问题
When I execute a simple x64 application with the following code, I get different results on Windows PCs with a i7-3770 and i7-4790 CPU.
#include <cmath>
#include <iostream>
#include <limits>
void main()
{
double val = exp(-10.240990982718174);
std::cout.precision(std::numeric_limits<double>::max_digits10);
std::cout << val;
}
Result on i7-3770:
3.5677476354876406e-05
Result on i7-4790:
3.5677476354876413e-05
When I modify the code to call
unsigned int control_word;
_controlfp_s(&control_word, _RC_UP, MCW_RC);
before the exp function call, both CPUs deliver the same results.
My questions:
- Does anyone have an idea for the reason of the differences between the i7-3770 and i7-4790?
- Is there a way to set the floating point precision or consistency in a Visual Studio 2015/2017 C++ project for the whole project and not only for the following function call? The "Floating Point Model" setting (/fp) does not have any influence on the results here.
回答1:
Assuming that double is coded using IEEE-754, and using this decimal to binary converter, you can see that:
3.5677476354876406e-05 is represented in hexa as 0x3F02B48CC0D0ABA8 3.5677476354876413e-05 is represented in hexa as 0x3F02B48CC0D0ABA9
which differ only in the last bit, probably due round error.
回答2:
I did some further investigations and I found out the following facts:
- the problem does also occur on Windows with a different compiler (Intel)
- on a linux system both values are equal
I also posted this question to the Visual Studio Community. I got the information, that Haswell and newer CPUs use FMA3. You can disable this feature with _set_FMA3_enable(0) at the beginning of the program. When I do this, the results are the same.
来源:https://stackoverflow.com/questions/45821588/c-exp-function-different-results-under-x64-on-i7-3770-and-i7-4790