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/* Alias analysis for GNU C

   Copyright (C) 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006,

   2007, 2008, 2009, 2010 Free Software Foundation, Inc.

   Contributed by John Carr (jfc@mit.edu).

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 "tm.h"

#include "rtl.h"

#include "tree.h"

#include "tm_p.h"

#include "function.h"

#include "alias.h"

#include "emit-rtl.h"

#include "regs.h"

#include "hard-reg-set.h"

#include "basic-block.h"

#include "flags.h"

#include "output.h"

#include "toplev.h"

#include "cselib.h"

#include "splay-tree.h"

#include "ggc.h"

#include "langhooks.h"

#include "timevar.h"

#include "target.h"

#include "cgraph.h"

#include "varray.h"

#include "tree-pass.h"

#include "ipa-type-escape.h"

#include "df.h"

#include "tree-ssa-alias.h"

#include "pointer-set.h"

#include "tree-flow.h"

/* The aliasing API provided here solves related but different problems:

   Say there exists (in c)

   struct X {

     struct Y y1;

     struct Z z2;

   } x1, *px1,  *px2;

   struct Y y2, *py;

   struct Z z2, *pz;

   py = &px1.y1;

   px2 = &x1;

   Consider the four questions:

   Can a store to x1 interfere with px2->y1?

   Can a store to x1 interfere with px2->z2?

   (*px2).z2

   Can a store to x1 change the value pointed to by with py?

   Can a store to x1 change the value pointed to by with pz?

   The answer to these questions can be yes, yes, yes, and maybe.

   The first two questions can be answered with a simple examination

   of the type system.  If structure X contains a field of type Y then

   a store thru a pointer to an X can overwrite any field that is

   contained (recursively) in an X (unless we know that px1 != px2).

   The last two of the questions can be solved in the same way as the

   first two questions but this is too conservative.  The observation

   is that in some cases analysis we can know if which (if any) fields

   are addressed and if those addresses are used in bad ways.  This

   analysis may be language specific.  In C, arbitrary operations may

   be applied to pointers.  However, there is some indication that

   this may be too conservative for some C++ types.

   The pass ipa-type-escape does this analysis for the types whose

   instances do not escape across the compilation boundary.

   Historically in GCC, these two problems were combined and a single

   data structure was used to represent the solution to these

   problems.  We now have two similar but different data structures,

   The data structure to solve the last two question is similar to the

   first, but does not contain have the fields in it whose address are

   never taken.  For types that do escape the compilation unit, the

   data structures will have identical information.

*/

/* The alias sets assigned to MEMs assist the back-end in determining

   which MEMs can alias which other MEMs.  In general, two MEMs in

   different alias sets cannot alias each other, with one important

   exception.  Consider something like:

     struct S { int i; double d; };

   a store to an `S' can alias something of either type `int' or type

   `double'.  (However, a store to an `int' cannot alias a `double'

   and vice versa.)  We indicate this via a tree structure that looks

   like:

           struct S

            /   \

           /     \

         |/_     _\|

         int    double

   (The arrows are directed and point downwards.)

    In this situation we say the alias set for `struct S' is the

   `superset' and that those for `int' and `double' are `subsets'.

   To see whether two alias sets can point to the same memory, we must

   see if either alias set is a subset of the other. We need not trace

   past immediate descendants, however, since we propagate all

   grandchildren up one level.

   Alias set zero is implicitly a superset of all other alias sets.

   However, this is no actual entry for alias set zero.  It is an

   error to attempt to explicitly construct a subset of zero.  */

struct GTY(()) alias_set_entry_d {

  /* The alias set number, as stored in MEM_ALIAS_SET.  */

  alias_set_type alias_set;

  /* Nonzero if would have a child of zero: this effectively makes this

     alias set the same as alias set zero.  */

  int has_zero_child;

  /* The children of the alias set.  These are not just the immediate

     children, but, in fact, all descendants.  So, if we have:

       struct T { struct S s; float f; }

     continuing our example above, the children here will be all of

     `int', `double', `float', and `struct S'.  */

  splay_tree GTY((param1_is (int), param2_is (int))) children;

};

typedef struct alias_set_entry_d *alias_set_entry;

static int rtx_equal_for_memref_p (const_rtx, const_rtx);

static int memrefs_conflict_p (int, rtx, int, rtx, HOST_WIDE_INT);

static void record_set (rtx, const_rtx, void *);

static int base_alias_check (rtx, rtx, enum machine_mode,

                             enum machine_mode);

static rtx find_base_value (rtx);

static int mems_in_disjoint_alias_sets_p (const_rtx, const_rtx);

static int insert_subset_children (splay_tree_node, void*);

static alias_set_entry get_alias_set_entry (alias_set_type);

static const_rtx fixed_scalar_and_varying_struct_p (const_rtx, const_rtx, rtx, rtx,

                                                    bool (*) (const_rtx, bool));

static int aliases_everything_p (const_rtx);

static bool nonoverlapping_component_refs_p (const_tree, const_tree);

static tree decl_for_component_ref (tree);

static rtx adjust_offset_for_component_ref (tree, rtx);

static int write_dependence_p (const_rtx, const_rtx, int);

static void memory_modified_1 (rtx, const_rtx, void *);

/* Set up all info needed to perform alias analysis on memory references.  */

/* Returns the size in bytes of the mode of X.  */

#define SIZE_FOR_MODE(X) (GET_MODE_SIZE (GET_MODE (X)))

/* Returns nonzero if MEM1 and MEM2 do not alias because they are in

   different alias sets.  We ignore alias sets in functions making use

   of variable arguments because the va_arg macros on some systems are

   not legal ANSI C.  */

#define DIFFERENT_ALIAS_SETS_P(MEM1, MEM2)                      \

  mems_in_disjoint_alias_sets_p (MEM1, MEM2)

/* Cap the number of passes we make over the insns propagating alias

   information through set chains.   10 is a completely arbitrary choice.  */

#define MAX_ALIAS_LOOP_PASSES 10

/* reg_base_value[N] gives an address to which register N is related.

   If all sets after the first add or subtract to the current value

   or otherwise modify it so it does not point to a different top level

   object, reg_base_value[N] is equal to the address part of the source

   of the first set.

   A base address can be an ADDRESS, SYMBOL_REF, or LABEL_REF.  ADDRESS

   expressions represent certain special values: function arguments and

   the stack, frame, and argument pointers.

   The contents of an ADDRESS is not normally used, the mode of the

   ADDRESS determines whether the ADDRESS is a function argument or some

   other special value.  Pointer equality, not rtx_equal_p, determines whether

   two ADDRESS expressions refer to the same base address.

   The only use of the contents of an ADDRESS is for determining if the

   current function performs nonlocal memory memory references for the

   purposes of marking the function as a constant function.  */

static GTY(()) VEC(rtx,gc) *reg_base_value;

static rtx *new_reg_base_value;

/* We preserve the copy of old array around to avoid amount of garbage

   produced.  About 8% of garbage produced were attributed to this

   array.  */

static GTY((deletable)) VEC(rtx,gc) *old_reg_base_value;

/* Static hunks of RTL used by the aliasing code; these are initialized

   once per function to avoid unnecessary RTL allocations.  */

static GTY (()) rtx static_reg_base_value[FIRST_PSEUDO_REGISTER];

#define REG_BASE_VALUE(X)                               \

  (REGNO (X) < VEC_length (rtx, reg_base_value)         \

   ? VEC_index (rtx, reg_base_value, REGNO (X)) : 0)

/* Vector indexed by N giving the initial (unchanging) value known for

   pseudo-register N.  This array is initialized in init_alias_analysis,

   and does not change until end_alias_analysis is called.  */

static GTY((length("reg_known_value_size"))) rtx *reg_known_value;

/* Indicates number of valid entries in reg_known_value.  */

static GTY(()) unsigned int reg_known_value_size;

/* Vector recording for each reg_known_value whether it is due to a

   REG_EQUIV note.  Future passes (viz., reload) may replace the

   pseudo with the equivalent expression and so we account for the

   dependences that would be introduced if that happens.

   The REG_EQUIV notes created in assign_parms may mention the arg

   pointer, and there are explicit insns in the RTL that modify the

   arg pointer.  Thus we must ensure that such insns don't get

   scheduled across each other because that would invalidate the

   REG_EQUIV notes.  One could argue that the REG_EQUIV notes are

   wrong, but solving the problem in the scheduler will likely give

   better code, so we do it here.  */

static bool *reg_known_equiv_p;

/* True when scanning insns from the start of the rtl to the

   NOTE_INSN_FUNCTION_BEG note.  */

static bool copying_arguments;

DEF_VEC_P(alias_set_entry);

DEF_VEC_ALLOC_P(alias_set_entry,gc);

/* The splay-tree used to store the various alias set entries.  */

static GTY (()) VEC(alias_set_entry,gc) *alias_sets;

/* Build a decomposed reference object for querying the alias-oracle

   from the MEM rtx and store it in *REF.

   Returns false if MEM is not suitable for the alias-oracle.  */

static bool

ao_ref_from_mem (ao_ref *ref, const_rtx mem)

{

  tree expr = MEM_EXPR (mem);

  tree base;

  if (!expr)

    return false;

  /* If MEM_OFFSET or MEM_SIZE are NULL punt.  */

  if (!MEM_OFFSET (mem)

      || !MEM_SIZE (mem))

    return false;

  ao_ref_init (ref, expr);

  /* Get the base of the reference and see if we have to reject or

     adjust it.  */

  base = ao_ref_base (ref);

  if (base == NULL_TREE)

    return false;

  /* If this is a pointer dereference of a non-SSA_NAME punt.

     ???  We could replace it with a pointer to anything.  */

  if (INDIRECT_REF_P (base)

      && TREE_CODE (TREE_OPERAND (base, 0)) != SSA_NAME)

    return false;

  /* The tree oracle doesn't like to have these.  */

  if (TREE_CODE (base) == FUNCTION_DECL

      || TREE_CODE (base) == LABEL_DECL)

    return false;

  /* If this is a reference based on a partitioned decl replace the

     base with an INDIRECT_REF of the pointer representative we

     created during stack slot partitioning.  */

  if (TREE_CODE (base) == VAR_DECL

      && ! TREE_STATIC (base)

      && cfun->gimple_df->decls_to_pointers != NULL)

    {

      void *namep;

      namep = pointer_map_contains (cfun->gimple_df->decls_to_pointers, base);

      if (namep)

        {

          ref->base_alias_set = get_alias_set (base);

          ref->base = build1 (INDIRECT_REF, TREE_TYPE (base), *(tree *)namep);

        }

    }

  ref->ref_alias_set = MEM_ALIAS_SET (mem);

  /* If the base decl is a parameter we can have negative MEM_OFFSET in

     case of promoted subregs on bigendian targets.  Trust the MEM_EXPR

     here.  */

  if (INTVAL (MEM_OFFSET (mem)) < 0

      && ((INTVAL (MEM_SIZE (mem)) + INTVAL (MEM_OFFSET (mem)))

          * BITS_PER_UNIT) == ref->size)

    return true;

  ref->offset += INTVAL (MEM_OFFSET (mem)) * BITS_PER_UNIT;

  ref->size = INTVAL (MEM_SIZE (mem)) * BITS_PER_UNIT;

  /* The MEM may extend into adjacent fields, so adjust max_size if

     necessary.  */

  if (ref->max_size != -1

      && ref->size > ref->max_size)

    ref->max_size = ref->size;

  /* If MEM_OFFSET and MEM_SIZE get us outside of the base object of

     the MEM_EXPR punt.  This happens for STRICT_ALIGNMENT targets a lot.  */

  if (MEM_EXPR (mem) != get_spill_slot_decl (false)

      && (ref->offset < 0

          || (DECL_P (ref->base)

              && (!host_integerp (DECL_SIZE (ref->base), 1)

                  || (TREE_INT_CST_LOW (DECL_SIZE ((ref->base)))

                      < (unsigned HOST_WIDE_INT)(ref->offset + ref->size))))))

    return false;

  return true;

}

/* Query the alias-oracle on whether the two memory rtx X and MEM may

   alias.  If TBAA_P is set also apply TBAA.  Returns true if the

   two rtxen may alias, false otherwise.  */

static bool

rtx_refs_may_alias_p (const_rtx x, const_rtx mem, bool tbaa_p)

{

  ao_ref ref1, ref2;

  if (!ao_ref_from_mem (&ref1, x)

      || !ao_ref_from_mem (&ref2, mem))

    return true;

  return refs_may_alias_p_1 (&ref1, &ref2, tbaa_p);

}

/* Returns a pointer to the alias set entry for ALIAS_SET, if there is

   such an entry, or NULL otherwise.  */

static inline alias_set_entry

get_alias_set_entry (alias_set_type alias_set)

{

  return VEC_index (alias_set_entry, alias_sets, alias_set);

}

/* Returns nonzero if the alias sets for MEM1 and MEM2 are such that

   the two MEMs cannot alias each other.  */

static inline int

mems_in_disjoint_alias_sets_p (const_rtx mem1, const_rtx mem2)

{

/* Perform a basic sanity check.  Namely, that there are no alias sets

   if we're not using strict aliasing.  This helps to catch bugs

   whereby someone uses PUT_CODE, but doesn't clear MEM_ALIAS_SET, or

   where a MEM is allocated in some way other than by the use of

   gen_rtx_MEM, and the MEM_ALIAS_SET is not cleared.  If we begin to

   use alias sets to indicate that spilled registers cannot alias each

   other, we might need to remove this check.  */

  gcc_assert (flag_strict_aliasing

              || (!MEM_ALIAS_SET (mem1) && !MEM_ALIAS_SET (mem2)));

  return ! alias_sets_conflict_p (MEM_ALIAS_SET (mem1), MEM_ALIAS_SET (mem2));

}

/* Insert the NODE into the splay tree given by DATA.  Used by

   record_alias_subset via splay_tree_foreach.  */

static int

insert_subset_children (splay_tree_node node, void *data)

{

  splay_tree_insert ((splay_tree) data, node->key, node->value);

  return 0;

}

/* Return true if the first alias set is a subset of the second.  */

bool

alias_set_subset_of (alias_set_type set1, alias_set_type set2)

{

  alias_set_entry ase;

  /* Everything is a subset of the "aliases everything" set.  */

  if (set2 == 0)

    return true;

  /* Otherwise, check if set1 is a subset of set2.  */

  ase = get_alias_set_entry (set2);

  if (ase != 0

      && (ase->has_zero_child

          || splay_tree_lookup (ase->children,

                                (splay_tree_key) set1)))

    return true;

  return false;

}

/* Return 1 if the two specified alias sets may conflict.  */

int

alias_sets_conflict_p (alias_set_type set1, alias_set_type set2)

{

  alias_set_entry ase;

  /* The easy case.  */

  if (alias_sets_must_conflict_p (set1, set2))

    return 1;

  /* See if the first alias set is a subset of the second.  */

  ase = get_alias_set_entry (set1);

  if (ase != 0

      && (ase->has_zero_child

          || splay_tree_lookup (ase->children,

                                (splay_tree_key) set2)))

    return 1;

  /* Now do the same, but with the alias sets reversed.  */

  ase = get_alias_set_entry (set2);

  if (ase != 0

      && (ase->has_zero_child

          || splay_tree_lookup (ase->children,

                                (splay_tree_key) set1)))

    return 1;

  /* The two alias sets are distinct and neither one is the

     child of the other.  Therefore, they cannot conflict.  */

  return 0;

}

static int

walk_mems_2 (rtx *x, rtx mem)

{

  if (MEM_P (*x))

    {

      if (alias_sets_conflict_p (MEM_ALIAS_SET(*x), MEM_ALIAS_SET(mem)))

        return 1;

      return -1;

    }

  return 0;

}

static int

walk_mems_1 (rtx *x, rtx *pat)

{

  if (MEM_P (*x))

    {

      /* Visit all MEMs in *PAT and check indepedence.  */

      if (for_each_rtx (pat, (rtx_function) walk_mems_2, *x))

        /* Indicate that dependence was determined and stop traversal.  */

        return 1;

      return -1;

    }

  return 0;

}

/* Return 1 if two specified instructions have mem expr with conflict alias sets*/

bool

insn_alias_sets_conflict_p (rtx insn1, rtx insn2)

{

  /* For each pair of MEMs in INSN1 and INSN2 check their independence.  */

  return  for_each_rtx (&PATTERN (insn1), (rtx_function) walk_mems_1,

                         &PATTERN (insn2));

}

/* Return 1 if the two specified alias sets will always conflict.  */

int

alias_sets_must_conflict_p (alias_set_type set1, alias_set_type set2)

{

  if (set1 == 0 || set2 == 0 || set1 == set2)

    return 1;

  return 0;

}

/* Return 1 if any MEM object of type T1 will always conflict (using the

   dependency routines in this file) with any MEM object of type T2.

   This is used when allocating temporary storage.  If T1 and/or T2 are

   NULL_TREE, it means we know nothing about the storage.  */

int

objects_must_conflict_p (tree t1, tree t2)

{

  alias_set_type set1, set2;

  /* If neither has a type specified, we don't know if they'll conflict

     because we may be using them to store objects of various types, for

     example the argument and local variables areas of inlined functions.  */

  if (t1 == 0 && t2 == 0)

    return 0;

  /* If they are the same type, they must conflict.  */

  if (t1 == t2

      /* Likewise if both are volatile.  */

      || (t1 != 0 && TYPE_VOLATILE (t1) && t2 != 0 && TYPE_VOLATILE (t2)))

    return 1;

  set1 = t1 ? get_alias_set (t1) : 0;

  set2 = t2 ? get_alias_set (t2) : 0;

  /* We can't use alias_sets_conflict_p because we must make sure

     that every subtype of t1 will conflict with every subtype of

     t2 for which a pair of subobjects of these respective subtypes

     overlaps on the stack.  */

  return alias_sets_must_conflict_p (set1, set2);

}

/* Return true if all nested component references handled by

   get_inner_reference in T are such that we should use the alias set

   provided by the object at the heart of T.

   This is true for non-addressable components (which don't have their

   own alias set), as well as components of objects in alias set zero.

   This later point is a special case wherein we wish to override the

   alias set used by the component, but we don't have per-FIELD_DECL

   assignable alias sets.  */

bool

component_uses_parent_alias_set (const_tree t)

{

  while (1)

    {

      /* If we're at the end, it vacuously uses its own alias set.  */

      if (!handled_component_p (t))

        return false;

      switch (TREE_CODE (t))

        {

        case COMPONENT_REF:

          if (DECL_NONADDRESSABLE_P (TREE_OPERAND (t, 1)))

            return true;

          break;

        case ARRAY_REF:

        case ARRAY_RANGE_REF:

          if (TYPE_NONALIASED_COMPONENT (TREE_TYPE (TREE_OPERAND (t, 0))))

            return true;

          break;

        case REALPART_EXPR:

        case IMAGPART_EXPR:

          break;

        default:

          /* Bitfields and casts are never addressable.  */

          return true;

        }

      t = TREE_OPERAND (t, 0);

      if (get_alias_set (TREE_TYPE (t)) == 0)

        return true;

    }

}

/* Return the alias set for the memory pointed to by T, which may be

   either a type or an expression.  Return -1 if there is nothing

   special about dereferencing T.  */

static alias_set_type

get_deref_alias_set_1 (tree t)

{

  /* If we're not doing any alias analysis, just assume everything

     aliases everything else.  */

  if (!flag_strict_aliasing)

    return 0;

  /* All we care about is the type.  */

  if (! TYPE_P (t))

    t = TREE_TYPE (t);

  /* If we have an INDIRECT_REF via a void pointer, we don't

     know anything about what that might alias.  Likewise if the

     pointer is marked that way.  */

  if (TREE_CODE (TREE_TYPE (t)) == VOID_TYPE

      || TYPE_REF_CAN_ALIAS_ALL (t))

    return 0;

  return -1;

}

/* Return the alias set for the memory pointed to by T, which may be

   either a type or an expression.  */

alias_set_type

get_deref_alias_set (tree t)

{

  alias_set_type set = get_deref_alias_set_1 (t);

  /* Fall back to the alias-set of the pointed-to type.  */

  if (set == -1)

    {

      if (! TYPE_P (t))

        t = TREE_TYPE (t);

      set = get_alias_set (TREE_TYPE (t));

    }

  return set;

}

/* Return the alias set for T, which may be either a type or an

   expression.  Call language-specific routine for help, if needed.  */

alias_set_type

get_alias_set (tree t)

{

  alias_set_type set;

  /* If we're not doing any alias analysis, just assume everything

     aliases everything else.  Also return 0 if this or its type is

     an error.  */

  if (! flag_strict_aliasing || t == error_mark_node

      || (! TYPE_P (t)

          && (TREE_TYPE (t) == 0 || TREE_TYPE (t) == error_mark_node)))

    return 0;

  /* We can be passed either an expression or a type.  This and the

     language-specific routine may make mutually-recursive calls to each other

     to figure out what to do.  At each juncture, we see if this is a tree

     that the language may need to handle specially.  First handle things that

     aren't types.  */

  if (! TYPE_P (t))

    {

      tree inner;

      /* Remove any nops, then give the language a chance to do

         something with this tree before we look at it.  */

      STRIP_NOPS (t);

      set = lang_hooks.get_alias_set (t);

      if (set != -1)

        return set;

      /* Retrieve the original memory reference if needed.  */

      if (TREE_CODE (t) == TARGET_MEM_REF)

        t = TMR_ORIGINAL (t);

      /* First see if the actual object referenced is an INDIRECT_REF from a

         restrict-qualified pointer or a "void *".  */

      inner = t;

      while (handled_component_p (inner))

        {

          inner = TREE_OPERAND (inner, 0);

          STRIP_NOPS (inner);

        }

      if (INDIRECT_REF_P (inner))

        {

          set = get_deref_alias_set_1 (TREE_OPERAND (inner, 0));

          if (set != -1)

            return set;

        }

      /* Otherwise, pick up the outermost object that we could have a pointer

         to, processing conversions as above.  */

      while (component_uses_parent_alias_set (t))

        {

          t = TREE_OPERAND (t, 0);

          STRIP_NOPS (t);

        }

      /* If we've already determined the alias set for a decl, just return

         it.  This is necessary for C++ anonymous unions, whose component

         variables don't look like union members (boo!).  */

      if (TREE_CODE (t) == VAR_DECL

          && DECL_RTL_SET_P (t) && MEM_P (DECL_RTL (t)))

        return MEM_ALIAS_SET (DECL_RTL (t));

      /* Now all we care about is the type.  */

      t = TREE_TYPE (t);

    }

  /* Variant qualifiers don't affect the alias set, so get the main

     variant.  */

  t = TYPE_MAIN_VARIANT (t);

  /* Always use the canonical type as well.  If this is a type that

     requires structural comparisons to identify compatible types

     use alias set zero.  */

  if (TYPE_STRUCTURAL_EQUALITY_P (t))

    {

      /* Allow the language to specify another alias set for this

         type.  */

      set = lang_hooks.get_alias_set (t);

      if (set != -1)

        return set;

      return 0;

    }

  t = TYPE_CANONICAL (t);

  /* Canonical types shouldn't form a tree nor should the canonical

     type require structural equality checks.  */

  gcc_assert (!TYPE_STRUCTURAL_EQUALITY_P (t) && TYPE_CANONICAL (t) == t);

  /* If this is a type with a known alias set, return it.  */

  if (TYPE_ALIAS_SET_KNOWN_P (t))

    return TYPE_ALIAS_SET (t);

  /* We don't want to set TYPE_ALIAS_SET for incomplete types.  */

  if (!COMPLETE_TYPE_P (t))

    {

      /* For arrays with unknown size the conservative answer is the

         alias set of the element type.  */

      if (TREE_CODE (t) == ARRAY_TYPE)

        return get_alias_set (TREE_TYPE (t));

      /* But return zero as a conservative answer for incomplete types.  */

      return 0;

    }

  /* See if the language has special handling for this type.  */

  set = lang_hooks.get_alias_set (t);

  if (set != -1)

    return set;

  /* There are no objects of FUNCTION_TYPE, so there's no point in

     using up an alias set for them.  (There are, of course, pointers

     and references to functions, but that's different.)  */

  else if (TREE_CODE (t) == FUNCTION_TYPE

           || TREE_CODE (t) == METHOD_TYPE)

    set = 0;

  /* Unless the language specifies otherwise, let vector types alias

     their components.  This avoids some nasty type punning issues in

     normal usage.  And indeed lets vectors be treated more like an

     array slice.  */