关于 VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT 的困惑

Question

When I read Vulkan Specs for VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT , it says:

VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT specifies the stage of the pipeline after blending .... This stage also includes subpass load and store operations,....

I am really confused as why this stage can include subpass load and store operation.

From my understanding, in a subpass that performs drawing on a color attachment:

• Subpass load operation happens first in submission order.
• Then , there is graphics pipeline ( vkCmdDraw ) submitted afterwards. Among all those graphics pipeline stages, there is a final color output stage that is after color blending. That stage is called VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT
• At the end, subpass store operation happens.

Since those three are very distinct stages all with their own purposes, how come they can be all specified in one VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT .

If put this stage in dstStageMask , is subpass load operation waiting for the srcStageMask or the color output stage of a graphics pipeline waiting for srcStageMask .

Similarly, if I put this stage in srcStageMask , is Vulkan waiting for previous subpass to perform store operation instead of the color output stage in a graphics pipeline?

Answer 1

I am really confused as why this stage can include subpass load and store operation.

It "can" do so by fiat; it does so because the standard says that it does so.

All of this stuff is an abstraction, a model of a conceptual GPU that doesn't necessarily exist in any particular hardware. The job of a Vulkan implementation is to translate this abstraction for their particular hardware.

Subpass load/store operations may or may not be a distinct thing for any particular piece of hardware. Some hardware has them; others do not.

If these processes are distinct for a particular GPU, and a user specifies a dependency through the color output stage, it is that implementation's job to include whatever is necessary to make that dependency work with both the usual color outputs and the subpass load/store hardware. That is, if there's separate caching for both operations (somehow), the implementation has to handle both sets of caches.

So the answer to this question:

If put this stage in dstStageMask , is subpass load operation waiting for the srcStageMask or the color output stage of a graphics pipeline waiting for srcStageMask .

is both . If these are two separate processes on a particular GPU, the implementation must make them appear as though they are the same process.

That being said, attachment load only happens before the first subpass that uses the attachment, and attachment store only happens after the last subpass that uses the attachment. So its not like this is a big deal that impacts every dependency.