MSL: Adjust FragCoord for sample-rate shading.

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.

1 files changed, 6 insertions, 0 deletions

diff --git a/spirv_msl.hpp b/spirv_msl.hpp
index 0b78df47..61d4fbea 100644
--- a/spirv_msl.hpp
+++ b/spirv_msl.hpp
@@ -401,6 +401,11 @@ public:
 		// different shaders for these three scenarios.
 		IndexType vertex_index_type = IndexType::None;
 
+		// If set, a dummy [[sample_id]] input is added to a fragment shader if none is present.
+		// This will force the shader to run at sample rate, assuming Metal does not optimize
+		// the extra threads away.
+		bool force_sample_rate_shading = false;
+
 		bool is_ios() const
 		{
 			return platform == iOS;
@@ -798,6 +803,7 @@ protected:
 	std::string to_sampler_expression(uint32_t id);
 	std::string to_swizzle_expression(uint32_t id);
 	std::string to_buffer_size_expression(uint32_t id);
+	bool is_sample_rate() const;
 	bool is_direct_input_builtin(spv::BuiltIn builtin);
 	std::string builtin_qualifier(spv::BuiltIn builtin);
 	std::string builtin_type_decl(spv::BuiltIn builtin, uint32_t id = 0);