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apsun/nantag.c

Last active Feb 27, 2021
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NaN tagging demo (64-bit only)
/* 64-bit tagged NaN demo.
*
* 63 51 48
* v v v
* SEEEEEEEEEEEMMMMMMMMMMMMMMMMMM...
* ~~~~
*/
#include <stdint.h>
#include <assert.h>
#include <math.h>
/*
* Our box type.
*/
typedef union {
double dbl;
uint64_t raw;
} box_t;
/*
* NaN value means the box holds a pointer.
*/
#define box_is_pointer(b) (isnan(b.dbl))
/*
* To turn the pointer into a NaN value, we set all the exponent bits and
* (at least) one mantissa bit.
*/
#define box_set_pointer(b, v) (b.raw = (uint64_t)(v) | 0x7ff8000000000000UL)
/*
* To turn the NaN value back into a pointer, we mask off all the top bits.
* Note that this doesn't work if the 48th bit of the pointer is set, since
* x64 requires that the top 16 bits be copies of the 48th bit.
*/
#define box_get_pointer(b) ((void *)(b.raw & 0x0000ffffffffffffUL))
/*
* For double values, we just set/get the double field directly.
*/
#define box_set_double(b, v) (b.dbl = v)
#define box_get_double(b) (b.dbl)
int
main(void)
{
/* Basic sanity checks */
assert(sizeof(double) == 8);
assert(sizeof(void *) == 8);
assert(sizeof(box_t) == 8);
box_t b;
int x = 42, *px = &x;
double y = 3.14159;
void *z = (void *)0x0000123456789abc;
/* Read/write pointer */
box_set_pointer(b, px);
assert(box_is_pointer(b));
assert(box_get_pointer(b) == px);
/* Read/write double */
box_set_double(b, y);
assert(!box_is_pointer(b));
assert(box_get_double(b) == y);
/* Read/write weird pointer */
box_set_pointer(b, z);
assert(box_is_pointer(b));
assert(box_get_pointer(b) == z);
return 0;
}
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