Visual C ++ 2015，舍入模式和优化问题 [英] Visual C++ 2015, rounding mode and optimization issue

问题描述

大家好，

我需要使用visual c ++以特定的舍入模式（朝向负无穷大）执行计算。我的程序的行为从vs2013改为vs2015，它似乎与在vs2013中没有执行的优化有关。

I need to perform computations with specific rounding mode (toward minus infinity) with visual c++. The behaviour of my program changed from vs2013 to vs2015 and it seems to be related to an optimization that was not performed in vs2013.

这是一个最小的例子（用cl example.cpp /编译） fp：strict / O2）：

Here is a minimal example (compiled with cl example.cpp /fp:strict /O2):

#include <iostream>
int main(){
    union{
        long long i;
        double d;
    } value;
    
    //rounding mode to minus infinity
    _MM_SET_ROUNDING_MODE((unsigned int) _MM_ROUND_DOWN);
    _control87(_DN_FLUSH | _RC_DOWN | _PC_53, _MCW_DN | _MCW_RC | _MCW_PC);
    
    value.i = 0x3ff0000000000000;
    double x = value.d;
    
    value.i = 0x7feffffc57ca82ae;
    double y = value.d;
    
    value.d = -((-x) + (-y));
    std::cout << std::hex << value.i << std::endl; //expect 7feffffc57ca82af
    
    value.d = x + y;
    std::cout << std::hex << value.i << std::endl; // expect 7feffffc57ca82ae
}

VS2010和2013得到预期的结果，但vs2015和vs2017总是显示7feffffc57ca82ae，好像在我的第一次计算中简化了减号。

VS2010 and 2013 get the expected results but vs2015 and vs2017 always display 7feffffc57ca82ae as if the minus signs were simplified in my first computation.

根据我对IEEE-754和/ fp：strict选项的理解，预期结果应该是vs2013的结果，而不是vs2015的结果。

In my understanding of IEEE-754 and the /fp:strict option, the expected results should be the results of vs2013 and not the results of vs2015.

如果我删除优化标志（/ O2），两个编译器都会返回预期值。

If I remove the optimization flag (/O2) both compilers return the expected values.

推荐答案

>根据我对IEEE-754和/ fp：strict选项的理解，预期结果应该是vs2013的结果，而不是vs2015的结果。

>In my understanding of IEEE-754 and the /fp:strict option, the expected results should be the results of vs2013 and not the results of vs2015.

使用VS2017帮助，发送反馈，报告问题工具向MS报告，以了解他们对此有何看法。

Report it to MS using the VS2017 Help, Send Feedback, Report a Problem facility to see what they have to say about it.

Dave

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