vk_rasterizer: Implement constant attributes

Constant attributes (in OpenGL known disabled attributes) are not
supported on Vulkan, even with extensions. To emulate this behavior we
return zero on reads from disabled vertex attributes in shader code.
This has no caching cost because attribute formats are not dynamic state
on Vulkan and we have to store it in the pipeline cache anyway.

- Fixes Animal Crossing: New Horizons terrain borders

src/video_core/renderer_vulkan/vk_pipeline_cache.cpp

@@ -312,7 +312,9 @@ VKPipelineCache::DecompileShaders(const GraphicsPipelineCacheKey& key) {
         ASSERT(point_size != 0.0f);
     }
     for (std::size_t i = 0; i < Maxwell::NumVertexAttributes; ++i) {
-        specialization.attribute_types[i] = fixed_state.vertex_input.attributes[i].Type();
+        const auto& attribute = fixed_state.vertex_input.attributes[i];
+        specialization.enabled_attributes[i] = attribute.enabled.Value() != 0;
+        specialization.attribute_types[i] = attribute.Type();
     }
     specialization.ndc_minus_one_to_one = fixed_state.rasterizer.ndc_minus_one_to_one;
 

src/video_core/renderer_vulkan/vk_rasterizer.cpp

@@ -875,7 +875,7 @@ void RasterizerVulkan::SetupVertexArrays(FixedPipelineState::VertexInput& vertex
 
     for (std::size_t index = 0; index < Maxwell::NumVertexAttributes; ++index) {
         const auto& attrib = regs.vertex_attrib_format[index];
-        if (!attrib.IsValid()) {
+        if (attrib.IsConstant()) {
             vertex_input.SetAttribute(index, false, 0, 0, {}, {});
             continue;
         }

src/video_core/renderer_vulkan/vk_shader_decompiler.cpp

@@ -731,8 +731,10 @@ private:
             if (!IsGenericAttribute(index)) {
                 continue;
             }
-
             const u32 location = GetGenericAttributeLocation(index);
+            if (!IsAttributeEnabled(location)) {
+                continue;
+            }
             const auto type_descriptor = GetAttributeType(location);
             Id type;
             if (IsInputAttributeArray()) {
@@ -976,6 +978,10 @@ private:
         return stage == ShaderType::TesselationControl;
     }
 
+    bool IsAttributeEnabled(u32 location) const {
+        return stage != ShaderType::Vertex || specialization.enabled_attributes[location];
+    }
+
     u32 GetNumInputVertices() const {
         switch (stage) {
         case ShaderType::Geometry:
@@ -1192,16 +1198,20 @@ private:
                 UNIMPLEMENTED_MSG("Unmanaged FrontFacing element={}", element);
                 return {v_float_zero, Type::Float};
             default:
-                if (IsGenericAttribute(attribute)) {
-                    const u32 location = GetGenericAttributeLocation(attribute);
-                    const auto type_descriptor = GetAttributeType(location);
-                    const Type type = type_descriptor.type;
-                    const Id attribute_id = input_attributes.at(attribute);
-                    const std::vector elements = {element};
-                    const Id pointer = ArrayPass(type_descriptor.scalar, attribute_id, elements);
-                    return {OpLoad(GetTypeDefinition(type), pointer), type};
+                if (!IsGenericAttribute(attribute)) {
+                    break;
                 }
-                break;
+                const u32 location = GetGenericAttributeLocation(attribute);
+                if (!IsAttributeEnabled(location)) {
+                    // Disabled attributes (also known as constant attributes) always return zero.
+                    return {v_float_zero, Type::Float};
+                }
+                const auto type_descriptor = GetAttributeType(location);
+                const Type type = type_descriptor.type;
+                const Id attribute_id = input_attributes.at(attribute);
+                const std::vector elements = {element};
+                const Id pointer = ArrayPass(type_descriptor.scalar, attribute_id, elements);
+                return {OpLoad(GetTypeDefinition(type), pointer), type};
             }
             UNIMPLEMENTED_MSG("Unhandled input attribute: {}", static_cast<u32>(attribute));
             return {v_float_zero, Type::Float};

src/video_core/renderer_vulkan/vk_shader_decompiler.h

@@ -88,7 +88,8 @@ struct Specialization final {
     u32 shared_memory_size{};
 
     // Graphics specific
-    std::optional<float> point_size{};
+    std::optional<float> point_size;
+    std::bitset<Maxwell::NumVertexAttributes> enabled_attributes;
     std::array<Maxwell::VertexAttribute::Type, Maxwell::NumVertexAttributes> attribute_types{};
     bool ndc_minus_one_to_one{};
 };