如何在舍入模式下将double转换为unsigned long long != FE_TOWARDZERO？

Question

如何在舍入模式下将double转换为unsigned long long != FE_TOWARDZERO?

补充：为什么没有ullrint功能？

Answer 1

标准 C99 数学函数 trunc()、ceil()、floor() 和 rint()（具有适当设置的舍入模式）可用于对 double 值使用所需的舍入模式转换为下一个整数值，然后通过强制转换转换为 unsigned long long。请注意，足够大的 double 值（幅度 >= 2⁵³ 的值）保证是整数，因此在这种方法中不存在双舍入问题。

或者（例如，当使用仅支持不提供这些标准数学函数的旧 ISO-C 标准的工具链时），可以通过一些简单的位操作轻松执行转换。以下示例代码演示了这两种方法。它假定 uint64_t 和 unsigned long long 在底层是相同的，并且 double 映射到 IEEE-754 binary64.

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <string.h>
#include <fenv.h>
#include <math.h>

#define RN (1)
#define RZ (2)
#define RU (3)
#define RD (4)
#define TEST_RND_MODE (RD)

uint64_t double_as_uint64 (double a)
{
    uint64_t r;
    memcpy (&r, &a, sizeof r);
    return r;
}

uint64_t double_to_uint64 (double a, int rnd_mode)
{
    const uint64_t uint64_nan = 0x8000000000000000ULL;
    const uint64_t uint64_msb = 0x8000000000000000ULL;
    const int dbl_expo_bias = 1023;
    const int dbl_expo_bits = 11;
    const int dbl_expo_mask = ((1 << dbl_expo_bits) - 1);
    const int dbl_mant_bits = 52; // stored
    uint64_t t = 0ULL;
    uint64_t ai = double_as_uint64 (a);
    uint64_t r;
    int shift;

    /* special case handling: season to taste */
    if (isnan (a)) return uint64_nan;
    if (a >= 0x1.0p64) return 0ULL;
    if (a <= 0.0) return 0ULL;

    shift = (int)(dbl_expo_bias + 63 - ((ai >> dbl_mant_bits) & dbl_expo_mask));
    r = ((ai << dbl_expo_bits) | uint64_msb);
    if (shift >= 64) {
        t = r >> (shift > 64);
        r = 0;
    } else if (shift) {
        t = r << (64 - shift);
        r = r >> shift;
    }
    if ((rnd_mode == FE_TONEAREST) && (t >= uint64_msb)) {
        r += (t == uint64_msb) ? (r & 1) : 1;
    } else if ((rnd_mode == FE_UPWARD) && t) {
        r++;
    }
    return r;
}

double uint64_as_double (uint64_t a)
{
    double r;
    memcpy (&r, &a, sizeof r);
    return r;
}

static uint64_t kiss64_x = 1234567890987654321ULL;
static uint64_t kiss64_c = 123456123456123456ULL;
static uint64_t kiss64_y = 362436362436362436ULL;
static uint64_t kiss64_z = 1066149217761810ULL;
static uint64_t kiss64_t;
#define MWC64  (kiss64_t = (kiss64_x << 58) + kiss64_c, \
                kiss64_c = (kiss64_x >> 6), kiss64_x += kiss64_t, \
                kiss64_c += (kiss64_x < kiss64_t), kiss64_x)
#define XSH64  (kiss64_y ^= (kiss64_y << 13), kiss64_y ^= (kiss64_y >> 17), \
                kiss64_y ^= (kiss64_y << 43))
#define CNG64  (kiss64_z = 6906969069ULL * kiss64_z + 1234567ULL)
#define KISS64 (MWC64 + XSH64 + CNG64)

int main (void)
{
    double x;
    uint64_t res, ref;

    do {
        x = fabs (uint64_as_double (KISS64));

#if TEST_RND_MODE == RZ
        res = double_to_uint64 (x, FE_TOWARDZERO);
        ref = (uint64_t)trunc(x);
#elif TEST_RND_MODE == RN
        res = double_to_uint64 (x, FE_TONEAREST);
        ref = (uint64_t)rint(x); // assumes current rounding mode is FE_TONEAREST!
#elif TEST_RND_MODE == RU 
        res = double_to_uint64 (x, FE_UPWARD);
        ref = (uint64_t)ceil(x);
#elif TEST_RND_MODE == RD
        res = double_to_uint64 (x, FE_DOWNWARD);
        ref = (uint64_t)floor(x);
#else
#error unsupported TEST_RND_MODE
#endif
        if (res != ref) {
            printf ("err @ % 22.13a: res=%016llx  ref=%016llx\n", x, res, ref);
            return EXIT_FAILURE;
        }
    } while (1);
    return EXIT_SUCCESS;
}

如何在舍入模式下将double转换为unsigned long long != FE_TOWARDZERO？

How to convert double to unsigned long long under rounding mode != FE_TOWARDZERO?

c

floating-point

double

type-conversion

unsigned-long-long-int