Fix VK_KHR_separate_depth_stencil_layouts

[mikes-lunarg]

Even though the test app captured and replayed without issue, the state view
showed that VkAttachmentDescriptionStencilLayout was not saved for the
render passes that included it. This appears to be caused by the api
file silently failing to modify renderPass.AttachmentDescriptions[i]
after initially setting it, so set it after processing the pNext chain
instead.

[mikes-lunarg]

@pmuetschard I'm still unsure why the original code was failing to update the render pass object, any thoughts?

[mark-lunarg]

@mikes-lunarg, how did you manage to notice the discrepancy -- examining the state in AGI perhaps?

[mikes-lunarg]

@mark-lunarg That's exactly how I found it. As an extra sanity check I went looking for the new extension data in the state tab of AGI and it wasn't there.

[AWoloszyn]

Map access operations return a value not a reference.
See https://github.com/google/agi/tree/master/gapil#map (although that could probably be a bit clearer)
So given that:

renderPass.AttachmentDescriptions[j] returns a value
renderPass.AttachmentDescriptions[j].StencilLayout gives you a temporary, then sets the value on that temporary, then discards it.

[mikes-lunarg]

Map access operations return a value not a reference. See https://github.com/google/agi/tree/master/gapil#map (although that could probably be a bit clearer) So given that:

renderPass.AttachmentDescriptions[j] returns a value renderPass.AttachmentDescriptions[j].StencilLayout gives you a temporary, then sets the value on that temporary, then discards it.

Makes sense, thanks for the info!

[yalcinmelihyasin]

Map access operations return a value not a reference. See https://github.com/google/agi/tree/master/gapil#map (although that could probably be a bit clearer) So given that:

renderPass.AttachmentDescriptions[j] returns a value renderPass.AttachmentDescriptions[j].StencilLayout gives you a temporary, then sets the value on that temporary, then discards it.

Are the a difference in handling lvalue and rvalue?

[yalcinmelihyasin]

I am having hard time to see how changing this solves the problem

[mikes-lunarg]

I think @AWoloszyn gave a pretty thorough answer, but the short version is:

I need to might need to modify this struct inside the pNext loop so I assign it to a local variable. Then, once I have the final version of the struct, I can put it in the map. Modifying it in-place in the map (the original code) won't work because the map access or will give me a copy of the data instead of a reference.

[yalcinmelihyasin]

Based on the change are these two examples are semantically different? @AWoloszyn

renderPass.AttachmentDescriptions[j] = AttachmentDescripton(...)

attachmentDescription := AttachmentDescripton(...)
renderPass.AttachmentDescriptions[j] = attachmentDescripton

[AWoloszyn]

Based on the change are these two examples are semantically different? @AWoloszyn

renderPass.AttachmentDescriptions[j] = AttachmentDescripton(...)

attachmentDescription := AttachmentDescripton(...)
renderPass.AttachmentDescriptions[j] = attachmentDescripton

Those are the same. What would be different is

a := A(...)
a.foo = Foo()
blah.a[j] = a

a := A(...)
blah.a[j] = a
blah.a[j].foo = Foo()

The difference is that

blah.a[j].foo = Foo()

is a no-op.

This is something very subtle and error prone are there any way to fix this?

[AWoloszyn]

Map access operations return a value not a reference. See https://github.com/google/agi/tree/master/gapil#map (although that could probably be a bit clearer) So given that:
renderPass.AttachmentDescriptions[j] returns a value renderPass.AttachmentDescriptions[j].StencilLayout gives you a temporary, then sets the value on that temporary, then discards it.

Are the a difference in handling lvalue and rvalue?

This is more about with what value map[] returns. In psuedo-C++ it would look something like:

template<typename K, typename V>
class map {
    V operator[](K key);
};

This differs from a STD unordered_map which looks like

template<typename K, typename V>
class map {
    V& operator[](K key);
};

I.e. the [] operator in GAPIL returns a COPY of the element in the map, not a REFERENCE to the element in the map.

[mikes-lunarg]

@yalcinmelihyasin What is our next step? Can we merge this fix or would you like to see additional changes?