[core] make vertex descriptors constexpr

3 files changed, 82 insertions, 65 deletions

diff --git a/src/mbgl/gfx/attribute.hpp b/src/mbgl/gfx/attribute.hpp
index 66bbf1ec5e..ec34a69a9a 100644
--- a/src/mbgl/gfx/attribute.hpp
+++ b/src/mbgl/gfx/attribute.hpp
@@ -4,6 +4,7 @@
 #include <mbgl/util/indexed_tuple.hpp>
 
 #include <array>
+#include <type_traits>
 #include <cstddef>
 
 #define MBGL_DEFINE_ATTRIBUTE(type_, n_, name_)                                                    \
@@ -14,6 +15,8 @@
         }                                                                                          \
     }
 
+#define MBGL_VERTEX_ALIGN __attribute__((aligned(4)))
+
 namespace mbgl {
 namespace gfx {
 
@@ -25,6 +28,12 @@ public:
     using Value = std::array<T, N>;
 };
 
+struct VertexDescriptor {
+    const uint8_t size;
+    const uint8_t count;
+    const uint8_t offsets[5];
+};
+
 // Attribute binding requires member offsets. The only standard way to
 // obtain an offset is the offsetof macro. The offsetof macro has defined
 // behavior only for standard layout types. That rules out std::tuple and
@@ -35,114 +44,123 @@ public:
 namespace detail {
 
 template <class...>
-class Vertex;
-
-template <>
-class Vertex<> {
-public:
-    using VertexType = Vertex<>;
-};
+struct Vertex;
 
 template <class A1>
-class Vertex<A1> {
-public:
+struct Vertex<A1> {
+    using Type = Vertex<A1>;
     A1 a1;
-
-    using VertexType = Vertex<A1>;
-    static const std::size_t attributeOffsets[1];
-};
+} MBGL_VERTEX_ALIGN;
 
 template <class A1, class A2>
-class Vertex<A1, A2> {
-public:
+struct Vertex<A1, A2> {
+    using Type = Vertex<A1, A2>;
     A1 a1;
     A2 a2;
-
-    using VertexType = Vertex<A1, A2>;
-    static const std::size_t attributeOffsets[2];
-};
+} MBGL_VERTEX_ALIGN;
 
 template <class A1, class A2, class A3>
-class Vertex<A1, A2, A3> {
-public:
+struct Vertex<A1, A2, A3> {
+    using Type = Vertex<A1, A2, A3>;
     A1 a1;
     A2 a2;
     A3 a3;
-
-    using VertexType = Vertex<A1, A2, A3>;
-    static const std::size_t attributeOffsets[3];
-};
+} MBGL_VERTEX_ALIGN;
 
 template <class A1, class A2, class A3, class A4>
-class Vertex<A1, A2, A3, A4> {
-public:
+struct Vertex<A1, A2, A3, A4> {
+    using Type = Vertex<A1, A2, A3, A4>;
     A1 a1;
     A2 a2;
     A3 a3;
     A4 a4;
-
-    using VertexType = Vertex<A1, A2, A3, A4>;
-    static const std::size_t attributeOffsets[4];
-};
+} MBGL_VERTEX_ALIGN;
 
 template <class A1, class A2, class A3, class A4, class A5>
-class Vertex<A1, A2, A3, A4, A5> {
-public:
+struct Vertex<A1, A2, A3, A4, A5> {
+    using Type = Vertex<A1, A2, A3, A4, A5>;
     A1 a1;
     A2 a2;
     A3 a3;
     A4 a4;
     A5 a5;
+} MBGL_VERTEX_ALIGN;
 
-    using VertexType = Vertex<A1, A2, A3, A4, A5>;
-    static const std::size_t attributeOffsets[5];
-};
+template <class>
+struct Descriptor;
 
 template <class A1>
-const std::size_t Vertex<A1>::attributeOffsets[1] = {
-    offsetof(VertexType, a1)
+struct Descriptor<Vertex<A1>> {
+    using Type = Vertex<A1>;
+    static_assert(sizeof(Type) < 256, "vertex type must be smaller than 256 bytes");
+    static_assert(std::is_standard_layout<Type>::value, "vertex type must use standard layout");
+    static constexpr const VertexDescriptor Data = { sizeof(Type), 1, {
+        offsetof(Type, a1),
+    }};
 };
 
 template <class A1, class A2>
-const std::size_t Vertex<A1, A2>::attributeOffsets[2] = {
-    offsetof(VertexType, a1),
-    offsetof(VertexType, a2)
+struct Descriptor<Vertex<A1, A2>> {
+    using Type = Vertex<A1, A2>;
+    static_assert(sizeof(Type) < 256, "vertex type must be smaller than 256 bytes");
+    static_assert(std::is_standard_layout<Type>::value, "vertex type must use standard layout");
+    static constexpr const VertexDescriptor Data = { sizeof(Type), 2, {
+        offsetof(Type, a1),
+        offsetof(Type, a2),
+    }};
 };
 
 template <class A1, class A2, class A3>
-const std::size_t Vertex<A1, A2, A3>::attributeOffsets[3] = {
-    offsetof(VertexType, a1),
-    offsetof(VertexType, a2),
-    offsetof(VertexType, a3)
+struct Descriptor<Vertex<A1, A2, A3>> {
+    using Type = Vertex<A1, A2, A3>;
+    static_assert(sizeof(Type) < 256, "vertex type must be smaller than 256 bytes");
+    static_assert(std::is_standard_layout<Type>::value, "vertex type must use standard layout");
+    static constexpr const VertexDescriptor Data = { sizeof(Type), 3, {
+        offsetof(Type, a1),
+        offsetof(Type, a2),
+        offsetof(Type, a3),
+    }};
 };
 
 template <class A1, class A2, class A3, class A4>
-const std::size_t Vertex<A1, A2, A3, A4>::attributeOffsets[4] = {
-    offsetof(VertexType, a1),
-    offsetof(VertexType, a2),
-    offsetof(VertexType, a3),
-    offsetof(VertexType, a4)
+struct Descriptor<Vertex<A1, A2, A3, A4>> {
+    using Type = Vertex<A1, A2, A3, A4>;
+    static_assert(sizeof(Type) < 256, "vertex type must be smaller than 256 bytes");
+    static_assert(std::is_standard_layout<Type>::value, "vertex type must use standard layout");
+    static constexpr const VertexDescriptor Data = { sizeof(Type), 4, {
+        offsetof(Type, a1),
+        offsetof(Type, a2),
+        offsetof(Type, a3),
+        offsetof(Type, a4),
+    }};
 };
 
 template <class A1, class A2, class A3, class A4, class A5>
-const std::size_t Vertex<A1, A2, A3, A4, A5>::attributeOffsets[5] = {
-    offsetof(VertexType, a1),
-    offsetof(VertexType, a2),
-    offsetof(VertexType, a3),
-    offsetof(VertexType, a4),
-    offsetof(VertexType, a5)
+struct Descriptor<Vertex<A1, A2, A3, A4, A5>> {
+    using Type = Vertex<A1, A2, A3, A4, A5>;
+    static_assert(sizeof(Type) < 256, "vertex type must be smaller than 256 bytes");
+    static_assert(std::is_standard_layout<Type>::value, "vertex type must use standard layout");
+    static constexpr const VertexDescriptor Data = { sizeof(Type), 5, {
+        offsetof(Type, a1),
+        offsetof(Type, a2),
+        offsetof(Type, a3),
+        offsetof(Type, a4),
+        offsetof(Type, a5),
+    }};
 };
 
 template <class... As>
-class Vertex<TypeList<As...>> {
-public:
-    using VertexType = Vertex<typename As::Type::Value...>;
+struct Vertex<TypeList<As...>> {
+    using Type = Vertex<typename As::Type::Value...>;
 };
 
 } // namespace detail
 
 template <class A>
-using Vertex = typename detail::Vertex<A>::VertexType;
+using Vertex = typename detail::Vertex<A>::Type;
+
+template <class V>
+constexpr const VertexDescriptor vertexDescriptor = detail::Descriptor<V>::Data;
 
 } // namespace gfx
 } // namespace mbgl
diff --git a/src/mbgl/gl/attribute.hpp b/src/mbgl/gl/attribute.hpp
index 95a9942381..e03e3ebf89 100644
--- a/src/mbgl/gl/attribute.hpp
+++ b/src/mbgl/gl/attribute.hpp
@@ -58,14 +58,13 @@ AttributeBinding attributeBinding(const VertexBuffer<Vertex>& buffer,
     static_assert(std::is_standard_layout<Vertex>::value, "vertex type must use standard layout");
     assert(attributeSize >= 1);
     assert(attributeSize <= 4);
-    assert(Vertex::attributeOffsets[attributeIndex] <= std::numeric_limits<uint32_t>::max());
-    static_assert(sizeof(Vertex) <= std::numeric_limits<uint32_t>::max(), "vertex too large");
+    assert(gfx::vertexDescriptor<Vertex>.count > attributeIndex);
     return AttributeBinding {
         DataTypeOf<typename AttributeType::ValueType>::value,
         static_cast<uint8_t>(attributeSize),
-        static_cast<uint32_t>(Vertex::attributeOffsets[attributeIndex]),
+        static_cast<uint32_t>(gfx::vertexDescriptor<Vertex>.offsets[attributeIndex]),
         buffer.buffer,
-        static_cast<uint32_t>(sizeof(Vertex)),
+        static_cast<uint32_t>(gfx::vertexDescriptor<Vertex>.size),
         0,
     };
 }
diff --git a/src/mbgl/programs/collision_box_program.cpp b/src/mbgl/programs/collision_box_program.cpp
index 57107db75d..869f4be61f 100644
--- a/src/mbgl/programs/collision_box_program.cpp
+++ b/src/mbgl/programs/collision_box_program.cpp
@@ -2,6 +2,6 @@
 
 namespace mbgl {
 
-static_assert(sizeof(CollisionBoxProgram::LayoutVertex) == 14, "expected CollisionBoxVertex size");
+static_assert(sizeof(CollisionBoxProgram::LayoutVertex) == 16, "expected CollisionBoxVertex size");
 
 } // namespace mbgl