These need to use arrayed texture types, or Metal will complain when
binding the resource. The target layer is addressed relative to the
Layer output by the vertex pipeline, or to the ViewIndex if in a
multiview pipeline. Unlike with the s/t coordinates, Vulkan does not
forbid non-zero layer coordinates here, though this cannot be expressed
in Vulkan GLSL.
Supporting 3D textures will require additional work. Part of the problem
is that Metal does not allow texture views to subset a 3D texture, so we
need some way to pass the base depth to the shader.
Some older iOS devices don't support layered rendering. In that case,
don't set `[[render_target_array_index]]`, because the compiler will
reject the shader in that case. The client will then have to unroll the
render pass manually.
In Metal render pipelines don't have an option to set a sampleMask
parameter, the only way to get that functionality is to set the
sample_mask output of the fragment shader to this value directly.
We also need to take care to combine the fixed sample mask with the
one that the shader might possibly output.
This should hopefully reduce underutilization of the GPU, especially on
GPUs where the thread execution width is greater than the number of
control points.
This also simplifies initialization by reading the buffer directly
instead of using Metal's vertex-attribute-in-compute support. It turns
out the only way in which shader stages are allowed to differ in their
interfaces is in the number of components per vector; the base type must
be the same. Since we are using the raw buffer instead of attributes, we
can now also emit arrays and matrices directly into the buffer, instead
of flattening them and then unpacking them. Structs are still flattened,
however; this is due to the need to handle vectors with fewer components
than were output, and I think handling this while also directly emitting
structs could get ugly.
Another advantage of this scheme is that the extra invocations needed to
read the attributes when there were more input than output points are
now no more. The number of threads per workgroup is now lcm(SIMD-size,
output control points). This should ensure we always process a whole
number of patches per workgroup.
To avoid complexity handling indices in the tessellation control shader,
I've also changed the way vertex shaders for tessellation are handled.
They are now compute kernels using Metal's support for vertex-style
stage input. This lets us always emit vertices into the buffer in order
of vertex shader execution. Now we no longer have to deal with indexing
in the tessellation control shader. This also fixes a long-standing
issue where if an index were greater than the number of vertices to
draw, the vertex shader would wind up writing outside the buffer, and
the vertex would be lost.
This is a breaking change, and I know SPIRV-Cross has other clients, so
I've hidden this behind an option for now. In the future, I want to
remove this option and make it the default.
On MSL, the compiler refuses to allow access chains into a normal vector type.
What happens in practice instead is a read-modify-write where a vector type is
loaded, modified and written back.
The workaround is to convert a vector into a pointer-to-scalar before
the access chain continues to add the scalar index.
Metal is picky about interface matching. If the types don't match
exactly, down to the number of vector components, Metal fails pipline
compilation. To support pipelines where the number of components
consumed by the fragment shader is less than that produced by the vertex
shader, we have to fix up the fragment shader to accept all the
components produced.
Like with `point_size` when not rendering points, Metal complains when
writing to a variable using the `[[depth]]` qualifier when no depth
buffer be attached. In that case, we must avoid emitting `FragDepth`,
just like with `PointSize`.
I assume it will also complain if there be no stencil attachment and the
shader write to `[[stencil]]`, or it write to `[[color(n)]]` but there
be no color attachment at n.
Limit inline blocks to one per descriptor set.
This should avoid the need for complicated code to calculate the
argument buffer ID stride of an inline uniform block. If there's demand
for more inline blocks, we can revisit this.
Here, the inline uniform block is explicit: we instantiate the buffer
block itself in the argument buffer, instead of a pointer to the buffer.
I just hope this will work with the `MTLArgumentDescriptor` API...
Note that Metal recursively assigns individual members of embedded
structs IDs. This means for automatic assignment that we have to
calculate the binding stride for a given buffer block. For MoltenVK,
we'll simply increment the ID by the size of the inline uniform block.
Then the later IDs will never conflict with the inline uniform block. We
can get away with this because Metal doesn't require that IDs be
contiguous, only monotonically increasing.
There was a hack to workaround a bug in DXC where control point -> patch
constant phase was passed in Function storage, but we have to use
Workgroup here. We will not support these kinds of hacks for invalid
SPIR-V, so hack the reference files to use the "proper" fix and remove
the hack for time being.
To support loading array of array properly in tessellation, we need a
rewrite of how tessellation access chains are handled.
The major change is to remove the implicit unflatten step inside
access_chain which does not take into account the case where you load
directly from a control point array variable.
We defer unflatten step until OpLoad time instead.
This fixes cases where we load array of {array,matrix,struct}.
Removes the hacky path for MSL access chain index workaround.
Add CompilerMSL::Options::texture_1D_as_2D.
Metal imposes significant restrictions on 1D textures, including not being
renderable, clearable, or permitting mipmaps. This option allows SPIR-V 1D
textures to be treated as 2D textures to permit this additional behaviour.
App must of course supply the textures to Metal as 2D textures.
There is an implicit tristate with {-1, 0, +1} values, but it was not
obvious how this was supposed to work before studying the implementation,
so refactor into a tristate enum class.
If there are enough members in an IAB, we cannot use the constant
address space as MSL compiler complains about there being too many
members. Support emitting the device address space instead.
