| Issue |
180957
|
| Summary |
flang-22 produces NaN and Inf when it should not, randomly
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
zaikunzhang
|
With flang-22, the following code generates NaN unexpectedly and randomly.
It can be reproduced by the following bash script.
```bash
rm -f a.out
uname -a
flang --version
flang -fstack-arrays -std=f2018 -O3 -ffast-math -g test_div_flang.f90
# Run a.out 5 times to demonstrate the random NaN generation behavior
for i in {1..5}; do
echo "Run $i:"
./a.out
done
```
N.B.:
0. The `-ffast-math` flag is being used. I know, this is not recommended in practice and it may give rise to non-mathematical results. However, the resutls should be deterministic.
1. Keeping `-ffast-math`, different combinations of the compilation flags will lead to different results.
2. Here are the results in my tests:
- [ubuntu_x86.log](https://github.com/user-attachments/files/25238070/ubuntu_x86.log)
- [ubuntu_arm.log](https://github.com/user-attachments/files/25238072/ubuntu_arm.log)
- [macos_x86.log](https://github.com/user-attachments/files/25238071/macos_x86.log)
- [macos_arm.log](https://github.com/user-attachments/files/25238073/macos_arm.log)
3. The above results can be reproduced by forking https://github.com/zequipe/flang_ranom_nan and run the GitHub Actions.
Thank you for taking a look.
Code:
```fortran
! test_div_flang.f90
program test_div_flang
use iso_fortran_env, only : RP => REAL32
implicit none
! The code may behave differently depending on the dimension of the array. Try them.
real(RP) :: a(14), b(14), c
a = [0., 0., 7.E-45, 7.E-45, 0., 5.E-45, 0., 5.E-45, 5.E-45, 0., 0., 0., 0., 5.E-45]
!real(RP) :: a(9), b(9), c
!a = [5.E-45, 0., 5.E-45, 5.E-45, 0., 0., 0., 0., 5.E-45]
!
!real(RP) :: a(8), b(8), c
!a = [0., 5.E-45, 5.E-45, 0., 0., 0., 0., 5.E-45]
b = a / maxval(abs(a))
c = maxval(abs(a))
print *, '>>> Dimension = ', size(a)
print *, a/maxval(abs(a)), '|', b
print *, (a/maxval(abs(a)))**2, '|', b**2
print * , '------------------'
print *, a, '|', a/maxval(abs(a)), '|', b
print *, a, '|', (a/maxval(abs(a)))**2, '|', b**2
print * , '=================='
print *, a/c, '|', b
print *, (a/c)**2, '|', b**2
print * , '------------------'
print *, a, '|', a/c, '|', b
print *, a, '|', (a/c)**2, '|', b**2
end program test_div_flang
```
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