Two performance optimizations for interpolated flex objects:
1. Hoist loop-invariant stride calculations in `mju_cellLookup` out of nested loops.
This reduces multiplications from 16 to 6 (linear) and 54 to 12 (quadratic)
per vertex.
2. Extract and consolidate optimized 3D interpolation logic into a new
reusable utility `mju_evalBasisArray` in `engine_util_misc.c`. This function
uses nested loops and precomputed 1D shape functions to avoid expensive
dynamic `phi` calls and branching, and leverages stack-buffered outputs to
eliminate compiler pointer-aliasing barriers. We propagate this optimization
to both kinematics (`mju_interpolate3D`) and constraint setup
(`engine_core_constraint.c`).
Together these changes yield a ~50% overall speedup in `mj_fwdKinematics` for interpolated flexes with ~10k vertices in the collision meshes and ~10 nodes in the deformation grid.
PiperOrigin-RevId: 922198093
Change-Id: I9c804e65cd532e2f9ac6e9a71d01c30cd64186fa
Alessio Quaglino
f6c85287bf