As there was no check for the lack of a `GuestTexture`/`GuestBuffer`, it would lead to UB when a texture/buffer that had no guest such as the `zeroTexture` from `GraphicsContext` would be marked as dirty they would cause a call to `NCE::RetrapRegions` with a `nullptr` handle that would be dereferenced and cause a segmentation fault.
In certain situations such as constant buffer updates, we desire to use the guest buffer as a shadow buffer forwarding all writes directly to it while we update the host using inline buffer updates so they happen in-sequence. This requires special behavior as we cannot let any synchronization operations take place as they would break the shadow buffer, as a result, an external synchronization flag has been added to prevent this from happening.
It should be noted that this flag is not respected for buffer recreation which will lead to UB, this can and will break updates in certain cases and this change isn't complete without buffer manager support.
Previously constant buffer updates would be handled on the CPU and only the end result would be synced to the GPU before execute. This caused issues as if the constant buffer contents was changed between each draw in a renderpass (e.g. text rendering) the draws themselves would only see the final resulting constant buffer. Fix this by updating cbufs on the GPU/CPU seperately, only ever syncing them back at the start or after a guest side CPU write, at the moment only a single word is updated at a time however this can be optimised in the future to batch all consecutive updates into one large one.
We require certain buffers to only be on the host while being accessible through the same abstractions as a guest buffer as they must be interchangeable in usage.
This commit encapsulates a complex sequence of cascading changes in the process of supporting overlaps for buffers:
* We determined that it is impossible to resolve overlaps with multiple intervals per buffer within the constraints of each overlap being a contiguous view, support for multiple intervals was therefore dropped. The older buffer manager code was entirely reworked to be simpler due to only handling one interval per buffer with code now being based off `IntervalMap` but tailored specifically for buffers.
* During overlap resolution, the problem of how existing views into the buffer being recreated would be updated, it had to be replaced with a larger buffer that could contain all overlaps and all existing views would need to be repointed to it. This was addressed by a buffer owning all views to itself, we could automatically recalculate the offset of all views and update the buffers with it.
* We still needed to update usage of existing views which was done by handling all access (such as inside a recorded draw) to buffer view properties via `BufferView::RegisterUsage` which dispatches a callback with the view and the corresponding backing buffer. This callback can be stored and called during overlap resolution with the new buffer.
* We had issues with lifetime of the buffer with the handle-like semantics of `BufferView` introduced in the last buffer-related commit, if we updated the view to be owned by a new buffer we'd need to extend the lifetime of the new buffer not the older one and the only way to do this was a proxy owner object `BufferDelegate` which holds a shared pointer to the real `Buffer` which in-turn holds a pointer to all `BufferDelegate` objects to update on repointing. A `BufferView` is effectively just a wrapper around `std::shared_ptr<BufferDelegate>` with more favorable semantics but generally just forwarding calls.
It should be additionally noted that to support usage of `RegisterUsage` the code around buffers in `GraphicsContext` was refactored to defer truly binding till the recording phase.
We wanted views to extend the lifetime of the underlying buffers and at the same time preserve all views until the destruction of the buffer to prevent recreation which might be costly in the future when we need `VkBufferView`s of the buffer but also require a centralized list of all views for recreation of the buffer. It also removes the inconsistency between `BufferView*` being returned in `GetXView` in `GraphicsContext`.
A large amount of Texture/Buffer views would expire before reuse could occur in `Texture::GetView`/`Buffer::GetView`. These can lead to a substantial memory allocation given enough time and they are now deleted during the lookup while iterating on all entries.
It should be noted that there are a lot of duplicate views that don't live long enough to be reused and the ultimate solution here is to make those views live long enough to be reused.
Similar to constant redundant synchronization for textures, there is a lot of redundant synchronization of buffers. Albeit, buffer synchronization is far cheaper than texture synchronization it still has associated costs which have now been reduced by only synchronizing on access.
This is a prerequisite to memory trapping as we need to write to the mirror to avoid a race condition with external threads writing to a texture/buffer while we do so ourselves for the sync on a read/write, it also avoids an additional `mprotect` to `-WX`/`RWX` on a read access.
An additional advantage for textures especially is that we now support split-mapping textures due to laying them out in a contiguous mirror and they will not require costly algorithmic changes. Buffers should also benefit from not needing to iterate over every region when they are split into multiple mappings.
Implements infrastructure for using guest buffers on the host for rendering, a `BufferManager` is still missing which'd handle mapping from guest buffers to host buffers and will be subsequently committed. It should be noted that `BufferView` is also disconnected from `Buffer` and shared for every instance with the same properties like `TextureView` is now.
