flang-new 19.0.0 bug: nan > 0, nan > 1

[zaikunzhang]

Hi,

This is a bug of flang-new 19.0.0, when invoked with -Ofast (This is not recommended in general. I know).

Code (latest version available at https://github.com/zequipe/test_compiler/blob/master/test_flang_nan.f90):

! test_flang_nan
program test_flang_nan
use ieee_arithmetic, only : ieee_value, ieee_signaling_nan, ieee_quiet_nan
! The problem occurs with REAL64 and REAL32, not REAL16 or REAL128.
use iso_fortran_env, only : RP => REAL32 !REAL64

implicit none
real(RP) :: nan

nan = ieee_value(0.0_RP, ieee_signaling_nan)
! nan = ieee_value(0.0_RP, ieee_quiet_nan)  ! The same problem

print *, nan > 0, nan > 1, nan < 0, nan < 1

if (nan > 0 .or. nan > 1 .or. nan < 0 .or. nan < 1) then
    error stop 1
end if

end program test_flang_nan

Test:

uname -a && flang --version && flang -Ofast test_flang_nan.f90 && ./a.out

Result:

Linux 6.5.0-28-generic #29~22.04.1-Ubuntu SMP PREEMPT_DYNAMIC Thu Apr  4 14:39:20 UTC 2 x86_64 x86_64 x86_64 GNU/Linux
flang-new version 19.0.0git (git@github.com:llvm/llvm-project.git 37c175af955f0aeab67e8c553a0a47b2ed0fdba2)
Target: x86_64-unknown-linux-gnu
Thread model: posix
 T T F F
Fortran ERROR STOP: code 1

IEEE arithmetic exceptions signaled: INVALID

Thank you for taking a look.

[hakostra]

Just a casual Fortran programmer here, I'm not a Flang/LLVM developer.

The first to do is to read the manpage for flang and what it says about -Ofast:

-Ofast in Flang means -O3 -ffast-math -fstack-arrays.

-ffast-math means the following:
-fno-honor-infinities
-fno-honor-nans
-fassociative-math
-freciprocal-math
-fapprox-func
-fno-signed-zeros
-ffp-contract=fast

In this very particular example you use NaN's. The -no-honor-nans flag description is:

Allow floating-point optimizations that assume arguments and results are not NaNs

I.e. you are, by using -Ofast, telling the compiler you do not have NaN's in your code. So I think the compiler is completely correct here. If you need standard-conforming maths, do not use -Ofast.