无符号与有符号整数的性能

2022-01-14 00:00:00 c integer int unsigned c++

使用无符号整数而不是有符号整数是否有任何性能增益/损失?

Is there any performance gain/loss by using unsigned integers over signed integers?

如果是这样，这是否也适用于短期和长期?

If so, does this goes for short and long as well?

推荐答案

使用 unsigned int 除以 2 的幂更快，因为它可以优化为单个移位指令.对于 signed int，它通常需要更多的机器指令，因为除法 向零舍入，但向右移动 向下.示例:

Division by powers of 2 is faster with unsigned int, because it can be optimized into a single shift instruction. With signed int, it usually requires more machine instructions, because division rounds towards zero, but shifting to the right rounds down. Example:

int foo(int x, unsigned y)
{
    x /= 8;
    y /= 8;
    return x + y;
}

这里是相关的 x 部分(签名除法):

Here is the relevant x part (signed division):

movl 8(%ebp), %eax
leal 7(%eax), %edx
testl %eax, %eax
cmovs %edx, %eax
sarl $3, %eax

这里是相关的 y 部分(无符号除法):

And here is the relevant y part (unsigned division):

movl 12(%ebp), %edx
shrl $3, %edx

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