Metal: Disable unused attribute slots.

src/libANGLE/renderer/metal/VertexArrayMtl.mm

@@ -139,7 +139,7 @@ inline size_t GetIndexCount(BufferMtl *srcBuffer, size_t offset, gl::DrawElement
 
 inline void SetDefaultVertexBufferLayout(mtl::VertexBufferLayoutDesc *layout)
 {
-    layout->stepFunction = MTLVertexStepFunctionConstant;
+    layout->stepFunction = mtl::kVertexStepFunctionInvalid;
     layout->stepRate     = 0;
     layout->stride       = 0;
 }
@@ -264,6 +264,9 @@ angle::Result VertexArrayMtl::setupDraw(const gl::Context *glContext,
     {
         mVertexArrayDirty = false;
 
+        const gl::AttributesMask &programActiveAttribsMask =
+            glContext->getState().getProgram()->getExecutable().getActiveAttribLocationsMask();
+
         const std::vector<gl::VertexAttribute> &attribs = mState.getVertexAttributes();
         const std::vector<gl::VertexBinding> &bindings  = mState.getVertexBindings();
 
@@ -280,6 +283,14 @@ angle::Result VertexArrayMtl::setupDraw(const gl::Context *glContext,
 
         for (uint32_t v = 0; v < mtl::kMaxVertexAttribs; ++v)
         {
+            if (!programActiveAttribsMask.test(v))
+            {
+                desc.attributes[v].format      = MTLVertexFormatInvalid;
+                desc.attributes[v].bufferIndex = 0;
+                desc.attributes[v].offset      = 0;
+                continue;
+            }
+
             const auto &attrib               = attribs[v];
             const gl::VertexBinding &binding = bindings[attrib.bindingIndex];
 
@@ -287,7 +298,8 @@ angle::Result VertexArrayMtl::setupDraw(const gl::Context *glContext,
             desc.attributes[v].format        = mCurrentArrayBufferFormats[v]->metalFormat;
 
             bool attribEnabled = attrib.enabled;
-            if (attribEnabled && !mCurrentArrayBuffers[v])
+            if (attribEnabled &&
+                (!mCurrentArrayBuffers[v] || !mCurrentArrayBuffers[v]->getCurrentBuffer()))
             {
                 // Disable it to avoid crash.
                 attribEnabled = false;

src/libANGLE/renderer/metal/mtl_common.h

@@ -132,6 +132,11 @@ constexpr uint32_t kDefaultUniformsBindingIndex = kDefaultAttribsBindingIndex +
 
 constexpr uint32_t kStencilMaskAll = 0xff;  // Only 8 bits stencil is supported
 
+// This special constant is used to indicate that a particular vertex descriptor's buffer layout
+// index is unused.
+constexpr MTLVertexStepFunction kVertexStepFunctionInvalid =
+    static_cast<MTLVertexStepFunction>(0xff);
+
 constexpr float kEmulatedAlphaValue = 1.0f;
 
 constexpr size_t kOcclusionQueryResultSize = sizeof(uint64_t);

src/libANGLE/renderer/metal/mtl_state_cache.mm

@@ -114,7 +114,12 @@ MTLVertexDescriptor *ToObjC(const VertexDesc &desc)
 
     for (uint8_t i = 0; i < desc.numBufferLayouts; ++i)
     {
-        [objCDesc.layouts setObject:ToObjC(desc.layouts[i]) atIndexedSubscript:i];
+        // Ignore if stepFunction is kVertexStepFunctionInvalid.
+        // If we don't set this slot, it will apparently be disabled by metal runtime.
+        if (desc.layouts[i].stepFunction != kVertexStepFunctionInvalid)
+        {
+            [objCDesc.layouts setObject:ToObjC(desc.layouts[i]) atIndexedSubscript:i];
+        }
     }
 
     return [objCDesc ANGLE_MTL_AUTORELEASE];