I think that comparing floating point numbers this way is wrong (well, at
least in C it is) because you can't never know how is the precision of this
comparison. It might be the case that in C++ the == operator is overloaded
and it performs correctly using some pre-defined precision constant. I'm
note sure about that.
if (b == c)
continue;
On Tue, Mar 1, 2016 at 9:04 AM, Benjamin Bihler <
[email protected]> wrote:
> Hello,
>
> I have found a behaviour of MinGW-W64 5.3.0 that disturbs me very much:
> the results of floating-point operations may differ, if they are run on
> different threads. This does not happen with Visual Studio 2015 and not
> with g++ 4.9.2 on my Debian Linux.
>
> Please consider the following program:
>
> -------------------------------------------------
> #include <iostream>
> #include <future>
> #include <cmath>
> #include <string>
> #include <thread>
>
> std::string getResult()
> {
> std::cout << "Thread id: " << std::this_thread::get_id() <<
> std::endl;
> std::string result = "Result:\n";
>
> double a, b, c;
> int i;
> for (i = 0, a = 1.0; i < 10000000; i++)
> {
> a *= 1.00000001;
> b = sqrt(a);
> c = pow(a, 0.5);
> if (b == c)
> continue;
> result += std::string("a: ") + std::to_string(a) + " b: "
> + std::to_string(b) + " c: " +
> std::to_string(c) + "\n";
> }
>
> return result;
> }
>
> int main()
> {
> std::string string1 = getResult();
>
> std::future<std::string> result = std::async(std::launch::async,
> getResult);
>
> std::string string2 = result.get();
>
> if (string1 != string2)
> {
> std::cout << "String 1 is: " << std::endl << string1 <<
> std::endl
> << " and string 2 is: " << std::endl <<
> string2 << std::endl;
> }
> else
> {
> std::cout << "The results are the same." << std::endl;
> }
>
> return 0;
> }
> -------------------------------------------------
>
> If I compile it with g++ -O0 -std=gnu++11 Main.cpp the output is
>
> -------------------------------------------------
> Thread id: 1
> Thread id: 2
> The results are the same.
> -------------------------------------------------
>
> If I compile it with g++ -O1 -std=gnu++11 Main.cpp the output is
> -------------------------------------------------
> Thread id: 1
> Thread id: 2
> String 1 is: ...
> -------------------------------------------------
>
> The same happens with "-O2".
>
> If I change the line
> std::future<std::string> result = std::async(std::launch::async,
> getResult);
> to
> std::future<std::string> result = std::async(getResult);
> then the result is independent from the optimization flag the following:
>
> -------------------------------------------------
> Thread id: 1
> Thread id: 1
> The results are the same.
> -------------------------------------------------
>
> What actually disturbs me, is that the results differ with asynchronous
> execution and optimization turned on. I tried parameters like -ffloat-store
> -msse2 -mfpmath=sse to have consistent floating-point operation results,
> but this won't help.
>
> Is this a bug? Or are there compiler flags that force the floating-point
> operation results to be consistent?
>
> Any help is greatly appreciated!
>
> Regards,
> Benjamin
>
>
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--
Daniel
"Let us change our traditional attitude to the construction of programs.
Instead of imagining that our main task is to instruct a computer what to
do, let us concentrate rather on explaining to human beings what we want a
computer to do." (Donald Knuth)
"Yes, technogeeks can be funny, even if only to each other." (
http://www.boogieonline.com/revolution/science/humor/)"
"Man is driven to create; I know I really love to create things. And while
I'm not good at painting, drawing, or music, I can write software."
(Yukihiro Matsumoto, a.k.a. ``Matz'')
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