gather_free_values! and gather_dirichlet_values! waste memory on unnecessary allocations

[Ady0333]

Summary

I noticed that gather_free_values! and gather_dirichlet_values! are allocating unnecessary temporary vectors in the UnconstrainedFESpace implementation.

Problem

Both functions delegate to gather_free_and_dirichlet_values!, which requires two output buffers. Since each function only needs one output, the other is allocated and immediately discarded:

• gather_free_values! allocates a throwaway dirichlet_values vector (~736 bytes on a 20×20 mesh)
• gather_dirichlet_values! allocates a throwaway free_values vector (~3 KB on a 20×20 mesh, 82 KB on a 100×100 mesh)

The gather_dirichlet_values! waste is particularly bad because it scales linearly with the free DOF count.

Impact

I'm working on a neural operator reduced-order model that calls interpolate! thousands of times in parameter sweeps. On a typical workflow with 10,000 samples:

• 20×20 mesh: ~187 MB wasted
• 100×100 mesh: ~780 MB wasted

This creates unnecessary GC pressure during snapshot generation.

Measurements

Function	Mesh Size	Current Allocation
gather_free_values!	20×20	4,320 bytes
gather_dirichlet_values!	20×20	6,624 bytes
gather_dirichlet_values!	100×100	82,064 bytes