32为整数保存32位乘法结果

32bit-multiply.cc

#include <cstdio>
#include <cstdlib>
#include <stdint.h>

struct UInt64 {
    uint32_t high;
    uint32_t low;
};

UInt64 multiply(uint32_t a, uint32_t b)
{
    uint16_t ah = (a >> 16) & 0xFFFF;
    uint16_t al = a & 0x0FFFF;
    uint16_t bh = (b >> 16) & 0xFFFF;
    uint16_t bl = b & 0x0FFFF;

    uint32_t h = ah * bh;
    uint32_t l = al * bl;
    uint32_t m0 = ah * bl;
    uint32_t m1 = al * bh;
    uint32_t m = m1 + m0;


    // 加法溢出
    if (m < m0 || m < m1) {
        h += (1 << 16);
    }

    struct UInt64 result;
    result.high = h + ((m >> 16) & 0x0FFFF);
    result.low = l + ((m & 0x0FFFF) << 16);

    // 加法溢出
    if (result.low < l || result.low < ((m & 0x0FFFF) << 16)) {
        result.high += 1;
    }

    return result;
}

int main()
{
    uint32_t a = 0xabcdef11;
    uint32_t b = 0xabcdef22;

    struct UInt64 pro = multiply(a, b);

    printf("%X %X\n", pro.high, pro.low);

    return 0;
}