topic IFX error with complex array in Intel® Fortran Compiler

IFX error with complex array

g_granucci — Thu, 29 Jan 2026 11:41:32 GMT

This simple program gives a wrong result if compiled with ifx (version 2025.3, but also with previous versions):

program cx implicit none complex, allocatable, dimension(:,:) :: a integer :: i,n n=2 allocate (a(-1:1,n)) a(0,:)=Cmplx(0,1) a(-1,:)=Cmplx(4,5) a(1,:)=Cmplx(2,3) do i=1,n write(*,*)i,a(0,i) end do end program cx

$ ./cx 1 (4.000000,5.000000) 2 (4.000000,5.000000)

instead of the expected result (0,1), obtained with ifort and gfortran.

Re: IFX error with complex array

MarcGrodent — Thu, 29 Jan 2026 12:45:16 GMT

Running the code in debug mode leads to correct results. The problem occurs in release mode, probably due to the code optimization...

I'm using version 2025.3.0

Re: IFX error with complex array

Mentzer__Stuart — Thu, 29 Jan 2026 17:35:06 GMT

Good catch! As @MarcGrodent noted it is optimization related: the bug shows up at /O2 and /O3 but not /O1.

Dumping the whole array shows that a(0,:) is treated as a(-1,:), both in the assignment and the elementwise write: the contents of a(0,:) when printing the whole array can contain (0,0) or uninitialized garbage values.

Re:IFX error with complex array

Igor_V_Intel — Tue, 03 Feb 2026 22:10:19 GMT

This loop optimizer bug is still in the latest compiler builds. I have escalated it to be fixed.

LLVM bisect tool points to the hir-post-vec-complete-unroll optimization pass, causing the problem. Note that with O1 results are correct. Looks like a full unrolling has a bug. I also tried to disable it, but then values are set to zero, which is also not correct.

$ ifx -O2 -fno-unroll-loops test.f90

$ ./a.out

1 (0.0000000E+00,0.0000000E+00)

2 (0.0000000E+00,0.0000000E+00)