Index-based mlebabeclap_adotx kernel

[ankithadas]

Summary

Rewrites mlebabeclap_adotx (2D and 3D) from a Box-based kernel that loops internally
(amrex::Loop(box, ncomp, ...)) into a per-cell (i,j,k,n) kernel, and switches its EB
geometry arguments from ten separate Array4s (flag, vfrc, apx/apy/apz,
fcx/fcy/fcz, ba, bc) to the existing EBData view, matching what
mlebabeclap_gsrb already does.

MLEBABecLap::Fapply now calls it through AMREX_HOST_DEVICE_PARALLEL_FOR_4D instead of
AMREX_LAUNCH_HOST_DEVICE_LAMBDA, and gets the geometry via factory->getEBData(mfi).

The arithmetic inside the kernel is untouched: the loop wrapper is removed and the body is
only re-indented, and the EB arrays are re-bound to identically named local references
(auto const& apx = ebdata.get<EBData_t::apx>(); etc.), so the expressions are character
for character the same. Results are bitwise unchanged.

Additional background

This is the first of several small PRs that split up the stale WIP #4922 ("GPU specific
kernels for MLEBABecLap"), as requested there. It supersedes the mlebabeclap_adotx part
of #4922. The templating on T that #4922 also introduced has been dropped, per the
review comment on that PR. The (i,j,k,n) form is the prerequisite for a later PR that
adds a fused (MultiFab-wide ParallelFor) GPU path for Fapply; that follow-up is not
included here.

Testing (all commands run from the repo root unless noted):

• cmake -S . -B build-split -DAMReX_SPACEDIM=3 -DAMReX_EB=ON -DAMReX_LINEAR_SOLVERS_EM=OFF -DAMReX_ENABLE_TESTS=ON -DAMReX_TEST_TYPE=Small -DAMReX_MPI=ON -DCMAKE_BUILD_TYPE=Release
  then cmake --build build-split -j8 and ctest --test-dir build-split --output-on-failure
  → builds clean, 9/9 tests pass.

• Tests/LinearSolvers/CellEB, make -j8 COMP=llvm USE_MPI=FALSE DIM=3 and DIM=2,
  run over seven configurations (sphere, sphere + eb_is_dirichlet=1, rotated_box,
  two_spheres, flower, two-level sphere, periodic sphere; n_cell=64, verbose=2).
  The MLMG/BiCGStab residual histories and the initial/final max, 1- and 2-norm residuals
  are bitwise identical to development in both 2D and 3D.

• CUDA build on an NVIDIA RTX A5000 (CUDA 13.2, gcc 11.4):
  Tests/LinearSolvers/CellEB, make -j8 COMP=gnu USE_MPI=FALSE USE_CUDA=TRUE CUDA_ARCH=86 DIM=3,
  same seven configurations. MLMG iteration counts are identical to development
  (9, 12, 11, 3, 11, 28, 11); the residual values differ only at the level of the
  run-to-run nondeterminism of the unmodified binary (GPU reductions are not
  bit-reproducible), and all solves converge to resid/resid0 ~ 1e-13.

• The same CellEB matrix was re-run on Linux/gcc 11.4 (make -j8 COMP=gnu USE_MPI=FALSE,
  DIM=3 and DIM=2) against a development build in a sibling worktree: residual
  histories again bitwise identical in both dimensions.

Performance

Neither timing changed measurably; this PR is a refactor, not an optimisation.

CPU, Tests/LinearSolvers/CellEB main3d.gnu.TEST.ex
(make -j8 COMP=gnu USE_MPI=FALSE DIM=3, gcc 11.4, single rank),
inputs n_cell=128 eb2.geom_type=sphere eb_is_dirichlet=1 verbose=1.
All binaries were built first and then timed interleaved in the same session
(3 reps each) on a shared machine, so the absolute numbers are inflated but the
comparison is fair. Best of 3, MLMG Timers: Solve [s]:

grids	development	this PR
max_grid_size=32	4.284	4.328
max_grid_size=64	4.534	4.554

GPU (NVIDIA RTX A5000, CUDA 13.2,
make -j8 COMP=gnu USE_MPI=FALSE USE_CUDA=TRUE CUDA_ARCH=86 DIM=3), same test at
n_cell=256, again built up front and timed interleaved, best of 3:

grids	development	this PR
max_grid_size=32 (512 boxes)	1.085	1.083
max_grid_size=64 (64 boxes)	0.834	0.833

MLMG converged in the same number of iterations in every one of these runs.

Checklist

The proposed changes:

• fix a bug or incorrect behavior in AMReX
• add new capabilities to AMReX
• changes answers in the test suite to more than roundoff level
• are likely to significantly affect the results of downstream AMReX users
• include documentation in the code and/or rst files, if appropriate

P.S Generated using Claude Code