MoltenVK Runtime User Guide

Copyright (c) 2014-2018 The Brenwill Workshop Ltd.

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Table of Contents

About This Document

This document describes how to integrate the MoltenVK runtime distribution package into a game or application, once MoltenVK has been built into a framework or library for iOS or macOS.

To learn how to use the MoltenVK open-source repository to build a MoltenVK runtime distribution package, see the main README.md document in the MoltenVK repository.

About MoltenVK

MoltenVK is an implementation of the Vulkan 1.0 graphics and compute API, that runs on Apple's Metal graphics and compute framework on both iOS and macOS.

MoltenVK allows you to use the Vulkan graphics and compute API to develop modern, cross-platform, high-performance graphical games and applications, and to run them across many platforms, including both iOS and macOS.

Metal uses a different shading language, the Metal Shading Language (MSL), than Vulkan, which uses SPIR-V. MoltenVK automatically converts your SPIR-V shaders to their MSL equivalents. This can be performed transparently at run time, using the Runtime Shader Conversion feature of MoltenVK, or at development time using the [MoltenVKShaderConverter] (#shader_converter_tool) tool provided with this MoltenVK distribution package.

To provide Vulkan capability to the iOS and macOS platforms, MoltenVK uses Apple's publicly available API‘s, including Metal. MoltenVK does not use any private or undocumented API calls or features, so your app will be compatible with all standard distribution channels, including *Apple’s App Store*.

Installing MoltenVK in Your Vulkan Application

Build and Runtime Requirements

At development time, MoltenVK references advanced OS frameworks during building.

  • Xcode 9 or above is required to build and link MoltenVK projects.

Once built, MoltenVK can be run on iOS or macOS devices that support Metal.

  • MoltenVK requires at least macOS 10.11 or iOS 9.
  • Information on macOS devices that are compatible with Metal can be found in this article.
  • Information on compatible iOS devices that are compatible with Metal can be found in this article.

Install as Static Library Framework

Installation of MoltenVK is straightforward and easy!

For most applications, you can install MoltenVK as a static library framework that will be embedded directly in your application executable, or a component library within your application. This is simple and straightforward, and is the recommended installation approach for all applications.

To add MoltenVK as a static library framework to your existing Vulkan application, follow the steps in this section. If you're new to MoltenVK, it is recommended that you start with a smaller project to help you understand the transition, and to reduce the possibility of needing to make modifications to your shaders to ensure their compatibility with the Metal environment.

  1. Open your application in Xcode and select your application's target in the Project Navigator panel.

  2. Open the Build Settings tab, and in the Framework Search Paths (aka FRAMEWORK_SEARCH_PATHS) setting:

    • If building for iOS, add an entry that points to the MoltenVK/iOS folder, found in the MoltenVK distribution package.
    • If building for macOS, add an entry that points to the MoltenVK/macOS folder, found in the MoltenVK distribution package.

  3. If using IOSurfaces on iOS, in the Build Settings tab, open the iOS Deployment Target (aka IPHONEOS_DEPLOYMENT_TARGET) setting, and ensure it is set to a value of iOS 11.0 or greater.

  4. On the Build Phases tab, open the Link Binary With Libraries list.

    1. Drag the MoltenVK/iOS/MoltenVK.framework or MoltenVK/macOS/MoltenVK.framework folder, found in the MoltenVK distribution package, to the Link Binary With Libraries list.
    2. Click the + button, and (selecting from the list of system libraries) add libc++.tbd.
    3. If you do not have the Link Frameworks Automatically (aka CLANG_MODULES_AUTOLINK) and Enable Modules (C and Objective-C (aka CLANG_ENABLE_MODULES) settings enabled, click the + button, and (selecting from the list of system frameworks) add the following frameworks:
      • Metal.framework
      • Foundation.framework.
      • QuartzCore.framework
      • IOKit.framework (macOS)
      • UIKit.framework (iOS)
      • IOSurface.framework (macOS, or iOS if IPHONEOS_DEPLOYMENT_TARGET is at least iOS 11.0)

  5. When a Metal app is running from Xcode, the default Scheme settings reduce performance. To improve performance and gain the benefits of Metal, perform the following in Xcode:

    1. Open the Scheme Editor for building your main application. You can do this by selecting Edit Scheme... from the Scheme drop-down menu, or select Product -> Scheme -> Edit Scheme... from the main menu.
    2. On the Info tab, set the Build Configuration to Release, and disable the Debug executable check-box.
    3. On the Options tab, disable both the Metal API Validation and GPU Frame Capture options. For optimal performance, you may also consider disabling the other simulation and debugging options on this tab. For further information, see the Xcode Scheme Settings and Performance section of Apple's Metal Programming Guide documentation.

Install as Dynamic Library

For some applications, you may prefer to install MoltenVK as a dynamic library. This is only recommended for developers who are used to working with dynamic libraries, and even then, the preferred approach is to link the MoltenVK static library framework into a dynamic library of your own creation, in order to give you the most flexibility for organizing your dynamic libraries.

In order to install MoltenVK as its own dynamic library in your application, follow these instructions:

  1. Open your application in Xcode and select your application's target in the Project Navigator panel.

  2. On the Build Settings tab:

    1. In the Header Search Paths (aka HEADER_SEARCH_PATHS) setting, add an entry that points to the MoltenVK/include folder, found in the MoltenVK distribution package.
    2. In the Library Search Paths (aka LIBRARY_SEARCH_PATHS) setting, add an entry that points to either the MoltenVK/iOS or MoltenVK/macOS folder, found in the MoltenVK distribution package.
    3. In the Runpath Search Paths (aka LD_RUNPATH_SEARCH_PATHS) setting, add a path that matches the library destination you established in Step 2 above. If the dynamic library is to be embedded within your application, you would typically set this value to either @executable_path or @loader_path. The libMoltenVK.dylib library is internally configured to be located at @rpath/libMoltenVK.dylib.

  3. If using IOSurfaces on iOS, in the Build Settings tab, open the iOS Deployment Target (aka IPHONEOS_DEPLOYMENT_TARGET) setting, and ensure it is set to a value of iOS 11.0 or greater.

  4. On the Build Phases tab, open the Link Binary With Libraries list.

    1. Drag the MoltenVK/iOS/libMoltenVK.dylib or MoltenVK/macOS/libMoltenVK.dylib file, found in the MoltenVK distribution package, to the Link Binary With Libraries list.
    2. Click the + button, and (selecting from the list of system libraries) add libc++.tbd.
    3. If you do not have the Link Frameworks Automatically (aka CLANG_MODULES_AUTOLINK) and Enable Modules (C and Objective-C (aka CLANG_ENABLE_MODULES) settings enabled, click the + button, and (selecting from the list of system frameworks) add the following frameworks:
      • Metal.framework
      • Foundation.framework.
      • QuartzCore.framework
      • IOKit.framework (macOS)
      • UIKit.framework (iOS)
      • IOSurface.framework (macOS, or iOS if IPHONEOS_DEPLOYMENT_TARGET is at least iOS 11.0)

  5. Arrange to install the libMoltenVK.dylib file in your application environment:

    • To copy the libMoltenVK.dylib file into your application or component library:

      1. On the Build Phases tab, add a new Copy Files build phase.
      2. Set the Destination into which you want to place the libMoltenVK.dylib file. Typically this will be Executables.
      3. Drag the MoltenVK/iOS/libMoltenVK.dylib or MoltenVK/macOS/libMoltenVK.dylib file to the Copy Files list in this new build phase.

    • Alternately, you may create your own installation mechanism to install the MoltenVK/iOS/libMoltenVK.dylib or MoltenVK/macOS/libMoltenVK.dylib file into a standard iOS or macOS system library folder on the user's device.

The Cube-iOS and Cube-macOS MoltenVK demo apps, found in the Demos.xcworkspace, located in the Demos folder within the MoltenVK distribution package, are simple examples of installing MoltenVK as a dynamic library embedded within an iOS or macOS application, respectively.

Interacting with the MoltenVK Runtime

You programmatically configure and interact with the MoltenVK runtime through function calls, enumeration values, and capabilities, in exactly the same way you do with other Vulkan implementations. The MoltenVK.framework contains several header files that define access to Vulkan and MoltenVK function calls.

In your application code, you access Vulkan features through the API defined in the standard vulkan.h header file. This file is included in the MoltenVK framework, and can be included in your source code files as follows:

#include <vulkan/vulkan.h>

In addition to the core Vulkan API, MoltenVK also supports the following Vulkan extensions:

  • VK_MVK_moltenvk
  • VK_KHR_swapchain
  • VK_KHR_surface
  • VK_MVK_ios_surface (iOS)
  • VK_MVK_macos_surface (macOS)
  • VK_AMD_negative_viewport_height
  • VK_IMG_format_pvrtc (iOS)

In order to visibly display your content on iOS or macOS, you must enable the VK_MVK_ios_surface or VK_MVK_macos_surface extension, respectively, and use the functions defined for that extension to create a Vulkan rendering surface.

You can enable each of these extensions by defining the VK_USE_PLATFORM_IOS_MVK or VK_USE_PLATFORM_MACOS_MVK guard macro in your compiler build settings. See the description of the mvk_vulkan.h file below for a convenient way to enable these extensions automatically.

MoltenVK Extension

The VK_MVK_moltenvk Vulkan extension provides functionality beyond the standard Vulkan API, to support configuration options, license registration, and behaviour that is specific to the MoltenVK implementation of Vulkan. You can access this functionality by including the vk_mvk_moltenvk.h header file in your code. The vk_mvk_moltenvk.h file also includes the API documentation for this VK_MVK_moltenvk extension.

The following API header files are included in the MoltenVK package, each of which can be included in your application source code as follows:

#include <MoltenVK/HEADER_FILE>

where HEADER_FILE is one of the following:

  • vk_mvk_moltenvk.h - Contains declarations and documentation for the functions, structures, and enumerations that define the behaviour of the VK_MVK_moltenvk Vulkan extension.

  • mvk_vulkan.h - This is a convenience header file that loads the vulkan.h header file with the appropriate MoltenVK Vulkan platform surface extension automatically enabled for iOS or macOS. Use this header file in place of the vulkan.h header file, where access to a MoltenVK platform surface extension is required.

    • When building for iOS, the mvk_vulkan.h header file automatically enables the VK_USE_PLATFORM_IOS_MVK build setting and VK_MVK_ios_surface Vulkan extension.
    • When building for macOS, the mvk_vulkan.h header file automatically enables the VK_USE_PLATFORM_MACOS_MVK build setting and VK_MVK_macos_surface Vulkan extension.

  • mvk_datatypes.h - Contains helpful functions for converting between Vulkan and Metal data types. You do not need to use this functionality to use MoltenVK, as MoltenVK converts between Vulkan and Metal datatypes automatically (using the functions declared in this header). These functions are exposed in this header for your own purposes such as interacting with Metal directly, or simply logging data values.

Metal Shading Language Shaders

Metal uses a different shader language than Vulkan. Vulkan uses the new SPIR-V Shading Language (SPIR-V), whereas Metal uses the Metal Shading Language (MSL).

MoltenVK provides several options for creating and running MSL versions of your existing SPIR-V shaders. The following options are presented in order of increasing sophistication and difficulty:

  • You can use the automatic Runtime Shader Conversion feature of MoltenVK to automatically and transparently convert your SPIR-V shaders to MSL at runtime, by simply loading your SPIR-V shaders as you always have, using the standard Vulkan vkCreateShaderModule() function. MoltenVK will automatically convert the SPIR-V code to MSL at runtime.

  • You can use the standard Vulkan vkCreateShaderModule() function to provide your own MSL shader code. To do so, set the value of the magic number element of the SPIR-V stream to one of the values in the MVKMSLMagicNumber enumeration found in the vk_mvk_moltenvk.h header file.

    The magic number element of the SPIR-V stream is the first element of the stream, and by setting the value of this element to either kMVKMagicNumberMSLSourceCode or kMVKMagicNumberMSLCompiledCode, on SPIR-V code that you submit to the vkCreateShaderModule() function, you are indicating that the remainder of the SPIR-V stream contains either MSL source code, or MSL compiled code, respectively.

  • You can use the MoltenVKShaderConverter command-line tool found in this MoltenVK distribution package to convert your SPIR-V shaders to MSL source code, offline at development time, in order to create the appropriate MSL code to load at runtime. The section below discusses how to use this tool in more detail.

You can mix and match these options in your application. For example, a convenient approach is to use Runtime Shader Conversion for most SPIR-V shaders, and provide pre-converted MSL shader source code for the odd SPIR-V shader that proves problematic for runtime conversion.

MoltenVKShaderConverter Shader Converter Tool

The MoltenVK distribution package includes the MoltenVKShaderConverter command line tool, which allows you to convert your SPIR-V shader source code to MSL at development time, and then supply the MSL code to MoltenVK using one of the methods described in the Metal Shading Language Shaders section above.

The MoltenVKShaderConverter tool uses the same conversion technology as the Runtime Shader Conversion feature of MoltenVK.

The MoltenVKShaderConverter tool has a number of options available from the command line:

  • The tool can be used to convert a single SPIR-V file to MSL, or an entire directory tree of SPIR-V files to MSL.

  • The tool can be used to convert a single OpenGL GLSL file, or an entire directory tree of GLSL files to either SPIR-V or MSL.

To see a complete list of options, run the MoltenVKShaderConverter tool from the command line with no arguments.

Troubleshooting Shader Conversion

The shader converter technology in MoltenVK is quite robust, and most SPIR-V shaders can be converted to MSL without any problems. In the case where a conversion issue arises, you can address the issue as follows:

  • Errors encountered during Runtime Shader Conversion are logged to the console.

  • To help understand conversion issues during Runtime Shader Conversion, you can enable the logging of the SPIR-V and MSL shader source code during conversion as follows:

      #include <MoltenVK/vk_mvk_moltenvk.h>
  ...
  MVKDeviceConfiguration mvkConfig;
  vkGetMoltenVKDeviceConfigurationMVK(vkDevice, &mvkConfig);
  mvkConfig.debugMode = true;
  vkSetMoltenVKDeviceConfigurationMVK(vkDevice, &mvkConfig);

    Performing these steps will enable debug mode in MoltenVK, which includes shader conversion logging, and causes both the incoming SPIR-V code and the converted MSL source code to be logged to the console (in human-readable form). This allows you to manually verify the conversions, and can help you diagnose issues that might occur during shader conversion.

  • For minor issues, you may be able to adjust your SPIR-V code so that it behaves the same under Vulkan, but is easier to automatically convert to MSL.

  • For more significant issues, you can use the MoltenVKShaderConverter tool to convert the shaders at development time, adjust the MSL code manually so that it compiles correctly, and use the MSL shader code instead of the SPIR-V code, using the techniques described in the Metal Shading Language Shaders section above.

  • You are also encouraged to report issues with shader conversion to the SPIRV-Cross project. MoltenVK and MoltenVKShaderConverter make use of SPIRV-Cross to convert SPIR-V shaders to MSL shaders.

Performance Considerations

This section discusses various options for improving performance when using MoltenVK.

Shader Loading Time

A number of steps is require to load and compile SPIR-V shaders into a form that Metal can use. Although the overall process is fast, the slowest step involves converting shaders from SPIR-V to MSL source code format.

If you have a lot of shaders, you can dramatically improve shader loading time by using the standard Vulkan pipeline cache feature, to serialize shaders and store them in MSL form offline. Loading MSL shaders via the pipeline cache serializing mechanism can be significantly faster than converting from SPIR-V to MSL each time.

In Vulkan, pipeline cache serialization for offline storage is available through the vkGetPipelineCacheData() and vkCreatePipelineCache() functions. Loading the pipeline cache from offline storage at app start-up time can dramatically improve both shader loading performance, and performance glitches and hiccups during runtime code if shader loading is performed then.

When using pipeline caching, nothing changes about how you load SPIR-V shader code. MoltenVK automatically detects that the SPIR-V was previously converted to MSL, and stored offline via the Vulkan pipeline cache serialization mechanism, and does not invoke the relatively expensive step of converting the SPIR-V to MSL again.

As a second shader loading performance option, Metal also supports pre-compiled shaders, which can improve shader loading and set-up performance, allowing you to reduce your scene loading time. See the Metal Shading Language Shaders and MoltenVKShaderConverter Shader Converter Tool sections above for more information about how to use the MoltenVKShaderConverter tool to create and load pre-compiled Metal shaders into MoltenVK. This behaviour is not standard Vulkan behaviour, and does not improve performance significantly. Your first choice should be to use offline storage of pipeline cache contents as described in the previous paragraphs.

Xcode Configuration

When a Metal app is running from Xcode, the default Scheme settings reduce performance. Be sure to follow the instructions for configuring your application's Scheme within Xcode, found in the in the installation section above.

Metal System Trace Tool

To help you get the best performance from your graphics app, the Xcode Instruments profiling tool includes the Metal System Trace template. This template can be used to provide detailed tracing of the CPU and GPU behaviour of your application, allowing you unprecedented performance measurement and tuning capabilities for apps using Metal.

Known MoltenVK Limitations

This section documents the known limitations in this version of MoltenVK.

  • MoltenVK is a Layer-0 driver implementation of Vulkan 1.0 Since it takes on the role of a driver in the Vulkan architecture, it does not load Vulkan Layers on its own. In order to use Vulkan layers such as the validation layers, use the Vulkan loader and layers from the LunarG Vulkan SDK.

The following Vulkan 1.0 features have not been implemented in this version of MoltenVK:

  • Tessellation and Geometry shader stages.

  • Events:

    • vkCreateEvent()
    • vkDestroyEvent()
    • vkGetEventStatus()
    • vkSetEvent()
    • vkResetEvent()
    • vkCmdSetEvent()
    • vkCmdResetEvent()
    • vkCmdWaitEvents()

  • Application-controlled memory allocations:

    • VkAllocationCallbacks are ignored

  • Sparse memory:

    • vkGetImageSparseMemoryRequirements()
    • vkGetPhysicalDeviceSparseImageFormatProperties()
    • vkQueueBindSparse()

  • Pipeline statistics query pool:

    • vkCreateQueryPool(VK_QUERY_TYPE_PIPELINE_STATISTICS)

  • VkImageViewCreateInfo::VkComponentMapping supports only the following per-texture swizzles:

    • VK_FORMAT_R8_UNORM: VkComponentMapping.r = VK_COMPONENT_SWIZZLE_R
    • VK_FORMAT_R8G8B8A8_UNORM <-> VK_FORMAT_B8G8R8A8_UNORM
    • VK_FORMAT_R8G8B8A8_SRGB <-> VK_FORMAT_B8G8R8A8_SRGB