Commits on Jan 31, 2024

1. [Impeller] Implement framebuffer-fetch via subpasses in Vulkan withou… …

   …t extensions. (#50154)
   
   * Subpasses are not exposed in the HAL and the need for subpasses in Vulkan can
     be determined based on the presence and use of input-attachments in the
     shaders. This information is already reflected by the compiler. Because of
     this, all references to subpasses have been removed from APIs above the HAL.
   * `RenderPassBuilderVK` is a lightweight object used to generate render passes
     to use either with the pipelines (compat, base, or per-subpass) or during
     rendering along with the framebuffer. Using the builder also sets up the
     right subpass dependencies. As long as the builder contains compatible
     attachments and subpass counts, different subpasses stamped by the builder
     (via the `Build` method) are guaranteed to be compatible per the rules in the
     spec.
   * Pass attachments are now in the `eGeneral` layout. There was no observable
     difference in performance when manually inserting the right transitions.
     Except, a lot of transitions needed to be inserted. If we need it, we can add
     it back in short order. I wouldn't be averse to adding it if reviewers
     insist.
   * Additional pipeline state objects need not be created as the sub-pass
     self-dependencies are sufficient to setup the render-pass.
   * Speaking of the `rasterization_order_attachment_access` extension, its use has
     been removed in this patch. I am prototyping adding it back to measure the
     overhead introduced by manual subpass management. If the overhead is
     measurable, we can use the extension on devices that have it as an added
     optimization.
   * The complexity of command encoding remains linear (to the number of commands)
     per pass.
   * This patch only works on a single color attachment being used as an input
     attachment. While this is sufficient for current use cases, the Metal
     implementation is significantly more capable since the multiple attachments
     and attachment types (depth) are already supported. Rounding out support for
     this is in progress.

   

   chinmaygarde authored Jan 31, 2024
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