2016-03-01 16:58 GMT+01:00 Daniel Franzini <[email protected]>:
> 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;
>
That is true in general, but here, it is used to compare the output of
identical code.
The fact that this produces different outcomes on different threads has to
do with Windows' floating point settings on thread creation and the way
MinGW-w64 handles it (C++ threads are handled through pthread):
https://sourceforge.net/p/mingw-w64/mailman/mingw-w64-public/thread/CAGGsPMz%3D5w-2X_7bGSgWjEzyQFuwtBVCR%2BOm%3Dr-OA_zp8E2KdQ%40mail.gmail.com/#msg33032222
more precisely:
https://sourceforge.net/p/mingw-w64/mailman/message/33080317/
I don't remember what the wanted behaviour is. I do know this is probably
the cause for your inconsistency.
Ruben
> 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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