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nanbox

A type that can store various types of data in 64-bits using NaN-boxing.

The header file nanbox.h defines a type nanbox_t which can be used to store either a double, a 32-bit integer, a pointer, a boolean, null or one of a few additional values named 'undefined', 'empty' and 'deleted' plus five additional 'auxillary' types of data of up to 48 bits. The encoding scheme differs between 32-bit and 64-bit platforms but the size of nanbox_t is always 64 bits.

How does it work?

NaN-boxing is a way to store various information in unused NaN-space in the IEEE754 representation.

Any value with the top 13 bits set represents a quiet NaN. The remaining bits are called the 'payload'. NaNs produced by hardware and C-library functions typically produce a payload of zero. We assume that all quiet NaNs with a non-zero payload can be used to encode whatever we want.

On 64-bit platforms, unused bits in pointers are also used to encode various information. The representation is inspired by that used by Webkit's JavaScriptCore. It should work on most 32-bit and 64-bit little endian and big endian machines. (See Testing below.)

Functions

A number of very short functions functions, all declared static inline, are defined to encode values as nanbox_t:

nanbox_t nanbox_from_double(double d);
nanbox_t nanbox_from_int(int32_t i);
nanbox_t nanbox_from_pointer(void* pointer);
nanbox_t nanbox_from_boolean(bool b);
nanbox_t nanbox_null(void);
nanbox_t nanbox_undefined(void);
nanbox_t nanbox_empty(void);
nanbox_t nanbox_deleted(void);
nanbox_t nanbox_true(void);   /* the same as nanbox_from_boolean(true) */
nanbox_t nanbox_false(void);  /* the same as nanbox_from_boolean(false) */

... to check the type:

bool nanbox_is_double(nanbox_t value);
bool nanbox_is_int(nanbox_t value);
bool nanbox_is_pointer(nanbox_t value);
bool nanbox_is_boolean(nanbox_t value);
bool nanbox_is_null(nanbox_t value);
bool nanbox_is_undefined(nanbox_t value);
bool nanbox_is_empty(nanbox_t value);
bool nanbox_is_deleted(nanbox_t value);
bool nanbox_is_true(nanbox_t value);
bool nanbox_is_false(nanbox_t value);
bool nanbox_is_number(nanbox_t value);  /* either int or double */
bool nanbox_is_undefined_or_null(nanbox_t value); /* either */
bool nanbox_is_aux(nanbox_t value);     /* auxillary space */

... and to decode the value:

double nanbox_to_double(nanbox_t value);
int32_t nanbox_to_int(nanbox_t value);
void* nanbox_to_pointer(nanbox_t value);
bool nanbox_to_boolean(nanbox_t value);
double nanbox_to_number(nanbox_t value); /* value can be int or double */

Before fetching the value using these functions, you should make sure the nanbox is holdig a value of the correct type, e.g. using the corresponding nanbox_is_... function. If the encoded value is not of the correct type, the results of the nanbox_to_... functions are undefined. If compiled with assertions, you will get a failed assertion when trying to fetch a value of the wrong type.

The 'empty' value

The 'empty' value is designed to used to represent empty slots in e.g. a hashtable. It is guarranteed to consist of a single repeated byte. This is to make sure memset can be used to set all the elements in an array of nanboxes to 'empty'. The macro NANBOX_EMPTY_BYTE represents the byte that, when repeated 8 times (64 bits), makes up an 'empty' value.

void foo(void) {
	nanbox_t boxes[100];
	// Initialize the boxes to empty values
	memset(boxes, NANBOX_EMPTY_BYTE, sizeof(nanbox_t) * 100);
	// ...
}

User-defined prefix instead of 'nanbox'

You can define NANBOX_PREFIX to the prefix you want, before including nanbox.h. Then, the functions and types will be e.g. bool myprefix_is_double(myprefix_t value), etc. By undefining NANBOX_H and redefining NANBOX_PREFIX (and possibly some of the other macros such as NANBOX_POINTER_TYPE) you can include nanbox.h multiple times to create multiple instances of nanbox type.

User-defined pointer type

When encoding and decoding pointers to/from a nanbox, the pointer type void* is used by default. This can be changed by defining NANBOX_POINTER_TYPE to the pointer type of choice, before including nanbox.h. The type must be a pointer type, because unused bits in the pointers are used to encode various data.

Auxillary data

Apart from doubles, pointers, ints, booleans, null, etc. there are still some bits left to store even more types of data in a nanbox. We call this 'auxillary space'. To check if the type of data in a nanbox is 'auxillary data', the function nanbox_is_aux can be used, but accessing the data itself requires some insight into the internal representation of the nanbox. nanbox_h is a union type, which means it can be accessed in multiple ways. The easiest way is to access the nanbox raw data is as a 64-bit integer using nanbox.as_int64. You can only store 64-bit integer value in the range NANBOX_MIN_AUX..NANBOX_MAX_AUX, which is the 'auxillary space'. You can store 5 * 248 distinct values in this range, or equivallently, 5 types of 48-bit values.

Another way to access the data is to use tag and payload. These each represent 32 bits of the nanbox data. If the a nanbox has its tag (nanbox.as_bits.tag) in the range NANBOX_MIN_AUX_TAG..NANBOX_MAX_AUX_TAG and a payload nanbox.as_bits.payload being any 32-bit integer value, then the nanbox data is in auxillary space.

Short strings

As an example of what auxillary data can be used for, the file nanbox_shortstring.h is included, which implements a scheme to store strings of up to 6 bytes in the auxillary space of a nanbox. The functions nanbox_is_shortstring, nanbox_shortstring_create, etc. are defined and a small demo program is included in shortstring_demo.c.

Testing

Tested on

  • x86-64 (Intel Core 2 Duo), Mac OS X (Darwin 10.0.8) in 64-bit and 32-bit mode, using gcc version 4.2.1 (Apple Inc. build 5664).

I would like to add more architectures to the above list, especially non-Intel ones such as ARM and big endian systems such as SPARC. If you test this on another architecture or with another compiler, please drop me a line!

To test with gcc, use the command gcc -std=c99 -Wall -pedantic -o test test.c to compile the test. It should produce no warnings. The executable test should run without outputting any errors.

On x86-64 platforms, it is also possible to test in 32-bit mode using the -m32 flag as in gcc -m32 -std=c99 -Wall -pedantic -o test test.c.

About

NaN-boxing in C (but not really NaN-boxing strictly speaking)

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