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Update documentation for descriptor set support in MSL/HLSL.

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Ancient outdated README.
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Hans-Kristian Arntzen 2022-02-04 12:51:32 +01:00
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README.md
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@ -381,10 +381,28 @@ for (auto &remap : compiler->get_combined_image_samplers())
If your target is Vulkan GLSL, `--vulkan-semantics` will emit separate image samplers as you'd expect.
The command line client calls `Compiler::build_combined_image_samplers` automatically, but if you're calling the library, you'll need to do this yourself.
#### Descriptor sets (Vulkan GLSL) for backends which do not support them (HLSL/GLSL/Metal)
#### Descriptor sets (Vulkan GLSL) for backends which do not support them (pre HLSL 5.1 / GLSL)
Descriptor sets are unique to Vulkan, so make sure that descriptor set + binding is remapped to a flat binding scheme (set always 0), so that other APIs can make sense of the bindings.
This can be done with `Compiler::set_decoration(id, spv::DecorationDescriptorSet)`.
This can be done with `Compiler::set_decoration(id, spv::DecorationDescriptorSet)`. For other backends like MSL and HLSL, descriptor sets
can be used, with some minor caveats, see below.
##### MSL 2.0+
Metal supports indirect argument buffers (--msl-argument-buffers). In this case, descriptor sets become argument buffers,
and bindings are mapped to [[id(N)]] within the argument buffer. One quirk is that arrays of resources consume multiple ids,
where Vulkan does not. This can be worked around either from shader authoring stage
or remapping bindings as needed to avoid the overlap.
There is also a rich API to declare remapping schemes which is intended to work like
the pipeline layout in Vulkan. See `CompilerMSL::add_msl_resource_binding`. Remapping combined image samplers for example
must be split into two bindings in MSL, so it's possible to declare an id for the texture and sampler binding separately.
##### HLSL - SM 5.1+
In SM 5.1+, descriptor set bindings are interpreted as register spaces directly. In HLSL however, arrays of resources consume
multiple binding slots where Vulkan does not, so there might be overlap if the SPIR-V was not authored with this in mind.
This can be worked around either from shader authoring stage (don't assign overlapping bindings)
or remap bindings in SPIRV-Cross as needed to avoid the overlap.
#### Linking by name for targets which do not support explicit locations (legacy GLSL/ESSL)