当从int到float和返回时，符号改变 [英] sign changes when going from int to float and back

问题描述

请考虑以下代码，这是我的实际问题的 SSCCE ：

Consider the following code, which is an SSCCE of my actual problem:

#include <iostream>

int roundtrip(int x)
{
    return int(float(x));
}

int main()
{
    int a = 2147483583;
    int b = 2147483584;
    std::cout << a << " -> " << roundtrip(a) << '\n';
    std::cout << b << " -> " << roundtrip(b) << '\n';
}

我的电脑上的输出（Xubuntu 12.04.3 LTS） p>

The output on my computer (Xubuntu 12.04.3 LTS) is:

2147483583 -> 2147483520
2147483584 -> -2147483648

注意正数 b 往返后结束负。这个行为是否明确？我会期望int-to-float round-tripping至少正确保存符号...

Note how the positive number b ends up negative after the roundtrip. Is this behavior well-specified? I would have expected int-to-float round-tripping to at least preserve the sign correctly...

Hm，在ideone 上，输出是不同的：

Hm, on ideone, the output is different:

2147483583 -> 2147483520
2147483584 -> 2147483647

g ++团队在此期间修复了错误还是两个输出完全有效？ p>

Did the g++ team fix a bug in the meantime, or are both outputs perfectly valid?

推荐答案

您的程序正在调用未定义的行为，因为在从浮点到整数的转换中出现溢出。您看到的只是x86处理器上的常见症状。

Your program is invoking undefined behavior because of an overflow in the conversion from floating-point to integer. What you see is only the usual symptom on x86 processors.

float > 2147483584

The float value nearest to 2147483584 is 2³¹ exactly (the conversion from integer to floating-point usually rounds to the nearest, which can be up, and is up in this case. To be specific, the behavior when converting from integer to floating-point is implementation-defined, most implementations define rounding as being "according to the FPU rounding mode", and the FPU's default rounding mode is to round to the nearest).

然后，当从代表2 ³¹ 的float的转换到 int 时，会发生溢出。这个溢出是未定义的行为。一些处理器引发异常，其他处理器饱和。通常由编译器生成的IA-32指令 cvttsd2si 在溢出的情况下总是返回 INT_MIN

Then, while converting from the float representing 2³¹ to int, an overflow occurs. This overflow is undefined behavior. Some processors raise an exception, others saturate. The IA-32 instruction cvttsd2si typically generated by compilers happens to always return INT_MIN in case of overflow, regardless of whether the float is positive or negative.

您不应该依赖这种行为，即使您知道您的目标是Intel处理器：当定位x86-64时，编译器可以发射从浮点到整数的转换，指令序列未定义行为的优点是返回结果，而不是您可能期望的目标整数类型。

You should not rely on this behavior even if you know you are targeting an Intel processor: when targeting x86-64, compilers can emit, for the conversion from floating-point to integer, sequences of instructions that take advantage of the undefined behavior to return results other than what you might otherwise expect for the destination integer type.

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