erroneous_paths_attribute))
 335             {
 336               insert_trap_and_remove_trailing_statements (&si,
 337                                                           null_pointer_node);
 338
 339               /* And finally, remove all outgoing edges from BB.  */
 340               edge e;
 341               for (edge_iterator ei = ei_start (bb->succs);
 342                    (e = ei_safe_edge (ei)); )
 343                 remove_edge (e);
 344
 345               /* Ignore any more operands on this statement and
 346                  continue the statement iterator (which should
 347                  terminate its loop immediately.  */
 348               cfg_altered = true;
 349               break;
 350             }
 351         }
 352     }
 353 }
 354 /* Search the function for statements which, if executed, would cause
 355    the program to fault such as a dereference of a NULL pointer.
 356
 357    Such a program can't be valid if such a statement was to execute
 358    according to ISO standards.
 359
 360    We detect explicit NULL pointer dereferences as well as those implied
 361    by a PHI argument having a NULL value which unconditionally flows into
 362    a dereference in the same block as the PHI.
 363
 364    In the former case we replace the offending statement with an
 365    unconditional trap and eliminate the outgoing edges from the statement's
 366    basic block.  This may expose secondary optimization opportunities.
 367
 368    In the latter case, we isolate the path(s) with the NULL PHI
 369    feeding the dereference.  We can then replace the offending statement
 370    and eliminate the outgoing edges in the duplicate.  Again, this may
 371    expose secondary optimization opportunities.
 372
 373    A warning for both cases may be advisable as well.
 374
 375    Other statically detectable violations of the ISO standard could be
 376    handled in a similar way, such as out-of-bounds array indexing.  */
 377
 378 static unsigned int
 379 gimple_ssa_isolate_erroneous_paths (void)
 380 {
 381   initialize_original_copy_tables ();
 382
 383   /* Search all the blocks for edges which, if traversed, will
 384      result in undefined behaviour.  */
 385   cfg_altered = false;
 386
 387   /* First handle cases where traversal of a particular edge
 388      triggers undefined behaviour.  These cases require creating
 389      duplicate blocks and thus new SSA_NAMEs.
 390
 391      We want that process complete prior to the phase where we start
 392      removing edges from the CFG.  Edge removal may ultimately result in
 393      removal of PHI nodes and thus releasing SSA_NAMEs back to the
 394      name manager.
 395
 396      If the two processes run in parallel we could release an SSA_NAME
 397      back to the manager but we could still have dangling references
 398      to the released SSA_NAME in unreachable blocks.
 399      that any released names not have dangling references in the IL.  */
 400   find_implicit_erroneous_behaviour ();
 401   find_explicit_erroneous_behaviour ();
 402
 403   free_original_copy_tables ();
 404
 405   /* We scramble the CFG and loop structures a bit, clean up
 406      appropriately.  We really should incrementally update the
 407      loop structures, in theory it shouldn't be that hard.  */
 408   if (cfg_altered)
 409     {
 410       free_dominance_info (CDI_DOMINATORS);
 411       free_dominance_info (CDI_POST_DOMINATORS);
 412       loops_state_set (LOOPS_NEED_FIXUP);
 413       return TODO_cleanup_cfg | TODO_update_ssa;
 414     }
 415   return 0;
 416 }
 417
 418 static bool
 419 gate_isolate_erroneous_paths (void)
 420 {
 421   /* If we do not have a suitable builtin function for the trap statement,
 422      then do not perform the optimization.  */
 423   return (flag_isolate_erroneous_paths_dereference != 0
 424           || flag_isolate_erroneous_paths_attribute != 0);
 425 }
 426
 427 namespace {
 428 const pass_data pass_data_isolate_erroneous_paths =
 429 {
 430   GIMPLE_PASS, /* type */
 431   "isolate-paths", /* name */
 432   OPTGROUP_NONE, /* optinfo_flags */
 433   true, /* has_gate */
 434   true, /* has_execute */
 435   TV_ISOLATE_ERRONEOUS_PATHS, /* tv_id */
 436   ( PROP_cfg | PROP_ssa ), /* properties_required */
 437   0, /* properties_provided */
 438   0, /* properties_destroyed */
 439   0, /* todo_flags_start */
 440   TODO_verify_ssa, /* todo_flags_finish */
 441 };
 442
 443 class pass_isolate_erroneous_paths : public gimple_opt_pass
 444 {
 445 public:
 446   pass_isolate_erroneous_paths (gcc::context *ctxt)
 447     : gimple_opt_pass (pass_data_isolate_erroneous_paths, ctxt)
 448   {}
 449
 450   /* opt_pass methods: */
 451   opt_pass * clone () { return new pass_isolate_erroneous_paths (m_ctxt); }
 452   bool gate () { return gate_isolate_erroneous_paths (); }
 453   unsigned int execute () { return gimple_ssa_isolate_erroneous_paths (); }
 454
 455 }; // class pass_isolate_erroneous_paths
 456 }
 457
 458 gimple_opt_pass *
 459 make_pass_isolate_erroneous_paths (gcc::context *ctxt)
 460 {
 461   return new pass_isolate_erroneous_paths (ctxt);
 462 }