How to verify safe automatic deallocation of Fortran allocatable arrays after a compiler upgrade?

Answer the exact question first

To check whether automatic deallocation of Fortran allocatable arrays still works after a compiler upgrade, build a small test that allocates an array inside a subroutine and lets the subroutine exit, then run the executable under a memory‑checking tool and compare the result with the old compiler.

Likely explanation vs confirmed facts

If the new compiler follows the Fortran 2003 rule, the allocatable will be deallocated automatically when it goes out of scope and no memory‑checking tool will report an unfreed block. If the upgrade introduced a regression, the tool may show a leak that was absent with the old compiler.

Steps needed for this case

1. Create a minimal test harness (module with a subroutine that allocates, uses, and does not explicitly deallocate an allocatable array; a main program that calls the subroutine).
2. Compile the harness with the new compiler using any available run‑time checks for allocatables (e.g. -fcheck=allocatable for gfortran, -check allocatable for ifort).
3. Run the resulting executable under a memory‑checking tool such as Valgrind (Linux) or Dr. Memory (Windows) and look for reports of unfreed heap blocks.
4. Repeat the same compile and run with the old compiler. If the old compiler shows no leaks while the new one does, the upgrade has likely broken the automatic deallocation guarantee.

Safe revert strategy

1. Keep the old compiler accessible (e.g., via a module file, container image, or separate build directory).
2. Pin the compiler version in your build system (explicit FC=… or module load).
3. Document the compiler versions, flags used, and any leak reports.
4. If a regression is confirmed, rebuild the legacy code with the old compiler until the issue is resolved upstream.

Question that changes the recommendation

Which specific compiler and version are you using, and which optimisation or run‑time flags are enabled during the build? Knowing this would let me suggest the exact flag to enable allocatable checking.