2017-02-20 Paul Thomas <pault@gcc.gnu.org>

[official-gcc.git] / gcc / gimple-ssa-warn-alloca.c

/* Warn on problematic uses of alloca and variable length arrays.
   Copyright (C) 2016-2017 Free Software Foundation, Inc.
   Contributed by Aldy Hernandez <aldyh@redhat.com>.

This file is part of GCC.

GCC is free software; you can redistribute it and/or modify it under
the terms of the GNU General Public License as published by the Free
Software Foundation; either version 3, or (at your option) any later
version.

GCC is distributed in the hope that it will be useful, but WITHOUT ANY
WARRANTY; without even the implied warranty of MERCHANTABILITY or
FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
for more details.

You should have received a copy of the GNU General Public License
along with GCC; see the file COPYING3.  If not see
<http://www.gnu.org/licenses/>.  */

#include "config.h"
#include "system.h"
#include "coretypes.h"
#include "backend.h"
#include "tree.h"
#include "gimple.h"
#include "tree-pass.h"
#include "ssa.h"
#include "gimple-pretty-print.h"
#include "diagnostic-core.h"
#include "fold-const.h"
#include "gimple-iterator.h"
#include "tree-ssa.h"
#include "params.h"
#include "tree-cfg.h"
#include "calls.h"
#include "cfgloop.h"
#include "intl.h"

const pass_data pass_data_walloca = {
  GIMPLE_PASS,
  "walloca",
  OPTGROUP_NONE,
  TV_NONE,
  PROP_cfg, // properties_required
  0,        // properties_provided
  0,        // properties_destroyed
  0,        // properties_start
  0,        // properties_finish
};

class pass_walloca : public gimple_opt_pass
{
public:
  pass_walloca (gcc::context *ctxt)
    : gimple_opt_pass(pass_data_walloca, ctxt), first_time_p (false)
  {}
  opt_pass *clone () { return new pass_walloca (m_ctxt); }
  void set_pass_param (unsigned int n, bool param)
    {
      gcc_assert (n == 0);
      first_time_p = param;
    }
  virtual bool gate (function *);
  virtual unsigned int execute (function *);

 private:
  // Set to TRUE the first time we run this pass on a function.
  bool first_time_p;
};

bool
pass_walloca::gate (function *fun ATTRIBUTE_UNUSED)
{
  // The first time this pass is called, it is called before
  // optimizations have been run and range information is unavailable,
  // so we can only perform strict alloca checking.
  if (first_time_p)
    return warn_alloca != 0;

  return warn_alloca_limit > 0 || warn_vla_limit > 0;
}

// Possible problematic uses of alloca.
enum alloca_type {
  // Alloca argument is within known bounds that are appropriate.
  ALLOCA_OK,

  // Alloca argument is KNOWN to have a value that is too large.
  ALLOCA_BOUND_DEFINITELY_LARGE,

  // Alloca argument may be too large.
  ALLOCA_BOUND_MAYBE_LARGE,

  // Alloca argument is bounded but of an indeterminate size.
  ALLOCA_BOUND_UNKNOWN,

  // Alloca argument was casted from a signed integer.
  ALLOCA_CAST_FROM_SIGNED,

  // Alloca appears in a loop.
  ALLOCA_IN_LOOP,

  // Alloca argument is 0.
  ALLOCA_ARG_IS_ZERO,

  // Alloca call is unbounded.  That is, there is no controlling
  // predicate for its argument.
  ALLOCA_UNBOUNDED
};

// Type of an alloca call with its corresponding limit, if applicable.
struct alloca_type_and_limit {
  enum alloca_type type;
  // For ALLOCA_BOUND_MAYBE_LARGE and ALLOCA_BOUND_DEFINITELY_LARGE
  // types, this field indicates the assumed limit if known or
  // integer_zero_node if unknown.  For any other alloca types, this
  // field is undefined.
  wide_int limit;
  alloca_type_and_limit ();
  alloca_type_and_limit (enum alloca_type type,
                         wide_int i) : type(type), limit(i) { }
  alloca_type_and_limit (enum alloca_type type) : type(type) { }
};

// NOTE: When we get better range info, this entire function becomes
// irrelevant, as it should be possible to get range info for an SSA
// name at any point in the program.
//
// We have a few heuristics up our sleeve to determine if a call to
// alloca() is within bounds.  Try them out and return the type of
// alloca call with its assumed limit (if applicable).
//
// Given a known argument (ARG) to alloca() and an EDGE (E)
// calculating said argument, verify that the last statement in the BB
// in E->SRC is a gate comparing ARG to an acceptable bound for
// alloca().  See examples below.
//
// If set, ARG_CASTED is the possible unsigned argument to which ARG
// was casted to.  This is to handle cases where the controlling
// predicate is looking at a casted value, not the argument itself.
//    arg_casted = (size_t) arg;
//    if (arg_casted < N)
//      goto bb3;
//    else
//      goto bb5;
//
// MAX_SIZE is WARN_ALLOCA= adjusted for VLAs.  It is the maximum size
// in bytes we allow for arg.

static struct alloca_type_and_limit
alloca_call_type_by_arg (tree arg, tree arg_casted, edge e, unsigned max_size)
{
  basic_block bb = e->src;
  gimple_stmt_iterator gsi = gsi_last_bb (bb);
  gimple *last = gsi_stmt (gsi);
  if (!last || gimple_code (last) != GIMPLE_COND)
    return alloca_type_and_limit (ALLOCA_UNBOUNDED);

  enum tree_code cond_code = gimple_cond_code (last);
  if (e->flags & EDGE_TRUE_VALUE)
    ;
  else if (e->flags & EDGE_FALSE_VALUE)
    cond_code = invert_tree_comparison (cond_code, false);
  else
    return alloca_type_and_limit (ALLOCA_UNBOUNDED);

  // Check for:
  //   if (ARG .COND. N)
  //     goto <bb 3>;
  //   else
  //     goto <bb 4>;
  //   <bb 3>:
  //   alloca(ARG);
  if ((cond_code == LE_EXPR
       || cond_code == LT_EXPR
       || cond_code == GT_EXPR
       || cond_code == GE_EXPR)
      && (gimple_cond_lhs (last) == arg
          || gimple_cond_lhs (last) == arg_casted))
    {
      if (TREE_CODE (gimple_cond_rhs (last)) == INTEGER_CST)
        {
          tree rhs = gimple_cond_rhs (last);
          int tst = wi::cmpu (wi::to_widest (rhs), max_size);
          if ((cond_code == LT_EXPR && tst == -1)
              || (cond_code == LE_EXPR && (tst == -1 || tst == 0)))
            return alloca_type_and_limit (ALLOCA_OK);
          else
            {
              // Let's not get too specific as to how large the limit
              // may be.  Someone's clearly an idiot when things
              // degrade into "if (N > Y) alloca(N)".
              if (cond_code == GT_EXPR || cond_code == GE_EXPR)
                rhs = integer_zero_node;
              return alloca_type_and_limit (ALLOCA_BOUND_MAYBE_LARGE, rhs);
            }
        }
      else
        return alloca_type_and_limit (ALLOCA_BOUND_UNKNOWN);
    }

  // Similarly, but check for a comparison with an unknown LIMIT.
  //   if (LIMIT .COND. ARG)
  //     alloca(arg);
  //
  //   Where LIMIT has a bound of unknown range.
  //
  // Note: All conditions of the form (ARG .COND. XXXX) where covered
  // by the previous check above, so we only need to look for (LIMIT
  // .COND. ARG) here.
  tree limit = gimple_cond_lhs (last);
  if ((gimple_cond_rhs (last) == arg
       || gimple_cond_rhs (last) == arg_casted)
      && TREE_CODE (limit) == SSA_NAME)
    {
      wide_int min, max;
      value_range_type range_type = get_range_info (limit, &min, &max);

      if (range_type == VR_UNDEFINED || range_type == VR_VARYING)
        return alloca_type_and_limit (ALLOCA_BOUND_UNKNOWN);

      // ?? It looks like the above `if' is unnecessary, as we never
      // get any VR_RANGE or VR_ANTI_RANGE here.  If we had a range
      // for LIMIT, I suppose we would have taken care of it in
      // alloca_call_type(), or handled above where we handle (ARG .COND. N).
      //
      // If this ever triggers, we should probably figure out why and
      // handle it, though it is likely to be just an ALLOCA_UNBOUNDED.
      return alloca_type_and_limit (ALLOCA_UNBOUNDED);
    }

  return alloca_type_and_limit (ALLOCA_UNBOUNDED);
}

// Return TRUE if SSA's definition is a cast from a signed type.
// If so, set *INVALID_CASTED_TYPE to the signed type.

static bool
cast_from_signed_p (tree ssa, tree *invalid_casted_type)
{
  gimple *def = SSA_NAME_DEF_STMT (ssa);
  if (def
      && !gimple_nop_p (def)
      && gimple_assign_cast_p (def)
      && !TYPE_UNSIGNED (TREE_TYPE (gimple_assign_rhs1 (def))))
    {
      *invalid_casted_type = TREE_TYPE (gimple_assign_rhs1 (def));
      return true;
    }
  return false;
}

// Return TRUE if X has a maximum range of MAX, basically covering the
// entire domain, in which case it's no range at all.

static bool
is_max (tree x, wide_int max)
{
  return wi::max_value (TREE_TYPE (x)) == max;
}

// Analyze the alloca call in STMT and return the alloca type with its
// corresponding limit (if applicable).  IS_VLA is set if the alloca
// call is really a BUILT_IN_ALLOCA_WITH_ALIGN, signifying a VLA.
//
// If the alloca call may be too large because of a cast from a signed
// type to an unsigned type, set *INVALID_CASTED_TYPE to the
// problematic signed type.

static struct alloca_type_and_limit
alloca_call_type (gimple *stmt, bool is_vla, tree *invalid_casted_type)
{
  gcc_assert (gimple_alloca_call_p (stmt));
  bool tentative_cast_from_signed = false;
  tree len = gimple_call_arg (stmt, 0);
  tree len_casted = NULL;
  wide_int min, max;
  struct alloca_type_and_limit ret = alloca_type_and_limit (ALLOCA_UNBOUNDED);

  gcc_assert (!is_vla || warn_vla_limit > 0);
  gcc_assert (is_vla || warn_alloca_limit > 0);

  // Adjust warn_alloca_max_size for VLAs, by taking the underlying
  // type into account.
  unsigned HOST_WIDE_INT max_size;
  if (is_vla)
    max_size = (unsigned HOST_WIDE_INT) warn_vla_limit;
  else
    max_size = (unsigned HOST_WIDE_INT) warn_alloca_limit;

  // Check for the obviously bounded case.
  if (TREE_CODE (len) == INTEGER_CST)
    {
      if (tree_to_uhwi (len) > max_size)
        return alloca_type_and_limit (ALLOCA_BOUND_DEFINITELY_LARGE, len);
      if (integer_zerop (len))
        return alloca_type_and_limit (ALLOCA_ARG_IS_ZERO);
      ret = alloca_type_and_limit (ALLOCA_OK);
    }
  // Check the range info if available.
  else if (value_range_type range_type = get_range_info (len, &min, &max))
    {
      if (range_type == VR_RANGE)
        {
          if (wi::leu_p (max, max_size))
            ret = alloca_type_and_limit (ALLOCA_OK);
          else
            {
              // A cast may have created a range we don't care
              // about.  For instance, a cast from 16-bit to
              // 32-bit creates a range of 0..65535, even if there
              // is not really a determinable range in the
              // underlying code.  In this case, look through the
              // cast at the original argument, and fall through
              // to look at other alternatives.
              //
              // We only look at through the cast when its from
              // unsigned to unsigned, otherwise we may risk
              // looking at SIGNED_INT < N, which is clearly not
              // what we want.  In this case, we'd be interested
              // in a VR_RANGE of [0..N].
              //
              // Note: None of this is perfect, and should all go
              // away with better range information.  But it gets
              // most of the cases.
              gimple *def = SSA_NAME_DEF_STMT (len);
              if (gimple_assign_cast_p (def)
                  && TYPE_UNSIGNED (TREE_TYPE (gimple_assign_rhs1 (def))))

                {
                  len_casted = gimple_assign_rhs1 (def);
                  range_type = get_range_info (len_casted, &min, &max);
                }
              // An unknown range or a range of the entire domain is
              // really no range at all.
              if (range_type == VR_VARYING
                  || (!len_casted && is_max (len, max))
                  || (len_casted && is_max (len_casted, max)))
                {
                  // Fall through.
                }
              else if (range_type == VR_ANTI_RANGE)
                return alloca_type_and_limit (ALLOCA_UNBOUNDED);
              else if (range_type != VR_VARYING)
                return
                  alloca_type_and_limit (ALLOCA_BOUND_MAYBE_LARGE, max);
            }
        }
      else if (range_type == VR_ANTI_RANGE)
        {
          // There may be some wrapping around going on.  Catch it
          // with this heuristic.  Hopefully, this VR_ANTI_RANGE
          // nonsense will go away, and we won't have to catch the
          // sign conversion problems with this crap.
          //
          // This is here to catch things like:
          // void foo(signed int n) {
          //   if (n < 100)
          //     alloca(n);
          //   ...
          // }
          if (cast_from_signed_p (len, invalid_casted_type))
            {
              // Unfortunately this also triggers:
              //
              // __SIZE_TYPE__ n = (__SIZE_TYPE__)blah;
              // if (n < 100)
              //   alloca(n);
              //
              // ...which is clearly bounded.  So, double check that
              // the paths leading up to the size definitely don't
              // have a bound.
              tentative_cast_from_signed = true;
            }
        }
      // No easily determined range and try other things.
    }

  // If we couldn't find anything, try a few heuristics for things we
  // can easily determine.  Check these misc cases but only accept
  // them if all predecessors have a known bound.
  basic_block bb = gimple_bb (stmt);
  if (ret.type == ALLOCA_UNBOUNDED)
    {
      ret.type = ALLOCA_OK;
      for (unsigned ix = 0; ix < EDGE_COUNT (bb->preds); ix++)
        {
          gcc_assert (!len_casted || TYPE_UNSIGNED (TREE_TYPE (len_casted)));
          ret = alloca_call_type_by_arg (len, len_casted,
                                         EDGE_PRED (bb, ix), max_size);
          if (ret.type != ALLOCA_OK)
            break;
        }
    }

  if (tentative_cast_from_signed && ret.type != ALLOCA_OK)
    return alloca_type_and_limit (ALLOCA_CAST_FROM_SIGNED);
  return ret;
}

// Return TRUE if the alloca call in STMT is in a loop, otherwise
// return FALSE. As an exception, ignore alloca calls for VLAs that
// occur in a loop since those will be cleaned up when they go out of
// scope.

static bool
in_loop_p (bool is_vla, gimple *stmt)
{
  basic_block bb = gimple_bb (stmt);
  if (bb->loop_father
      && bb->loop_father->header != ENTRY_BLOCK_PTR_FOR_FN (cfun))
    {
      // Do not warn on VLAs occurring in a loop, since VLAs are
      // guaranteed to be cleaned up when they go out of scope.
      // That is, there is a corresponding __builtin_stack_restore
      // at the end of the scope in which the VLA occurs.
      tree fndecl = gimple_call_fn (stmt);
      while (TREE_CODE (fndecl) == ADDR_EXPR)
        fndecl = TREE_OPERAND (fndecl, 0);
      if (DECL_BUILT_IN_CLASS (fndecl) == BUILT_IN_NORMAL
          && is_vla
          && DECL_FUNCTION_CODE (fndecl) == BUILT_IN_ALLOCA_WITH_ALIGN)
        return false;

      return true;
    }
  return false;
}

unsigned int
pass_walloca::execute (function *fun)
{
  basic_block bb;
  FOR_EACH_BB_FN (bb, fun)
    {
      for (gimple_stmt_iterator si = gsi_start_bb (bb); !gsi_end_p (si);
           gsi_next (&si))
        {
          gimple *stmt = gsi_stmt (si);
          location_t loc = gimple_location (stmt);

          if (!gimple_alloca_call_p (stmt))
            continue;
          gcc_assert (gimple_call_num_args (stmt) >= 1);

          bool is_vla = gimple_alloca_call_p (stmt)
            && gimple_call_alloca_for_var_p (as_a <gcall *> (stmt));

          // Strict mode whining for VLAs is handled by the front-end,
          // so we can safely ignore this case.  Also, ignore VLAs if
          // the user doesn't care about them.
          if (is_vla
              && (warn_vla > 0 || !warn_vla_limit))
            continue;

          if (!is_vla && (warn_alloca || !warn_alloca_limit))
            {
              if (warn_alloca)
                warning_at (loc, OPT_Walloca, G_("use of %<alloca%>"));
              continue;
            }

          tree invalid_casted_type = NULL;
          struct alloca_type_and_limit t
            = alloca_call_type (stmt, is_vla, &invalid_casted_type);

          // Even if we think the alloca call is OK, make sure it's
          // not in a loop.
          if (t.type == ALLOCA_OK && in_loop_p (is_vla, stmt))
            t = alloca_type_and_limit (ALLOCA_IN_LOOP);

          enum opt_code wcode
            = is_vla ? OPT_Wvla_larger_than_ : OPT_Walloca_larger_than_;
          char buff[WIDE_INT_MAX_PRECISION / 4 + 4];
          switch (t.type)
            {
            case ALLOCA_OK:
              break;
            case ALLOCA_BOUND_MAYBE_LARGE:
              if (warning_at (loc, wcode,
                              is_vla ? G_("argument to variable-length array "
                                          "may be too large")
                              : G_("argument to %<alloca%> may be too large"))
                  && t.limit != integer_zero_node)
                {
                  print_decu (t.limit, buff);
                  inform (loc, G_("limit is %u bytes, but argument "
                                  "may be as large as %s"),
                          is_vla ? warn_vla_limit : warn_alloca_limit, buff);
                }
              break;
            case ALLOCA_BOUND_DEFINITELY_LARGE:
              if (warning_at (loc, wcode,
                              is_vla ? G_("argument to variable-length array "
                                          "is too large")
                              : G_("argument to %<alloca%> is too large"))
                  && t.limit != integer_zero_node)
                {
                  print_decu (t.limit, buff);
                  inform (loc, G_("limit is %u bytes, but argument is %s"),
                          is_vla ? warn_vla_limit : warn_alloca_limit, buff);
                }
              break;
            case ALLOCA_BOUND_UNKNOWN:
              warning_at (loc, wcode,
                          is_vla ? G_("variable-length array bound is unknown")
                          : G_("%<alloca%> bound is unknown"));
              break;
            case ALLOCA_UNBOUNDED:
              warning_at (loc, wcode,
                          is_vla ? G_("unbounded use of variable-length array")
                          : G_("unbounded use of %<alloca%>"));
              break;
            case ALLOCA_IN_LOOP:
              gcc_assert (!is_vla);
              warning_at (loc, wcode, G_("use of %<alloca%> within a loop"));
              break;
            case ALLOCA_CAST_FROM_SIGNED:
              gcc_assert (invalid_casted_type != NULL_TREE);
              warning_at (loc, wcode,
                          is_vla ? G_("argument to variable-length array "
                                      "may be too large due to "
                                      "conversion from %qT to %qT")
                          : G_("argument to %<alloca%> may be too large "
                               "due to conversion from %qT to %qT"),
                          invalid_casted_type, size_type_node);
              break;
            case ALLOCA_ARG_IS_ZERO:
              warning_at (loc, wcode,
                          is_vla ? G_("argument to variable-length array "
                                      "is zero")
                          : G_("argument to %<alloca%> is zero"));
              break;
            default:
              gcc_unreachable ();
            }
        }
    }
  return 0;
}

gimple_opt_pass *
make_pass_walloca (gcc::context *ctxt)
{
  return new pass_walloca (ctxt);
}