LLVM only spack-stack using no ifort (ifx, icc and icpc and MPI wrappers around these) generally needed on RDHPCS platforms for UFS LLVM testing.

[GeorgeVandenberghe-NOAA]

THere is a general need to upgrade the compiler level and build LLVM only stacks on all NOAA platforms because as of issue creation date, Intel classic ifort is a year out of support (Dec 2024) and NOAA applications need spack-stack depencencies to build with LLVM. This writer (George V) has built his own stacks to advance such testing but for more general app testing we need spack-stack envrionments. UFS_Weather-Model coupled though does not build until intel/2025.2 or higher is available and the GOCART component does not build until intel/2025.3 is made available so systems such a spack-stack is built on need to be at these levels of Intel. LLVM model and CLASSIC dependencies is fully supported by the compiler but very difficult to implement with spack-stack.

[climbfuji]

FYI neptune-env alreayd builds with LLVM, and NEPTUNE runs with it (clang, clang++, flang-new). That was a non-trivial effort. Our NOAA (EMC, EPIC) colleagues will have to take care of the rest that is needed for the UFS. Not in spack-stack-2.0.0, but maybe as soon as 2.1.0 in February 2026.