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Improvements from TheForge (see description)
The work was performed by collaboration of TheForge and Google. I am merely splitting it up into smaller PRs and cleaning it up. This is the most "risky" PR so far because the previous ones have been miscellaneous stuff aimed at either [improve debugging](godotengine#90993) (e.g. device lost), [improve Android experience](godotengine#96439) (add Swappy for better Frame Pacing + Pre-Transformed Swapchains for slightly better performance), or harmless [ASTC improvements](godotengine#96045) (better performance by simply toggling a feature when available). However this PR contains larger modifications aimed at improving performance or reducing memory fragmentation. With greater modifications, come greater risks of bugs or breakage. Changes introduced by this PR: ## Transient memory TBDR GPUs (e.g. most of Android + iOS + M1 Apple) support rendering to Render Targets that are not backed by actual GPU memory (everything stays in cache). This works as long as load action isn't `LOAD`, and store action must be `DONT_CARE`. This saves VRAM (it also makes painfully obvious when a mistake introduces a performance regression). Of particular usefulness is when doing MSAA and keeping the raw MSAA content is not necessary. ## Immutable samplers Some GPUs get faster when the sampler settings are hard-coded into the GLSL shaders (instead of being dynamically bound at runtime). This required changes to the GLSL shaders, PSO creation routines, Descriptor creation routines, and Descriptor binding routines. ### Toggle - `bool immutable_samplers_enabled = true` Setting it to false enforces the old behavior. Useful for debugging bugs and regressions. Immutable samplers requires that the samplers stay... immutable, hence this boolean is useful if the promise gets broken. We might want to turn this into a `GLOBAL_DEF` setting. ## Linear Descriptor Pools Instead of creating dozen/hundreds/thousands of `VkDescriptorSet` every frame that need to be freed individually when they are no longer needed, they all get freed at once by resetting the whole pool. Once the whole pool is no longer in use by the GPU, it gets reset and its memory recycled. Descriptor sets that are created to be kept around for longer or forever (i.e. not created and freed within the same frame) **must not** use linear pools. There may be more than one pool per frame. How many pools per frame Godot ends up with depends on its capacity, and that is controlled by `rendering/rendering_device/vulkan/max_descriptors_per_pool`. - **Possible improvement for later:** It should be possible for Godot to adapt to how many descriptors per pool are needed on a per-key basis (i.e. grow their capacity like `std::vector` does) after rendering a few frames; which would be better than the current solution of having a single global value for all pools (`max_descriptors_per_pool`) that the user needs to tweak. ### Toggle - `bool linear_descriptor_pools_enabled = true` Setting it to false enforces the old behavior. Useful for debugging bugs and regressions. Setting it to false is required when workarounding driver bugs (e.g. Adreno 730). ## Reset Command Pools A ridiculous optimization. Ridiculous because the original code should've done this in the first place. Previously Godot was doing the following: 1. Create a command buffer **pool**. One per frame. 2. Create multiple command buffers from the pool in point 1. 3. Call `vkBeginCommandBuffer` on the cmd buffer in point 2. This resets the cmd buffer because Godot requests the `VK_COMMAND_POOL_CREATE_RESET_COMMAND_BUFFER_BIT` flag. 4. Add commands to the cmd buffers from point 2. 5. Submit those commands. 6. On frame N + 2, recycle the buffer pool and cmd buffers from pt 1 & 2, and repeat from step 3. The problem here is that step 3 resets each command buffer individually. Initially Godot used to have 1 cmd buffer per pool, thus the impact is very low. But not anymore (specially with Adreno workarounds to force splitting compute dispatches into a new cmd buffer, more on this later). However Godot keeps around a very low amount of command buffers per frame. The recommended method is to reset the whole pool, to reset all cmd buffers at once. Hence the new steps would be: 1. Create a command buffer **pool**. One per frame. 2. Create multiple command buffers from the pool in point 1. 3. Call `vkBeginCommandBuffer` on the cmd buffer in point 2, which is already reset/empty (see step 6). 4. Add commands to the cmd buffers from point 2. 5. Submit those commands. 6. On frame N + 2, recycle the buffer pool and cmd buffers from pt 1 & 2, call `vkResetCommandPool` and repeat from step 3. **Possible issues:** @DarioSamo added `transfer_worker` which creates a command buffer pool: ```cpp transfer_worker->command_pool = driver->command_pool_create(transfer_queue_family, RDD::COMMAND_BUFFER_TYPE_PRIMARY); ``` As expected, validation was complaining that command buffers were being reused without being reset (that's good, we now know Validation Layers will warn us of wrong use). I fixed it by adding: ```cpp void RenderingDevice::_wait_for_transfer_worker(TransferWorker *p_transfer_worker) { driver->fence_wait(p_transfer_worker->command_fence); driver->command_pool_reset(p_transfer_worker->command_pool); // ! New line ! ``` **Secondary cmd buffers are subject to the same issue but I didn't alter them. I talked this with Dario and he is aware of this.** Secondary cmd buffers are currently disabled due to other issues (it's disabled on master). ### Toggle - `bool RenderingDeviceCommons::command_pool_reset_enabled` Setting it to false enforces the old behavior. Useful for debugging bugs and regressions. There's no other reason for this boolean. Possibly once it becomes well tested, the boolean could be removed entirely. ## Descriptor set batched binding Adds `command_bind_render_uniform_sets` and `add_draw_list_bind_uniform_sets` (+ compute variants). It performs the same as `add_draw_list_bind_uniform_set` (notice singular vs plural), but on multiple consecutive uniform sets, thus reducing graph and draw call overhead. ### Toggle - `bool descriptor_set_batching = true;` Setting it to false enforces the old behavior. Useful for debugging bugs and regressions. There's no other reason for this boolean. Possibly once it becomes well tested, the boolean could be removed entirely. ## Do not wait so long for swapchain Godot currently does the following: 1. Fill the entire cmd buffer with commands. 2. `submit()` - Wait with a semaphore for the swapchain. - Trigger a semaphore to indicate when we're done (so the swapchain can submit). 3. `present()` The optimization opportunity here is that 95% of Godot's rendering is done offscreen. Then a fullscreen pass copies everything to the swapchain. Godot doesn't practically render directly to the swapchain. The problem with this is that the GPU has to wait for the swapchain to be released **to start anything**, when we could start *much earlier*. Only the final blit pass must wait for the swapchain. TheForge changed it to the following (more complicated, I'm simplifying the idea): 1. Fill the entire cmd buffer with commands. 2. In `screen_prepare_for_drawing` do `submit()` - There are no semaphore waits for the swapchain. - Trigger a semaphore to indicate when we're done. 3. Fill a new cmd buffer that only does the final blit to the swapchain. 4. `submit()` - Wait with a semaphore for the submit() from step 2. - Wait with a semaphore for the swapchain (so the swapchain can submit). - Trigger a semaphore to indicate when we're done (so the swapchain can submit). 5. `present()` Dario discovered this problem independently while working on a different platform. **However TheForge's solution had to be rewritten from scratch:** The complexity to achieve the solution was high and quite difficult to maintain with the way Godot works now (after Übershaders PR). But on the other hand, re-implementing the solution became much simpler because Dario already had to do something similar: To fix an Adreno 730 driver bug, he had to implement splitting command buffers. **This is exactly what we need!**. Thus it was re-written using this existing functionality for a new purpose. To achieve this, I added a new argument, `bool p_split_cmd_buffer`, to `RenderingDeviceGraph::add_draw_list_begin`, which is only set to true by `RenderingDevice::draw_list_begin_for_screen`. The graph will split the draw list into its own command buffer. ### Toggle - `bool split_swapchain_into_its_own_cmd_buffer = true;` Setting it to false enforces the old behavior. This might be necessary for consoles which follow an alternate solution to the same problem. If not, then we should consider removing it. ## Free Shader memory PR godotengine#90993 added `shader_destroy_modules()` but it was not actually in use. This PR adds several places where `shader_destroy_modules()` is called after initialization to free up memory of SPIR-V structures that are no longer needed.
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