[Bug c++/110321] New: Double precision comaprison does not seem to work well when -std=c++17 option is selected

[Bug c++/110321] New: Double precision comaprison does not seem to work well when -std=c++17 option is selected

[serge.ayoun at outlook dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110321

            Bug ID: 110321
           Summary: Double precision comaprison does not seem to work well
                    when -std=c++17 option is selected
           Product: gcc
           Version: 13.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: serge.ayoun at outlook dot com
  Target Milestone: ---

The following code:

int main(int, char**)
{ 
    double a = 2.2;
    if ( a == 2.2) return 1; 
    return 0; 
}


compiled and link with the following instruction:

g++ -O0 -m32 -std=c++17

with gcc 11.1, returns 1 as expected
with gcc 13.1, returns 0.

when -std=c++17 is removed, both return 0 as expected.

for exact reproduction See:

https://godbolt.org/#z:OYLghAFBqd5QCxAYwPYBMCmBRdBLAF1QCcAaPECAMzwBtMA7AQwFtMQByARg9KtQYEAysib0QXACx8BBAKoBnTAAUAHpwAMvAFYTStJg1DIApACYAQuYukl9ZATwDKjdAGFUtAK4sGe1wAyeAyYAHI%2BAEaYxHoADqgKhE4MHt6%2BcQlJAkEh4SxRMVy2mPaOAkIETMQEqT5%2BRXaYDskVVQQ5YZHRegqV1bXpDX3twZ353VwAlLaoXsTI7BzBBADULEzBEMukK8gIVQBUB5MmGgCCJgDsFiunZysPK%2BizEfQrTLcAzAAiK2YAdGYTJ8rOdHis8FQVhB3l9vsDfgCzJMVsRMAQ5gwVlxgTc7uC0RjiFiNLjbucrt8ONNaJwAKy8PwcLSkVCcNzWawrBSzeaYW5mT48UgETTU6YAaxAkku/zpAE4uBoAGzyz6fDRceXyy7Kun6TiSRli1mcXgKEAaEVi6ZwWAwRAoVAsWJ0aLkShoF1umLALhC0hYABueAWADU8JgAO4AeVijE4wpotAI0QtEAiJoiwSqAE9E7xs8xiLmYxFtE1RdxeF62IIYwxaPnmbwsBEvMA3GJaBbq4HMOsjOIW4G8GjmkHML2WZhVE0vKmC%2BRBCUTbQ8BFiHmPFgTQRiHgWAXplQDMAFBHo3GE33%2BIIRGJ2FIZIJFCp1CPdEUDEYUJzLPoG4WpA0yoLEZQML2AC0MYaCsUEsJ8Zjwb06AIqYljWFwly8Kgk7EAeWDARA0yNM0zgQK4Az1KQgSjHkBQZIkEHUUxWQMB0DETMUpQtMMrENCUlZ8W0nFdIUtj8Z4dQ9MMYnjIUpG8gsEg0vSxojmyHArKoAAcypQcqkgrMAyDINinz/Fw0K4IQJACkKky8FWWiTJKIB0latIcEapBHpI8r/Boum6ZcZjypqyqXPpdKXKQTIslp5qWtaLa2g6EBIF6rr0GQFAQNlPogH6XBFMGoaYJesbxkySZ0KmxDppmI5FnmS6tSWZYVg4S61owBANk2Jpth2Xa0D2S5YIOwDDiy%2BDjo4k7Trws7zoufbLKuI7rpu24YIsLL7oex58GeF6RtVN5JrID7iM%2Bd7yEoagmroZj6IYxj/jYO3EaB4HJNBsHwYhyFQah6FfdhuH4YRU7wKRQnkX4lEMO40mDLRqPyYxRTxMxyQCaQePsdj3FkRBrT9OjNHkyJ1SkxJvRtITTP0/R4mqTMcwqVMBocAy8UmlpOn6YZxmmeZpVWTZ%2BBEMQDlTM5NrTAgmBMFgMQkXzvlHp5/yKlwZiXBo8q6Uh/qSJ8guaWatgpS54qkFKdIApIwVqiqqq6RoGiCvq3mfBpiW2w7bl82YQe4SHyukPhiTOJIQA%3D

Thanks,
Serge

[Bug c++/110321] Double precision comaprison does not seem to work well when -std=c++17 option is selected

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110321

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
As described in the manual, sincve GCC 13.1 -fexcess-precision=standard is
enabled by -std=c++NN (but not by -std=gnu++NN).

This means the literal 2.2 is represented with excess precision, and so is not
equal to the same value converted to double.

You can use (a == (double)2.2) to make the comparison work.

[Bug c++/110321] Double precision comaprison does not seem to work well when -std=c++17 option is selected

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110321

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |INVALID
             Status|UNCONFIRMED                 |RESOLVED

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Invalid as explained.
More over this is basically bug 323 in reverse.

[Bug c++/110321] Double precision comaprison does not seem to work well when -std=c++17 option is selected

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110321

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|INVALID                     |FIXED

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #1)
> You can use (a == (double)2.2) to make the comparison work.

Or use -std=gnu++17, or use -std=c++17 -fexcess-precision=fast

[Bug c++/110321] Double precision comaprison does not seem to work well when -std=c++17 option is selected

[serge.ayoun at outlook dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110321

--- Comment #4 from Serge Ayoun <serge.ayoun at outlook dot com> ---
Thanks guys for your help