topic Re:std::isfinite(INFINITY) returns true on Linux in Intel® oneAPI DPC++/C++ Compiler

std::isfinite(INFINITY) returns true on Linux

AJIOB — Fri, 25 Mar 2022 13:38:02 GMT

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

Re: std::isfinite(INFINITY) returns true on Linux

NoorjahanSk_Intel — Mon, 28 Mar 2022 09:20:30 GMT

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.

Re: std::isfinite(INFINITY) returns true on Linux

AJIOB — Mon, 28 Mar 2022 10:21:05 GMT

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

Re:std::isfinite(INFINITY) returns true on Linux

NoorjahanSk_Intel — Fri, 01 Apr 2022 07:38:16 GMT

Hi,

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

Thanks & Regards,

Noorjahan

Re:std::isfinite(INFINITY) returns true on Linux

DMITRY_T_Intel — Fri, 01 Apr 2022 13:17:56 GMT

Hi Alex,

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

Thanks!

Re: std::isfinite(INFINITY) returns true on Linux

VinInn — Sun, 03 Apr 2022 12:20:02 GMT

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';

Re:std::isfinite(INFINITY) returns true on Linux

DMITRY_T_Intel — Tue, 05 Apr 2022 04:54:28 GMT

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!

Re: std::isfinite(INFINITY) returns true on Linux

lpsinger — Mon, 26 Jun 2023 21:05:34 GMT

The old compiler, icc, also had -fp-model=fast (or its equivalent) as the default, did it not? And yet isfinite does work on icc even in this mode.

I have a program that needs to have isfinite working. It runs about half as fast when I compile it with icx in precise mode as with icc in fast mode. I am eager to switch from icc to icx, but given this performance hit it is not an option for me right now.

Are there any prospects for fixing isfinite in icx in the fast mode?

Re: std::isfinite(INFINITY) returns true on Linux

lpsinger — Mon, 26 Jun 2023 21:39:16 GMT

...My apologies. This behavior is consistent with the most recent versions of gcc and clang in fast math mode. The solution is to add the compiler option -fno-finite-math-only, which is supported by gcc, clang, and icx.