std::isfinite(INFINITY) returns true on Linux

[AJIOB]

Hello

I'm using Intel oneAPI 2022.1.3 from APT on Ubuntu 20.04 LTS x64 English (Linux-based).

I got example code of std::isfinite() from cppreference, compile them with DPC++ 2022.0.0.20211123 and it works incorrectly on Linux: not as C++ standard wants.

Linux build & execution log was attached.

Linux output:

isfinite(NaN) = true
isfinite(Inf) = true
isfinite(0.0) = true
isfinite(exp(800)) = true
isfinite(DBL_MIN/2.0) = true

Windows version of DPC++ generates correct output:

isfinite(NaN) = false
isfinite(Inf) = false
isfinite(0.0) = true
isfinite(exp(800)) = false
isfinite(DBL_MIN/2.0) = true

Example code was attached as a test.cpp file in test.zip too.

Code was compiled with command dpcpp test.cpp -o test

How can I fix that behavior?

With regards,
Alex

[NoorjahanSk_Intel]

Hi,

Thanks for reaching out to us.

>>How can I fix that behavior?

Could you please try with below command as it worked (as expected) from our end.

dpcpp test.cpp -fp-model=precise -o test

Please find the below screenshot for more details.

"-fp-model=precise" disables the optimization for floating-point calculations, so that the result won't be changed.

Please refer to below link for more details.

https://www.intel.com/content/www/us/en/develop/documentation/oneapi-dpcpp-cpp-compiler-dev-guide-and-reference/top/compiler-reference/compiler-options/compiler-option-details/floating-point-options/fp-model-fp.html

Thanks & regards,

Noorjahan.

[AJIOB]

Hi Noorjahan,

Yes, you are right, that flag was helpful for fixing behavior on Linux.

But I need to have maximum speed for Linux & Windows with equal & correct behaviour.

What about fixing an optimized version?

With regards,
Alex

[VinInn]

I advice to test this version [1] as the original has the risk that everything is computed at compile time (try in godbolt for instance)

with Ofast (aka finite-math) one DOES expect isfinite(NAN) to be true

with latest gcc I get

c++ -O3 isfinite.cpp; ./a.out
isfinite(NaN) = false
isfinite(Inf) = false
isfinite(0.0) = true
isfinite(exp(800)) = false
isfinite(DBL_MIN/2.0) = true
x = 0
isfinite(0.f/x) = false
isfinite(1.f/x) = false
isfinite(x) = true
isfinite(exp(x)) = true
isfinite(DBL_MIN/x) = false

c++ -Ofast isfinite.cpp
 ./a.out

isfinite(NaN) = false
isfinite(Inf) = false
isfinite(0.0) = true
isfinite(exp(800)) = true
isfinite(DBL_MIN/2.0) = true
x = 0
isfinite(0.f/x) = true
isfinite(1.f/x) = true
isfinite(x) = true
isfinite(exp(x)) = true
isfinite(DBL_MIN/x) = true

[1]

#include <iostream>
#include <cmath>
#include <cfloat>

int main(int argc, char**)
{

float x = 1.f -float(argc);

std::cout << std::boolalpha
<< "isfinite(NaN) = " << std::isfinite(NAN) << '\n'
<< "isfinite(Inf) = " << std::isfinite(INFINITY) << '\n'
<< "isfinite(0.0) = " << std::isfinite(0.0) << '\n'
<< "isfinite(exp(800)) = " << std::isfinite(std::exp(800)) << '\n'
<< "isfinite(DBL_MIN/2.0) = " << std::isfinite(DBL_MIN/2.0) << '\n';

std::cout << std::boolalpha
<< "x = " << x << '\n'
<< "isfinite(0.f/x) = " << std::isfinite(0.f/x) << '\n'
<< "isfinite(1.f/x) = " << std::isfinite(1.f/x) << '\n'
<< "isfinite(x) = " << std::isfinite(0.0) << '\n'
<< "isfinite(exp(x)) = " << std::isfinite(std::exp(x)) << '\n'
<< "isfinite(DBL_MIN/x) = " << std::isfinite(DBL_MIN/x) << '\n';

[NoorjahanSk_Intel]

Hi,

We are working on your issue. We will get back to you soon.

Thanks & Regards,

Noorjahan

[DMITRY_T_Intel]

Hi Alex,

I escalated this issue to development. I will update it as soon as I get any information.

Thanks!

[DMITRY_T_Intel]

Hi Alex,

I got reply from developers:

"Use of dpcpp or icpx uses Intel defaults, which includes -fp-model=fast. This behavior is expected."

Thanks!