Currently, when a mesh slab overflows, we recreate the allocator and
reinsert all the meshes that were in it in an arbitrary order. This can
result in the meshes moving around. Before `MeshInputUniform`s were
retained, this was slow but harmless, because the `MeshInputUniform`
that contained the positions of the vertex and index data in the slab
would be recreated every frame. However, with mesh retention, there's no
guarantee that the `MeshInputUniform`, which could be cached from the
previous frame, will reflect the new position of the mesh data within
the buffer if that buffer happened to grow. This manifested itself as
seeming mesh data corruption when adding many meshes dynamically to the
scene.
There are three possible ways that I could have fixed this that I can
see:
1. Invalidate and rebuild all the `MeshInputUniform`s belonging to all
meshes in a slab when that mesh grows.
2. Introduce a second layer of indirection so that the
`MeshInputUniform` points to a *mesh allocation table* that contains the
current locations of the data of each mesh.
3. Avoid moving meshes when reallocating the buffer.
To be efficient, option (1) would require scanning meshes to see if
their positions changed, a la
`mark_meshes_as_changed_if_their_materials_changed`. Option (2) would
add more runtime indirection and would require additional bookkeeping on
the part of the allocator.
Therefore, this PR chooses option (3), which was remarkably simple to
implement. The key is that the offset allocator happens to allocate
addresses from low addresses to high addresses. So all we have to do is
to *conceptually* allocate the full 512 MiB mesh slab as far as the
offset allocator is concerned, and grow the underlying backing store
from 1 MiB to 512 MiB as needed. In other words, the allocator now
allocates *virtual* GPU memory, and the actual backing slab resizes to
fit the virtual memory. This ensures that the location of mesh data
remains constant for the lifetime of the mesh asset, and we can remove
the code that reinserts meshes one by one when the slab grows in favor
of a single buffer copy.
Closes#17766.
ce433955e6