c - IEEE 754位操作舍入错误

不使用强制转换或库的功能，我必须使用位操作将整数强制转换为浮点。下面是我正在处理的代码。它基于我在Cast Integer to Float using Bit Manipulation breaks on some integers in C中找到的代码。我遇到的问题涉及IEEE 754中的舍入标准。更具体地说，我的代码向0舍入，但应该向偶数舍入。我需要做什么改变？

unsigned inttofloat(int x) {
    int bias = 127;
    int man;
    int exp = bias + 31; //8-bit exp
    int count = 0;
    int tmin = 1 << 31;
    int manpattern = 0x7FFFFF;

    int sign = 0;

    if (x == 0){
        return 0;
    }
    else if (x == tmin){
        return 0xcf << 24;
    }

    if (x < 0) {
        sign = tmin;
        x = ~x + 1; // makes x negative so that we can accurately represent it later on.
    }

    while((x & tmin) == 0){
        exp--;
        x <<= 1;
        count++;
    }

    exp <<= 23;
    man = (x >> 8) & manpattern;

    return (sign | exp | man);
}

最佳答案

若要向最接近的方向取整，请将(x >> 8)替换为：

unsigned u = x;  // avoid any potential signed shifting issues
unsigned lease_significant_bit = (u >> 8) & 1;
unsigned round_bit = (u >> 7) & 1; // Most significant bit shifted out
unsigned sticky_bit_flag = !!(u & 0x7F);  // All other bits shifts out

// OP's shifted answer.
u = (u >> 8):

// round away if more than half-way or
//  if at half-way and number is odd
u += (round_bit & sticky_bit_flag) | (round_bit & lease_significant_bit);

让OP简化
请注意，u += 1可能会一直传播并需要指数增长。

关于c - IEEE 754位操作舍入错误，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/42031305/