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  • Hash : f5986fbb
    Author : Charlie Lao
    Date : 2023-07-11T12:11:20

    Vulkan: Dont break RP if there is actual render feedback loop
    
    There is a bit terminology confusion here that will be fixed in next CL.
    If a depth attachment is read only, then there is no feedback loop, we
    should not call feedback loop for read only depth attachment. The real
    depth render feedback loop mode is formed when we write to depth and
    sample from depth at the same time. In this condition, the content is
    undefined per OpenGLES spec section 9.3.1
    (https://registry.khronos.org/OpenGL/specs/es/3.2/es_spec_3.2.pdf). The
    shouldSwitchToReadOnlyDepthStencilFeedbackLoopMode() implementation
    handles the usage case that the same render pass has depth write and
    then switch to read only. Under this usage there is no actual feedback
    loop, and we should still work properly by end current render pass and
    start a new render pass with read only depth attachment. This
    implementation also treating the actual feedback loop case exactly the
    same way by ending render pass first, even though this is undefined
    behavior.  gangstar_vegas has the exact this undefined behavior usage
    case, where it write and sample from depth buffer at the same draw call.
    Native driver is not ending the render pass but ANGLE currently does.
    This puts ANGLE into worse performance. Since this is undefined
    behavior, either way is correct. This CL checks if there is an actual
    feedback loop in the current render pass and if yes, we adopt the native
    driver's behavior that keep the current render pass going. This improves
    gangstar_vegas frame time from 4.365ms to 3.89ms, and interestingly,
    yield the same golden image.
    
    Bug: b/289436017
    Change-Id: Ifc04ecd8ad6455a88e8615bd5452b9cce88c6687
    Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4679361
    Reviewed-by: Yuxin Hu <yuxinhu@google.com>
    Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
    Commit-Queue: Charlie Lao <cclao@google.com>
    

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