clang18 adds warnings when variable length arrays are placed on the stack. HOOMD-blue code did so in a variety of places. I replaced these with std::vector in the MPI code. Benchmark before and after (mpirun -n 24 python3 -m hoomd_benchmarks.md_pair_lj --device CPU -v -N 64000 --warmup_steps 10000 --benchmark_steps 1000 --repeat 10):

• Before: 269 TPS
• After: 269 TPS
  The MPI std::vector allocations are once per timestep (or less often?), so they do not impose an overhead.

I used a large fixed size stack array in the convex polyhedron support function. This code is called in the innermost loop, so std::vector will severely impact performance here. I added an error check to prevent stack overflows. The max size is 4096 vertices which should be much larger than any reasonable use-case for convex polyhedron overlap checks.
Benchmark (python3 -m hoomd_benchmarks.hpmc_octahedron --device=CPU -v -N 1000 --repeat 10)

• Before: 866 sweeps/s
• After: 919 sweeps/s
  With the now force inlined methods, the fixes stack allocation occurs only once per timestep. Even when including the forceinline changes, the fixed size stack allocation is faster than the variable length arrays.