| Issue |
180374
|
| Summary |
[flang] Incorrect `cshift` results under optimization for 2D arrays
|
| Labels |
flang
|
| Assignees |
|
| Reporter |
gha3mi
|
Hi!
I’ve run into an issue with the intrinsic `cshift` in **flang 21.1.8** that only appears when optimization is enabled.
With `-g` (or `-O0`) the code behaves correctly, but with any `-O*` the results are incorrect.
**Compiler**
```
flang version 21.1.8
Target: x86_64-conda-linux-gnu
```
**Minimal reproducer**
```fortran
program cshift_bug
implicit none
integer :: a(0:6,3), b(0:6,3)
a = reshape([ &
1,2,3, 4,5,6, 7,8,9, 0,0,0, 9,8,7, 6,5,4, 3,2,1 ], &
shape(a), order=[2,1])
b = cshift(a, 7) ! shift by extent → must be identity
if (any(b /= a)) error stop "cshift(a,7) /= a"
end program
```
**Build/run**
```sh
flang -g cshift_bug.f90 && ./a.out # OK
flang -O3 cshift_bug.f90 && ./a.out # FAIL
```
**Observed behavior**
With `-O*`, `cshift(a,7)` produces constant or undefined rows instead of returning the original array.
**Expected behavior**
Shifting by the extent of the dimension should return the input array unchanged.
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