Flang Binary Release 2019-03-29: NaN > 0

[zaikunzhang]

This is a bug in Flang Binary Release 2019-03-29, which evaluates NaN > 0 to .TRUE..

Since I have not succeeded in building Flang, and there is no up-to-date binary release, I am still using this old version.
I am not sure whether the same problem exists in the current version. It would be great if someone can check it.

Code:

program test_nan_flang                                                                                                               
use, intrinsic :: ieee_arithmetic, only : ieee_value, ieee_quiet_nan, ieee_is_nan                                                    
                                                                                                                                     
implicit none                                                                                                                        
                                                                                                                                     
real :: a                                                                                                                            
                                                                                                                                     
a = ieee_value(a, ieee_quiet_nan)                                                                                                    
                                                                                                                                     
print *, a, ieee_is_nan(a), a > 0, a >= a, a == a, a <= a                                                                                                                                            
                                                                                                                                     
end program test_nan_flang 

Result:

$  uname -a && flang --version && flang -O1 test_nan_flang.f90 && ./a.out             
Linux 5.4.0-135-generic #152-Ubuntu SMP Wed Nov 23 20:19:22 UTC 2022 x86_64 x86_64 x86_64 GNU/Linux
clang version 7.0.1 
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
             NaN  T  T  T  T  T 

Thank you very much!

[bryanpkc]

@zaikunzhang I tried your test in a recent build of classic-flang-llvm-project/release_14x with flang/legacy (sorry I don't have the exact commit hashes), and it produces this output:

$ ./a.out
             NaN  T  F  F  F  F

So I don't think this problem exists in the latest version of the code. I will double-check with release_15x and master.

[zaikunzhang]

Thank you @bryanpkc for checking this. It is great to know that the latest version does not have the same issue.

[zaikunzhang]

I hope to mention that the bug arises only if flang is invoked with -O or higher optimization options. Without -O, the result is

./a.out 
             NaN  T  F  T  T  T

which is wrong as well, but NaN > 0 is evaluated to .FALSE..

[bryanpkc]

I ran with release_15x and master as well, with -O2, and the output is still correct.

[zaikunzhang]

Hi @bryanpkc . Thank you again for checking this. I have succeeded in compiling release_15x and made the test. As you observed, the bug does not affect release_15x, which is great. Thanks.