The changes in 9a0dc20821, moving the Hessian from the stack to the arena, should be rolled back: they prevent future threading over islands. Unlike the stack, arena allocations are not thread-friendly. The reason for the original move was to save memory, but the savings are small: `O(ctx->nH)` and only linear in `nv`. The significant reduction of the Cholesky factor, from quadratic in `nv` to quadratic in the largest dof island — the reduction afforded by 2dd518734f — remains in place.
Also refactor and improve readability.
PiperOrigin-RevId: 685717356
Change-Id: Ia9ec3e44a62d459a3b9cffb578cf6479d5aa1d7f
Yuval Tassa
7ec94f46d4