Firstly, never flatten inputs or outputs in multi-patch mode.
The main scenario where we do need to care is Block IO.
In this case, we should only flatten the top-level member, and after
that we use access chains as normal.
Using structs in Input storage class is now possible as well. We don't
need to consider per-location fixups at all here. In Vulkan, IO structs
must match exactly. Only plain vectors can have smaller vector sizes as
a special case.
Subsequent stages can legally attempt to read from these variables,
which causes compilation failure.
Always make sure we emit user outputs in vertex shaders if they are
active in the entry point.
In Metal, the `[[position]]` input to a fragment shader remains at
fragment center, even at sample rate, like OpenGL and Direct3D. In
Vulkan, however, when the fragment shader runs at sample rate, the
`FragCoord` builtin moves to the sample position in the framebuffer,
instead of the fragment center. To account for this difference, adjust
the `FragCoord`, if present, by the sample position. The -0.5 offset is
because the fragment center is at (0.5, 0.5).
Also, add an option to force sample-rate shading in a fragment shader.
Since Metal has no explicit control for this, this is done by adding a
dummy `[[sample_id]]` which is otherwise unused, if none is already
present. This is intended to be used from e.g. MoltenVK when a
pipeline's `minSampleShading` value is nonzero.
Instead of checking if any `Input` variables have `Sample`
interpolation, I've elected to check that the `SampleRateShading`
capability is present. Since `SampleId`, `SamplePosition`, and the
`Sample` interpolation decoration require this cap, this should be
equivalent for any valid SPIR-V module. If this isn't acceptable, let me
know.
We have been interchanging spv and SPIRV_Cross_ for a while, which
causes weirdness since we don't explicitly ban SPIRV_Cross identifiers,
as these identifiers are generally used for interface variable
workarounds.
New in MSL 2.3 is a template that can be used in the place of a scalar
type in a stage-in struct. This template has methods which interpolate
the varying at the given points. Curiously, you can't set interpolation
attributes on such a varying; perspective-correctness is encoded in the
type, while interpolation must be done using one of the methods. This
makes using this somewhat awkward from SPIRV-Cross, requiring us to jump
through a bunch of hoops to make this all work.
Using varyings from functions in particular is a pain point, requiring
us to pass the stage-in struct itself around. An alternative is to pass
references to the interpolants; except this will fall over badly with
composite types, which naturally must be flattened. As with
tessellation, dynamic indexing isn't supported with pull-model
interpolation. This is because of the need to reference the original
struct member in order to call one of the pull-model interpolation
methods on it. Also, this is done at the variable level; this means that
if one varying in a struct is used with the pull-model functions, then
the entire struct is emitted as pull-model interpolants.
For some reason, this was not documented in the MSL spec, though there
is a property on `MTLDevice`, `supportsPullModelInterpolation`,
indicating support for this, which *is* documented. This does not appear
to be implemented yet for AMD: it returns `NO` from
`supportsPullModelInterpolation`, and pipelines with shaders using the
templates fail to compile. It *is* implemeted for Intel. It's probably
also implemented for Apple GPUs: on Apple Silicon, OpenGL calls down to
Metal, and it wouldn't be possible to use the interpolation functions
without this implemented in Metal.
Based on my testing, where SPIR-V and GLSL have the offset relative to
the pixel center, in Metal it appears to be relative to the pixel's
upper-left corner, as in HLSL. Therefore, I've added an offset 0.4375,
i.e. one half minus one sixteenth, to all arguments to
`interpolate_at_offset()`.
This also fixes a long-standing bug: if a pull-model interpolation
function is used on a varying, make sure that varying is declared. We
were already doing this only for the AMD pull-model function,
`interpolateAtVertexAMD()`; for reasons which are completely beyond me,
we weren't doing this for the base interpolation functions. I also note
that there are no tests for the interpolation functions for GLSL or
HLSL.
I kept the code to replace constant zero arguments, because `Bias` and
`Grad` still have some problems on desktop GPUs.
`Bias` works on AMD GPUs. `Grad` does not. Both work on Intel. Still
needs testing on NV. It will definitely work with Apple GPUs.
`half` cannot be bitcasted to `float`, because the two types are not the
same size. Use an expanding cast instead.
We were already doing this for stores to the tessellation levels; why I
didn't also do this for loads is beyond me.
Fix reversed coordinates: `y` should be used to calculate the row
address. Align row address to the row stride.
I've made the row alignment a function constant; this makes it possible
to override it at pipeline compile time.
Honestly, I don't know how this worked at all for Epic. It definitely
didn't work in the CTS prior to this.
MSL 2.3 has everything needed to support this extension on all
platforms. The existing `discard_fragment()` function was given demote
semantics, similar to Direct3D, and the `simd_is_helper_thread()`
function was finally added to iOS.
I've left the old test alone. Should I remove it in favor of these?
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.
Account for a non-zero base instance when calculating the view index and
the "real" instance index. Before, it was likely broken with a non-zero
base instance, since the calculated instance index could be less than
the base instance.
- Do not silently drop reserved identifiers in the parser. This makes it
possible to reflect identifiers which are reserved by the
cross-compiler module.
- Instead of dropping the name, emit _RESERVED_IDENTIFIER_FIXUP in the
source to make it clear that a name has been rewritten.
- Document what is reserved and not.
Prior to this point, we were treating them as flattened, as they are in
old-style tessellation control shaders, and still are for structs in
new-style shaders. This is not true for outputs; output composites are
not flattened at all. This semantic mismatch broke a Vulkan CTS test.
It should now pass.
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.
