implicit subpass dependency and barriers

Question

I must copy a color attachment into a buffer. Do I need an image memory barrier between the end of the render pass and the copy operation, to ensure visibility to the transfer?

The render pass has a single subpass with a single attachment. The VkAttachmentDescription.finalLayout differs from the VkAttachmentReference.layout, so an implicit subpass dependency with dstSubpass = VK_SUBPASS_EXTERNAL does indeed exists.

My confusion arises from the description of such implicit external dependency: it has a dstAccessMask = 0 and dstStageMask = VK_PIPELINE_STAGE_BOTTOM_OF_PIPE_BIT. Since the transfer stage happens before the end of pipe pseudostage, does it mean I need to specify a barrier? Or does the implicit dependency only affect operations in the render pass, so I need a barrier anyway? As a side question, what is the meaning of 0 as a source or destination access mask?

Validation layers do not report any issue either with or without a barrier, and the output is as expected, but I am not sure it is correct though.

Answer 1

Yes, you need a dependency between _______ and _______ in Vulkan.

Implicit dependency exists in the spec only for formal reasons. Without it you would not know when the layout transition happens when used e.g. with a Semaphore. But in practice the implicit dependency is no-op. Its dst half does not cover anything.

The meaning of 0 in access mask is "no access". E.g. VK_PIPELINE_STAGE_BOTTOM_OF_PIPE_BIT stage does not read nor write anything, so the access flag accompanying it should be 0.