revise comments in libguile/tags.h

[bpt/guile.git] / libguile / tags.h

/* classes: h_files */

#ifndef SCM_TAGS_H
#define SCM_TAGS_H

/* Copyright (C) 1995,1996,1997,1998,1999,2000,2001,2002,2003,2004,2008,2009,2010,2011
 * Free Software Foundation, Inc.
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public License
 * as published by the Free Software Foundation; either version 3 of
 * the License, or (at your option) any later version.
 *
 * This library 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
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
 * 02110-1301 USA
 */

\f

/** This file defines the format of SCM values and cons pairs.
 ** It is here that tag bits are assigned for various purposes.
 **/

/* picks up scmconfig.h too */
#include "libguile/__scm.h"

\f

/* In the beginning was the Word:
 *
 * For the representation of scheme objects and their handling, Guile provides
 * two types:  scm_t_bits and SCM.
 *
 * - scm_t_bits values can hold bit patterns of non-objects and objects:
 *
 *   Non-objects -- in this case the value may not be changed into a SCM value
 *   in any way.
 *
 *   Objects -- in this case the value may be changed into a SCM value using
 *   the SCM_PACK macro.
 *
 * - SCM values can hold proper scheme objects only.  They can be changed into
 *   a scm_t_bits value using the SCM_UNPACK macro.
 *
 * When working in the domain of scm_t_bits values, programmers must keep
 * track of any scm_t_bits value they create that is not a proper scheme
 * object.  This makes sure that in the domain of SCM values developers can
 * rely on the fact that they are dealing with proper scheme objects only.
 * Thus, the distinction between scm_t_bits and SCM values helps to identify
 * those parts of the code where special care has to be taken not to create
 * bad SCM values.
 */

/* For dealing with the bit level representation of scheme objects we define
 * scm_t_bits:
 */

typedef scm_t_intptr  scm_t_signed_bits;
typedef scm_t_uintptr scm_t_bits;

#define SCM_T_SIGNED_BITS_MAX SCM_T_INTPTR_MAX
#define SCM_T_SIGNED_BITS_MIN SCM_T_INTPTR_MIN
#define SCM_T_BITS_MAX        SCM_T_UINTPTR_MAX


/* But as external interface, we define SCM, which may, according to the
 * desired level of type checking, be defined in several ways:
 */
#if (SCM_DEBUG_TYPING_STRICTNESS == 2)
typedef union SCM { struct { scm_t_bits n; } n; } SCM;
#   define SCM_UNPACK(x) ((x).n.n)
#   define SCM_PACK(x) ((SCM) { { (scm_t_bits) (x) } })
#elif (SCM_DEBUG_TYPING_STRICTNESS == 1)
/* This is the default, which provides an intermediate level of compile time
 * type checking while still resulting in very efficient code.
 */
    typedef struct scm_unused_struct { char scm_unused_field; } *SCM;

/*
  The 0?: constructions makes sure that the code is never executed,
  and that there is no performance hit.  However, the alternative is
  compiled, and does generate a warning when used with the wrong
  pointer type.

  The Tru64 and ia64-hp-hpux11.23 compilers fail on `case (0?0=0:x)'
  statements, so for them type-checking is disabled.  */
#if defined __DECC || defined __HP_cc
#   define SCM_UNPACK(x) ((scm_t_bits) (x))
#else
#   define SCM_UNPACK(x) ((scm_t_bits) (0? (*(SCM*)0=(x)): x))
#endif

/*
  There is no typechecking on SCM_PACK, since all kinds of types
  (unsigned long, void*) go in SCM_PACK
 */
#   define SCM_PACK(x) ((SCM) (x))

#else
/* This should be used as a fall back solution for machines on which casting
 * to a pointer may lead to loss of bit information, e. g. in the three least
 * significant bits.
 */
    typedef scm_t_bits SCM;
#   define SCM_UNPACK(x) (x)
#   define SCM_PACK(x) ((SCM) (x))
#endif


/* SCM values can not be compared by using the operator ==.  Use the following
 * macro instead, which is the equivalent of the scheme predicate 'eq?'.
 */
#define scm_is_eq(x, y) (SCM_UNPACK (x) == SCM_UNPACK (y))

\f

/* Representation of scheme objects:
 *
 * Guile's type system is designed to work on systems where scm_t_bits
 * and SCM variables consist of at least 32 bits.  The objects that a
 * SCM variable can represent belong to one of the following two major
 * categories:
 *
 * - Immediates -- meaning that the SCM variable contains an entire
 *   Scheme object.  That means, all the object's data (including the
 *   type tagging information that is required to identify the object's
 *   type) must fit into 32 bits.
 *
 * - Heap objects -- meaning that the SCM variable holds a pointer into
 *   the heap.  On systems where a pointer needs more than 32 bits this
 *   means that scm_t_bits and SCM variables need to be large enough to
 *   hold such pointers.  In contrast to immediates, the data associated
 *   with a heap object can consume arbitrary amounts of memory.
 *
 * The 'heap' is the memory area that is under control of Guile's
 * garbage collector.  It holds allocated memory of various sizes.  The
 * impact on the runtime type system is that Guile needs to be able to
 * determine the type of an object given the pointer.  Usually the way
 * that Guile does this is by storing a "type tag" in the first word of
 * the object.
 *
 * Some objects are common enough that they get special treatment.
 * Since Guile guarantees that the address of a GC-allocated object on
 * the heap is 8-byte aligned, Guile can play tricks with the lower 3
 * bits.  That is, since heap objects encode a pointer to an
 * 8-byte-aligned pointer, the three least significant bits of a SCM can
 * be used to store additional information.  The bits are used to store
 * information about the object's type and thus are called tc3-bits,
 * where tc stands for type-code.
 *
 * For a given SCM value, the distinction whether it holds an immediate
 * or heap object is based on the tc3-bits (see above) of its scm_t_bits
 * equivalent: If the tc3-bits equal #b000, then the SCM value holds a
 * heap object, and the scm_t_bits variable's value is just the pointer
 * to the heap cell.
 *
 * Summarized, the data of a scheme object that is represented by a SCM
 * variable consists of a) the SCM variable itself, b) in case of heap
 * objects memory that the SCM object points to, c) in case of heap
 * objects potentially additional data outside of the heap (like for
 * example malloc'ed data), and d) in case of heap objects potentially
 * additional data inside of the heap, since data stored in b) and c)
 * may hold references to other cells.
 *
 *
 * Immediates
 *
 * Operations on immediate objects can typically be processed faster than on
 * heap objects.  The reason is that the object's data can be extracted
 * directly from the SCM variable (or rather a corresponding scm_t_bits
 * variable), instead of having to perform additional memory accesses to
 * obtain the object's data from the heap.  In order to get the best possible
 * performance frequently used data types should be realized as immediates.
 * This is, as has been mentioned above, only possible if the objects can be
 * represented with 32 bits (including type tagging).
 *
 * In Guile, the following data types and special objects are realized as
 * immediates: booleans, characters, small integers (see below), the empty
 * list, the end of file object, the 'unspecified' object (which is delivered
 * as a return value by functions for which the return value is unspecified),
 * a 'nil' object used in the elisp-compatibility mode and certain other
 * 'special' objects which are only used internally in Guile.
 *
 * Integers in Guile can be arbitrarily large.  On the other hand, integers
 * are one of the most frequently used data types.  Especially integers with
 * less than 32 bits are commonly used.  Thus, internally and transparently
 * for application code guile distinguishes between small and large integers.
 * Whether an integer is a large or a small integer depends on the number of
 * bits needed to represent its value.  Small integers are those which can be
 * represented as immediates.  Since they don't require more than a fixed
 * number of bits for their representation, they are also known as 'fixnums'.
 *
 * The tc3-combinations #b010 and #b110 are used to represent small integers,
 * which allows to use the most significant bit of the tc3-bits to be part of
 * the integer value being represented.  This means that all integers with up
 * to 30 bits (including one bit for the sign) can be represented as
 * immediates.  On systems where SCM and scm_t_bits variables hold more than
 * 32 bits, the amount of bits usable for small integers will even be larger.
 * The tc3-code #b100 is shared among booleans, characters and the other
 * special objects listed above.
 *
 *
 * Heap Objects
 *
 * All object types not mentioned above in the list of immedate objects
 * are represented as heap objects.  The amount of memory referenced by
 * a heap object depends on the object's type, namely on the set of
 * attributes that have to be stored with objects of that type.  Every
 * heap object type is allowed to define its own layout and
 * interpretation of the data stored in its cell (with some
 * restrictions, see below).
 *
 * One of the design goals of guile's type system is to make it possible
 * to store a scheme pair with as little memory usage as possible.  The
 * minimum amount of memory that is required to store two scheme objects
 * (car and cdr of a pair) is the amount of memory required by two
 * scm_t_bits or SCM variables.  Therefore pairs in guile are stored in
 * two words, and are tagged with a bit pattern in the SCM value, not
 * with a type tag on the heap.
 *
 *
 * Garbage collection
 *
 * During garbage collection, unreachable objects on the heap will be
 * freed.  To determine the set of reachable objects, by default, the GC
 * just traces all words in all heap objects.  It is possible to
 * register custom tracing ("marking") procedures.
 *
 * If an object is unreachable, by default, the GC just notes this fact
 * and moves on.  Later allocations will clear out the memory associated
 * with the object, and re-use it.  It is possible to register custom
 * finalizers, however.
 *
 *
 * Run-time type introspection
 *
 * Guile's type system is designed to make it possible to determine a
 * the type of a heap object from the object's first scm_t_bits
 * variable.  (Given a SCM variable X holding a heap object, the macro
 * SCM_CELL_TYPE(X) will deliver the corresponding object's first
 * scm_t_bits variable.)
 *
 * If the object holds a scheme pair, then we already know that the
 * first scm_t_bits variable of the cell will hold a scheme object with
 * one of the following tc3-codes: #b000 (heap object), #b010 (small
 * integer), #b110 (small integer), #b100 (non-integer immediate).  All
 * these tc3-codes have in common, that their least significant bit is
 * #b0.  This fact is used by the garbage collector to identify cells
 * that hold pairs.  The remaining tc3-codes are assigned as follows:
 * #b001 (class instance or, more precisely, a struct, of which a class
 * instance is a special case), #b011 (closure), #b101/#b111 (all
 * remaining heap object types).
 *
 *
 * Summary of type codes of scheme objects (SCM variables)
 *
 * Here is a summary of tagging bits as they might occur in a scheme object.
 * The notation is as follows: tc stands for type code as before, tc<n> with n
 * being a number indicates a type code formed by the n least significant bits
 * of the SCM variables corresponding scm_t_bits value.
 *
 * Note that (as has been explained above) tc1==1 can only occur in the first
 * scm_t_bits variable of a cell belonging to a heap object that is
 * not a pair.  For an explanation of the tc tags with tc1==1, see the next
 * section with the summary of the type codes on the heap.
 *
 * tc1:
 *   0:  For scheme objects, tc1==0 must be fulfilled.
 *  (1:  This can never be the case for a scheme object.)
 *
 * tc2:
 *   00:  Either a heap object or some non-integer immediate
 *  (01:  This can never be the case for a scheme object.)
 *   10:  Small integer
 *  (11:  This can never be the case for a scheme object.)
 *
 * tc3:
 *   000:  a heap object (pair, closure, class instance etc.)
 *  (001:  This can never be the case for a scheme object.)
 *   010:  an even small integer (least significant bit is 0).
 *  (011:  This can never be the case for a scheme object.)
 *   100:  Non-integer immediate
 *  (101:  This can never be the case for a scheme object.)
 *   110:  an odd small integer (least significant bit is 1).
 *  (111:  This can never be the case for a scheme object.)
 *
 * The remaining bits of the heap objects form the pointer to the heap
 * cell.  The remaining bits of the small integers form the integer's
 * value and sign.  Thus, the only scheme objects for which a further
 * subdivision is of interest are the ones with tc3==100.
 *
 * tc8 (for objects with tc3==100):
 *   00000-100:  special objects ('flags')
 *   00001-100:  characters
 *   00010-100:  unused
 *   00011-100:  unused
 *
 *
 * Summary of type codes on the heap
 *
 * Here is a summary of tagging in scm_t_bits values as they might occur in
 * the first scm_t_bits variable of a heap cell.
 *
 * tc1:
 *   0:  the cell belongs to a pair.
 *   1:  the cell belongs to a non-pair.
 *
 * tc2:
 *   00:  the cell belongs to a pair with no short integer in its car.
 *   01:  the cell belongs to a non-pair (struct or some other heap object).
 *   10:  the cell belongs to a pair with a short integer in its car.
 *   11:  the cell belongs to a non-pair (closure or some other heap object).
 *
 * tc3:
 *   000:  the cell belongs to a pair with a heap object in its car.
 *   001:  the cell belongs to a struct
 *   010:  the cell belongs to a pair with an even short integer in its car.
 *   011:  the cell belongs to a closure
 *   100:  the cell belongs to a pair with a non-integer immediate in its car.
 *   101:  the cell belongs to some other heap object.
 *   110:  the cell belongs to a pair with an odd short integer in its car.
 *   111:  the cell belongs to some other heap object.
 *
 * tc7 (for tc3==1x1):
 *   See below for the list of types.  Note the special case of scm_tc7_vector
 *   and scm_tc7_wvect:  vectors and weak vectors are treated the same in many
 *   cases.  Thus, their tc7-codes are chosen to only differ in one bit.  This
 *   makes it possible to check an object at the same time for being a vector
 *   or a weak vector by comparing its tc7 code with that bit masked (using
 *   the TYP7S macro).  Three more special tc7-codes are of interest:
 *   numbers, ports and smobs in fact each represent collections of types,
 *   which are subdivided using tc16-codes.
 *
 * tc16 (for tc7==scm_tc7_smob):
 *   The largest part of the space of smob types is not subdivided in a
 *   predefined way, since smobs can be added arbitrarily by user C code.
 */

\f

/* Checking if a SCM variable holds an immediate or a heap object:
 * This check can either be performed by checking for tc3==000 or tc3==00x,
 * since for a SCM variable it is known that tc1==0.  */
#define SCM_IMP(x)              (6 & SCM_UNPACK (x))
#define SCM_NIMP(x)             (!SCM_IMP (x))

/* Checking if a SCM variable holds an immediate integer: See numbers.h for
 * the definition of the following macros: SCM_I_FIXNUM_BIT,
 * SCM_MOST_POSITIVE_FIXNUM, SCM_I_INUMP, SCM_I_MAKINUM, SCM_I_INUM.  */

/* Checking if a SCM variable holds a pair (for historical reasons, in Guile
 * also known as a cons-cell): This is done by first checking that the SCM
 * variable holds a heap object, and second, by checking that tc1==0 holds
 * for the SCM_CELL_TYPE of the SCM variable.  
*/

#define SCM_I_CONSP(x)  (!SCM_IMP (x) && ((1 & SCM_CELL_TYPE (x)) == 0))

\f

/* Definitions for tc2: */

#define scm_tc2_int              2


/* Definitions for tc3: */

#define SCM_ITAG3(x)             (7 & SCM_UNPACK (x))
#define SCM_TYP3(x)              (7 & SCM_CELL_TYPE (x))

#define scm_tc3_cons             0
#define scm_tc3_struct           1
#define scm_tc3_int_1            (scm_tc2_int + 0)
#define scm_tc3_unused           3
#define scm_tc3_imm24            4
#define scm_tc3_tc7_1            5
#define scm_tc3_int_2            (scm_tc2_int + 4)
#define scm_tc3_tc7_2            7


/* Definitions for tc7: */

#define SCM_ITAG7(x)            (127 & SCM_UNPACK (x))
#define SCM_TYP7(x)             (0x7f &        SCM_CELL_TYPE (x))
#define SCM_TYP7S(x)            ((0x7f & ~2) & SCM_CELL_TYPE (x))

#define scm_tc7_symbol          5
#define scm_tc7_variable        7

/* couple */
#define scm_tc7_vector          13
#define scm_tc7_wvect           15

#define scm_tc7_string          21
#define scm_tc7_number          23
#define scm_tc7_stringbuf       39
#define scm_tc7_bytevector      77

#define scm_tc7_pointer         31
#define scm_tc7_hashtable       29
#define scm_tc7_fluid           37
#define scm_tc7_dynamic_state   45

#define scm_tc7_frame           47
#define scm_tc7_objcode         53
#define scm_tc7_vm              55
#define scm_tc7_vm_cont         71

#define scm_tc7_prompt          61
#define scm_tc7_with_fluids     63
#define scm_tc7_unused_19       69
#define scm_tc7_program         79
#define scm_tc7_unused_9        85
#define scm_tc7_unused_10       87
#define scm_tc7_unused_20       93
#define scm_tc7_unused_11       95
#define scm_tc7_unused_12       101
#define scm_tc7_unused_18       103
#define scm_tc7_unused_13       109
#define scm_tc7_unused_14       111
#define scm_tc7_unused_15       117
#define scm_tc7_unused_16       119

/* There are 256 port subtypes.  */
#define scm_tc7_port            125

/* There are 256 smob subtypes.  [**] If you change scm_tc7_smob, you must
 * also change the places it is hard coded in this file and possibly others.
 * Dirk:FIXME:: Any hard coded reference to scm_tc7_smob must be replaced by a
 * symbolic reference.  */
#define scm_tc7_smob            127 /* DO NOT CHANGE [**] */


/* Definitions for tc16: */
#define SCM_TYP16(x)            (0xffff & SCM_CELL_TYPE (x))
#define SCM_TYP16_PREDICATE(tag, x) (!SCM_IMP (x) && SCM_TYP16 (x) == (tag))


\f

/* {Immediate Values}
 */

enum scm_tc8_tags
{
  scm_tc8_flag = scm_tc3_imm24 + 0x00,  /* special objects ('flags') */
  scm_tc8_char = scm_tc3_imm24 + 0x08,  /* characters */
  scm_tc8_unused_0 = scm_tc3_imm24 + 0x10,
  scm_tc8_unused_1 = scm_tc3_imm24 + 0x18
};

#define SCM_ITAG8(X)            (SCM_UNPACK (X) & 0xff)
#define SCM_MAKE_ITAG8_BITS(X, TAG) (((X) << 8) + TAG)
#define SCM_MAKE_ITAG8(X, TAG)  (SCM_PACK (SCM_MAKE_ITAG8_BITS (X, TAG)))
#define SCM_ITAG8_DATA(X)       (SCM_UNPACK (X) >> 8)

\f

/* Flags (special objects).  The indices of the flags must agree with the
 * declarations in print.c: iflagnames.  */

#define SCM_IFLAGP(n)    (SCM_ITAG8 (n) == scm_tc8_flag)
#define SCM_MAKIFLAG_BITS(n)  (SCM_MAKE_ITAG8_BITS ((n), scm_tc8_flag))
#define SCM_IFLAGNUM(n)  (SCM_ITAG8_DATA (n))

/*
 * IMPORTANT NOTE regarding IFLAG numbering!!!
 *
 * Several macros depend upon careful IFLAG numbering of SCM_BOOL_F,
 * SCM_BOOL_T, SCM_ELISP_NIL, SCM_EOL, and the two SCM_XXX_*_DONT_USE
 * constants.  In particular:
 *
 * - SCM_BOOL_F and SCM_BOOL_T must differ in exactly one bit position.
 *   (used to implement scm_is_bool_and_not_nil, aka scm_is_bool)
 *
 * - SCM_ELISP_NIL and SCM_BOOL_F must differ in exactly one bit position.
 *   (used to implement scm_is_false_or_nil and
 *    scm_is_true_and_not_nil)
 *
 * - SCM_ELISP_NIL and SCM_EOL must differ in exactly one bit position.
 *   (used to implement scm_is_null_or_nil)
 *
 * - SCM_ELISP_NIL, SCM_BOOL_F, SCM_EOL, SCM_XXX_ANOTHER_LISP_FALSE_DONT_USE
 *   must all be equal except for two bit positions.
 *   (used to implement scm_is_lisp_false)
 *
 * - SCM_ELISP_NIL, SCM_BOOL_F, SCM_BOOL_T, SCM_XXX_ANOTHER_BOOLEAN_DONT_USE_0
 *   must all be equal except for two bit positions.
 *   (used to implement scm_is_bool_or_nil)
 *
 * These properties allow the aforementioned macros to be implemented
 * by bitwise ANDing with a mask and then comparing with a constant,
 * using as a common basis the macro SCM_MATCHES_BITS_IN_COMMON,
 * defined below.  The properties are checked at compile-time using
 * `verify' macros near the top of boolean.c and pairs.c.
 */
#define SCM_BOOL_F_BITS         SCM_MAKIFLAG_BITS (0)
#define SCM_ELISP_NIL_BITS      SCM_MAKIFLAG_BITS (1)

#define SCM_BOOL_F              SCM_PACK (SCM_BOOL_F_BITS)
#define SCM_ELISP_NIL           SCM_PACK (SCM_ELISP_NIL_BITS)

#ifdef BUILDING_LIBGUILE
#define SCM_XXX_ANOTHER_LISP_FALSE_DONT_USE     SCM_MAKIFLAG_BITS (2)
#endif

#define SCM_EOL_BITS            SCM_MAKIFLAG_BITS (3)
#define SCM_BOOL_T_BITS         SCM_MAKIFLAG_BITS (4)

#define SCM_EOL                 SCM_PACK (SCM_EOL_BITS)
#define SCM_BOOL_T              SCM_PACK (SCM_BOOL_T_BITS)

#ifdef BUILDING_LIBGUILE
#define SCM_XXX_ANOTHER_BOOLEAN_DONT_USE_0      SCM_MAKIFLAG_BITS (5)
#define SCM_XXX_ANOTHER_BOOLEAN_DONT_USE_1      SCM_MAKIFLAG_BITS (6)
#define SCM_XXX_ANOTHER_BOOLEAN_DONT_USE_2      SCM_MAKIFLAG_BITS (7)
#endif

#define SCM_UNSPECIFIED_BITS    SCM_MAKIFLAG_BITS (8)
#define SCM_UNDEFINED_BITS      SCM_MAKIFLAG_BITS (9)
#define SCM_EOF_VAL_BITS        SCM_MAKIFLAG_BITS (10)

#define SCM_UNSPECIFIED         SCM_PACK (SCM_UNSPECIFIED_BITS)
#define SCM_UNDEFINED           SCM_PACK (SCM_UNDEFINED_BITS)
#define SCM_EOF_VAL             SCM_PACK (SCM_EOF_VAL_BITS)

/* When a variable is unbound this is marked by the SCM_UNDEFINED
 * value.  The following is an unbound value which can be handled on
 * the Scheme level, i.e., it can be stored in and retrieved from a
 * Scheme variable.  This value is only intended to mark an unbound
 * slot in GOOPS.  It is needed now, but we should probably rewrite
 * the code which handles this value in C so that SCM_UNDEFINED can be
 * used instead.  It is not ideal to let this kind of unique and
 * strange values loose on the Scheme level.  */
#define SCM_UNBOUND_BITS        SCM_MAKIFLAG_BITS (11)
#define SCM_UNBOUND             SCM_PACK (SCM_UNBOUND_BITS)

#define SCM_UNBNDP(x)           (scm_is_eq ((x), SCM_UNDEFINED))

/*
 * SCM_MATCHES_BITS_IN_COMMON(x,a,b) returns 1 if and only if x
 * matches both a and b in every bit position where a and b are equal;
 * otherwise it returns 0.  Bit positions where a and b differ are
 * ignored.
 *
 * This is used to efficiently compare against two values which differ
 * in exactly one bit position, or against four values which differ in
 * exactly two bit positions.  It is the basis for the following
 * macros:
 *
 *   scm_is_null_or_nil,
 *   scm_is_false_or_nil,
 *   scm_is_true_and_not_nil,
 *   scm_is_lisp_false,
 *   scm_is_lisp_true,
 *   scm_is_bool_and_not_nil (aka scm_is_bool)
 *   scm_is_bool_or_nil.
 */
#define SCM_MATCHES_BITS_IN_COMMON(x,a,b)                               \
  ((SCM_UNPACK(x) & ~(SCM_UNPACK(a) ^ SCM_UNPACK(b))) ==                \
   (SCM_UNPACK(a) & SCM_UNPACK(b)))

/*
 * These macros are used for compile-time verification that the
 * constants have the properties needed for the above macro to work
 * properly.
 */
#ifdef BUILDING_LIBGUILE
#define SCM_WITH_LEAST_SIGNIFICANT_1_BIT_CLEARED(x)  ((x) & ((x)-1))
#define SCM_HAS_EXACTLY_ONE_BIT_SET(x)                                  \
  ((x) != 0 && SCM_WITH_LEAST_SIGNIFICANT_1_BIT_CLEARED (x) == 0)
#define SCM_HAS_EXACTLY_TWO_BITS_SET(x)                                 \
  (SCM_HAS_EXACTLY_ONE_BIT_SET (SCM_WITH_LEAST_SIGNIFICANT_1_BIT_CLEARED (x)))

#define SCM_BITS_DIFFER_IN_EXACTLY_ONE_BIT_POSITION(a,b)                \
  (SCM_HAS_EXACTLY_ONE_BIT_SET ((a) ^ (b)))
#define SCM_BITS_DIFFER_IN_EXACTLY_TWO_BIT_POSITIONS(a,b,c,d)           \
  (SCM_HAS_EXACTLY_TWO_BITS_SET (((a) ^ (b)) |                          \
                                 ((b) ^ (c)) |                          \
                                 ((c) ^ (d))))
#endif /* BUILDING_LIBGUILE */
\f

/* Dispatching aids:

   When switching on SCM_TYP7 of a SCM value, use these fake case
   labels to catch types that use fewer than 7 bits for tagging.  */

/* For cons pairs with immediate values in the CAR
 */

#define scm_tcs_cons_imcar \
       scm_tc2_int + 0:   case scm_tc2_int + 4:   case scm_tc3_imm24 + 0:\
  case scm_tc2_int + 8:   case scm_tc2_int + 12:  case scm_tc3_imm24 + 8:\
  case scm_tc2_int + 16:  case scm_tc2_int + 20:  case scm_tc3_imm24 + 16:\
  case scm_tc2_int + 24:  case scm_tc2_int + 28:  case scm_tc3_imm24 + 24:\
  case scm_tc2_int + 32:  case scm_tc2_int + 36:  case scm_tc3_imm24 + 32:\
  case scm_tc2_int + 40:  case scm_tc2_int + 44:  case scm_tc3_imm24 + 40:\
  case scm_tc2_int + 48:  case scm_tc2_int + 52:  case scm_tc3_imm24 + 48:\
  case scm_tc2_int + 56:  case scm_tc2_int + 60:  case scm_tc3_imm24 + 56:\
  case scm_tc2_int + 64:  case scm_tc2_int + 68:  case scm_tc3_imm24 + 64:\
  case scm_tc2_int + 72:  case scm_tc2_int + 76:  case scm_tc3_imm24 + 72:\
  case scm_tc2_int + 80:  case scm_tc2_int + 84:  case scm_tc3_imm24 + 80:\
  case scm_tc2_int + 88:  case scm_tc2_int + 92:  case scm_tc3_imm24 + 88:\
  case scm_tc2_int + 96:  case scm_tc2_int + 100: case scm_tc3_imm24 + 96:\
  case scm_tc2_int + 104: case scm_tc2_int + 108: case scm_tc3_imm24 + 104:\
  case scm_tc2_int + 112: case scm_tc2_int + 116: case scm_tc3_imm24 + 112:\
  case scm_tc2_int + 120: case scm_tc2_int + 124: case scm_tc3_imm24 + 120

/* For cons pairs with heap objects in the SCM_CAR
 */
#define scm_tcs_cons_nimcar \
       scm_tc3_cons + 0:\
  case scm_tc3_cons + 8:\
  case scm_tc3_cons + 16:\
  case scm_tc3_cons + 24:\
  case scm_tc3_cons + 32:\
  case scm_tc3_cons + 40:\
  case scm_tc3_cons + 48:\
  case scm_tc3_cons + 56:\
  case scm_tc3_cons + 64:\
  case scm_tc3_cons + 72:\
  case scm_tc3_cons + 80:\
  case scm_tc3_cons + 88:\
  case scm_tc3_cons + 96:\
  case scm_tc3_cons + 104:\
  case scm_tc3_cons + 112:\
  case scm_tc3_cons + 120

/* For structs
 */
#define scm_tcs_struct \
       scm_tc3_struct + 0:\
  case scm_tc3_struct + 8:\
  case scm_tc3_struct + 16:\
  case scm_tc3_struct + 24:\
  case scm_tc3_struct + 32:\
  case scm_tc3_struct + 40:\
  case scm_tc3_struct + 48:\
  case scm_tc3_struct + 56:\
  case scm_tc3_struct + 64:\
  case scm_tc3_struct + 72:\
  case scm_tc3_struct + 80:\
  case scm_tc3_struct + 88:\
  case scm_tc3_struct + 96:\
  case scm_tc3_struct + 104:\
  case scm_tc3_struct + 112:\
  case scm_tc3_struct + 120

\f

#endif  /* SCM_TAGS_H */

/*
  Local Variables:
  c-file-style: "gnu"
  End:
*/