stor-layout.c

GUN开源阻止下的编译器GCC

/* C-compiler utilities for types and variables storage layout
   Copyright (C) 1987, 88, 92, 93, 94, 1995 Free Software Foundation, Inc.

This file is part of GNU CC.

GNU CC 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 2, or (at your option)
any later version.

GNU CC 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 GNU CC; see the file COPYING.  If not, write to
the Free Software Foundation, 59 Temple Place - Suite 330,
Boston, MA 02111-1307, USA.  */


#include "config.h"
#include <stdio.h>

#include "tree.h"
#include "flags.h"
#include "function.h"

#define CEIL(x,y) (((x) + (y) - 1) / (y))

/* Data type for the expressions representing sizes of data types.
   It is the first integer type laid out.
   In C, this is int.  */

tree sizetype;

/* An integer constant with value 0 whose type is sizetype.  */

tree size_zero_node;

/* An integer constant with value 1 whose type is sizetype.  */

tree size_one_node;

/* If nonzero, this is an upper limit on alignment of structure fields.
   The value is measured in bits.  */
int maximum_field_alignment;

/* If non-zero, the alignment of a bitstring or (power-)set value, in bits.
   May be overridden by front-ends.  */
int set_alignment = 0;

#define GET_MODE_ALIGNMENT(MODE)   \
  MIN (BIGGEST_ALIGNMENT, 	   \
       MAX (1, (GET_MODE_UNIT_SIZE (MODE) * BITS_PER_UNIT)))

static enum machine_mode smallest_mode_for_size  PROTO((unsigned int,
							enum mode_class));
static tree layout_record	PROTO((tree));
static void layout_union	PROTO((tree));

/* SAVE_EXPRs for sizes of types and decls, waiting to be expanded.  */

static tree pending_sizes;

/* Nonzero means cannot safely call expand_expr now,
   so put variable sizes onto `pending_sizes' instead.  */

int immediate_size_expand;

tree
get_pending_sizes ()
{
  tree chain = pending_sizes;
  tree t;

  /* Put each SAVE_EXPR into the current function.  */
  for (t = chain; t; t = TREE_CHAIN (t))
    SAVE_EXPR_CONTEXT (TREE_VALUE (t)) = current_function_decl;
  pending_sizes = 0;
  return chain;
}

void
put_pending_sizes (chain)
     tree chain;
{
  if (pending_sizes)
    abort ();

  pending_sizes = chain;
}

/* Given a size SIZE that may not be a constant, return a SAVE_EXPR
   to serve as the actual size-expression for a type or decl.  */

tree
variable_size (size)
     tree size;
{
  /* If the language-processor is to take responsibility for variable-sized
     items (e.g., languages which have elaboration procedures like Ada),
     just return SIZE unchanged.  Likewise for self-referential sizes.  */
  if (TREE_CONSTANT (size)
      || global_bindings_p () < 0 || contains_placeholder_p (size))
    return size;

  size = save_expr (size);

  if (global_bindings_p ())
    {
      if (TREE_CONSTANT (size))
	error ("type size can't be explicitly evaluated");
      else
	error ("variable-size type declared outside of any function");

      return size_int (1);
    }

  if (immediate_size_expand)
    /* NULL_RTX is not defined; neither is the rtx type. 
       Also, we would like to pass const0_rtx here, but don't have it.  */
    expand_expr (size, expand_expr (integer_zero_node, NULL_PTR, VOIDmode, 0),
		 VOIDmode, 0);
  else
    pending_sizes = tree_cons (NULL_TREE, size, pending_sizes);

  return size;
}

#ifndef MAX_FIXED_MODE_SIZE
#define MAX_FIXED_MODE_SIZE GET_MODE_BITSIZE (DImode)
#endif

/* Return the machine mode to use for a nonscalar of SIZE bits.
   The mode must be in class CLASS, and have exactly that many bits.
   If LIMIT is nonzero, modes of wider than MAX_FIXED_MODE_SIZE will not
   be used.  */

enum machine_mode
mode_for_size (size, class, limit)
     unsigned int size;
     enum mode_class class;
     int limit;
{
  register enum machine_mode mode;

  if (limit && size > MAX_FIXED_MODE_SIZE)
    return BLKmode;

  /* Get the first mode which has this size, in the specified class.  */
  for (mode = GET_CLASS_NARROWEST_MODE (class); mode != VOIDmode;
       mode = GET_MODE_WIDER_MODE (mode))
    if (GET_MODE_BITSIZE (mode) == size)
      return mode;

  return BLKmode;
}

/* Similar, but never return BLKmode; return the narrowest mode that
   contains at least the requested number of bits.  */

static enum machine_mode
smallest_mode_for_size (size, class)
     unsigned int size;
     enum mode_class class;
{
  register enum machine_mode mode;

  /* Get the first mode which has at least this size, in the
     specified class.  */
  for (mode = GET_CLASS_NARROWEST_MODE (class); mode != VOIDmode;
       mode = GET_MODE_WIDER_MODE (mode))
    if (GET_MODE_BITSIZE (mode) >= size)
      return mode;

  abort ();
}

/* Return the value of VALUE, rounded up to a multiple of DIVISOR.  */

tree
round_up (value, divisor)
     tree value;
     int divisor;
{
  return size_binop (MULT_EXPR,
		     size_binop (CEIL_DIV_EXPR, value, size_int (divisor)),
		     size_int (divisor));
}

/* Set the size, mode and alignment of a ..._DECL node.
   TYPE_DECL does need this for C++.
   Note that LABEL_DECL and CONST_DECL nodes do not need this,
   and FUNCTION_DECL nodes have them set up in a special (and simple) way.
   Don't call layout_decl for them.

   KNOWN_ALIGN is the amount of alignment we can assume this
   decl has with no special effort.  It is relevant only for FIELD_DECLs
   and depends on the previous fields.
   All that matters about KNOWN_ALIGN is which powers of 2 divide it.
   If KNOWN_ALIGN is 0, it means, "as much alignment as you like":
   the record will be aligned to suit.  */

void
layout_decl (decl, known_align)
     tree decl;
     unsigned known_align;
{
  register tree type = TREE_TYPE (decl);
  register enum tree_code code = TREE_CODE (decl);
  int spec_size = DECL_FIELD_SIZE (decl);

  if (code == CONST_DECL)
    return;

  if (code != VAR_DECL && code != PARM_DECL && code != RESULT_DECL
      && code != FIELD_DECL && code != TYPE_DECL)
    abort ();

  if (type == error_mark_node)
    {
      type = void_type_node;
      spec_size = 0;
    }

  /* Usually the size and mode come from the data type without change.  */

  DECL_MODE (decl) = TYPE_MODE (type);
  TREE_UNSIGNED (decl) = TREE_UNSIGNED (type);
  if (DECL_SIZE (decl) == 0)
    DECL_SIZE (decl) = TYPE_SIZE (type);

  if (code == FIELD_DECL && DECL_BIT_FIELD (decl))
    {
      if (spec_size == 0 && DECL_NAME (decl) != 0)
	abort ();

      /* Size is specified number of bits.  */
      DECL_SIZE (decl) = size_int (spec_size);
    }
  /* Force alignment required for the data type.
     But if the decl itself wants greater alignment, don't override that.
     Likewise, if the decl is packed, don't override it.  */
  else if (DECL_ALIGN (decl) == 0
	   || (! DECL_PACKED (decl) &&  TYPE_ALIGN (type) > DECL_ALIGN (decl)))
    DECL_ALIGN (decl) = TYPE_ALIGN (type);

  /* See if we can use an ordinary integer mode for a bit-field.  */
  /* Conditions are: a fixed size that is correct for another mode
     and occupying a complete byte or bytes on proper boundary.  */
  if (code == FIELD_DECL)
    {
      DECL_BIT_FIELD_TYPE (decl) = DECL_BIT_FIELD (decl) ? type : 0;
      if (maximum_field_alignment != 0)
	DECL_ALIGN (decl) = MIN (DECL_ALIGN (decl), maximum_field_alignment);
      else if (flag_pack_struct)
	DECL_ALIGN (decl) = MIN (DECL_ALIGN (decl), BITS_PER_UNIT);
    }

  if (DECL_BIT_FIELD (decl)
      && TYPE_SIZE (type) != 0
      && TREE_CODE (TYPE_SIZE (type)) == INTEGER_CST)
    {
      register enum machine_mode xmode
	= mode_for_size (TREE_INT_CST_LOW (DECL_SIZE (decl)), MODE_INT, 1);

      if (xmode != BLKmode
	  && known_align % GET_MODE_ALIGNMENT (xmode) == 0)
	{
	  DECL_ALIGN (decl) = MAX (GET_MODE_ALIGNMENT (xmode),
				   DECL_ALIGN (decl));
	  DECL_MODE (decl) = xmode;
	  DECL_SIZE (decl) = size_int (GET_MODE_BITSIZE (xmode));
	  /* This no longer needs to be accessed as a bit field.  */
	  DECL_BIT_FIELD (decl) = 0;
	}
    }

  /* Evaluate nonconstant size only once, either now or as soon as safe.  */
  if (DECL_SIZE (decl) != 0 && TREE_CODE (DECL_SIZE (decl)) != INTEGER_CST)
    DECL_SIZE (decl) = variable_size (DECL_SIZE (decl));
}

/* Lay out a RECORD_TYPE type (a C struct).
   This means laying out the fields, determining their positions,
   and computing the overall size and required alignment of the record.
   Note that if you set the TYPE_ALIGN before calling this
   then the struct is aligned to at least that boundary.

   If the type has basetypes, you must call layout_basetypes
   before calling this function.

   The return value is a list of static members of the record.
   They still need to be laid out.  */

static tree
layout_record (rec)
     tree rec;
{
  register tree field;
#ifdef STRUCTURE_SIZE_BOUNDARY
  unsigned record_align = MAX (STRUCTURE_SIZE_BOUNDARY, TYPE_ALIGN (rec));
#else
  unsigned record_align = MAX (BITS_PER_UNIT, TYPE_ALIGN (rec));
#endif
  /* These must be laid out *after* the record is.  */
  tree pending_statics = NULL_TREE;
  /* Record size so far is CONST_SIZE + VAR_SIZE bits,
     where CONST_SIZE is an integer
     and VAR_SIZE is a tree expression.
     If VAR_SIZE is null, the size is just CONST_SIZE.
     Naturally we try to avoid using VAR_SIZE.  */
  register int const_size = 0;
  register tree var_size = 0;
  /* Once we start using VAR_SIZE, this is the maximum alignment
     that we know VAR_SIZE has.  */
  register int var_align = BITS_PER_UNIT;


  for (field = TYPE_FIELDS (rec); field; field = TREE_CHAIN (field))
    {
      register int known_align = var_size ? var_align : const_size;
      register int desired_align;

      /* If FIELD is static, then treat it like a separate variable,
	 not really like a structure field.
	 If it is a FUNCTION_DECL, it's a method.
	 In both cases, all we do is lay out the decl,
	 and we do it *after* the record is laid out.  */

      if (TREE_STATIC (field))
	{
	  pending_statics = tree_cons (NULL_TREE, field, pending_statics);
	  continue;
	}
      /* Enumerators and enum types which are local to this class need not
	 be laid out.  Likewise for initialized constant fields.  */
      if (TREE_CODE (field) != FIELD_DECL)
	continue;

      /* Lay out the field so we know what alignment it needs.
	 For a packed field, use the alignment as specified,
	 disregarding what the type would want.  */
      if (DECL_PACKED (field))
	desired_align = DECL_ALIGN (field);
      layout_decl (field, known_align);
      if (! DECL_PACKED (field))
	desired_align = DECL_ALIGN (field);
      /* Some targets (i.e. VMS) limit struct field alignment
	 to a lower boundary than alignment of variables.  */
#ifdef BIGGEST_FIELD_ALIGNMENT
      desired_align = MIN (desired_align, BIGGEST_FIELD_ALIGNMENT);
#endif

      /* Record must have at least as much alignment as any field.
	 Otherwise, the alignment of the field within the record
	 is meaningless.  */

#ifndef PCC_BITFIELD_TYPE_MATTERS
      record_align = MAX (record_align, desired_align);
#else
      if (PCC_BITFIELD_TYPE_MATTERS && TREE_TYPE (field) != error_mark_node
	  && DECL_BIT_FIELD_TYPE (field)
	  && ! integer_zerop (TYPE_SIZE (TREE_TYPE (field))))
	{
	  /* For these machines, a zero-length field does not
	     affect the alignment of the structure as a whole.
	     It does, however, affect the alignment of the next field
	     within the structure.  */
	  if (! integer_zerop (DECL_SIZE (field)))
	    record_align = MAX (record_align, desired_align);
	  else if (! DECL_PACKED (field))
	    desired_align = TYPE_ALIGN (TREE_TYPE (field));
	  /* A named bit field of declared type `int'
	     forces the entire structure to have `int' alignment.  */
	  if (DECL_NAME (field) != 0)
	    {
	      int type_align = TYPE_ALIGN (TREE_TYPE (field));
	      if (maximum_field_alignment != 0)
		type_align = MIN (type_align, maximum_field_alignment);
	      else if (flag_pack_struct)
		type_align = MIN (type_align, BITS_PER_UNIT);

	      record_align = MAX (record_align, type_align);
	    }
	}
      else
	record_align = MAX (record_align, desired_align);
#endif

      /* Does this field automatically have alignment it needs
	 by virtue of the fields that precede it and the record's
	 own alignment?  */

      if (const_size % desired_align != 0
	  || (var_align % desired_align != 0
	      && var_size != 0))
	{
	  /* No, we need to skip space before this field.
	     Bump the cumulative size to multiple of field alignment.  */

	  if (var_size == 0
	      || var_align % desired_align == 0)
	    const_size
	      = CEIL (const_size, desired_align) * desired_align;
	  else
	    {
	      if (const_size > 0)
		var_size = size_binop (PLUS_EXPR, var_size,
				       size_int (const_size));
	      const_size = 0;
	      var_size = round_up (var_size, desired_align);
	      var_align = MIN (var_align, desired_align);
	    }