Copyright (c) 2014-2019 The Brenwill Workshop Ltd.
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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 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*.
MoltenVK references the latest Apple SDK frameworks. To access these frameworks when building your app, and to avoid build errors, be sure to use the latest publicly available version of Xcode.
Note: To support
IOSurfaces, any app that uses MoltenVK, must be built with a minimum iOS Deployment Target (aka
IPHONEOS_DEPLOYMENT_TARGET) build setting of
iOS 11.0or greater.
Once built, your app integrating the MoltenVK libraries can be run on iOS or macOS devices that support Metal.
Installation of MoltenVK is straightforward and easy!
Depending on your build and deployment needs, you can install MoltenVK as a static framework, static library, or dynamic library, by following the steps in this section. If you are unsure about which linking and deployment option you need, follow the steps for installing a static framework, as it is the simplest to install.
Open your application in Xcode and select your application's target in the Project Navigator panel.
Open the Build Settings tab.
If installing MoltenVK as a static framework in your application:
FRAMEWORK_SEARCH_PATHS) setting, add an entry that points to one of the following folders:
If installing MoltenVK as a static library in your application:
LIBRARY_SEARCH_PATHS) setting, add an entry that points to one of the following folders:
HEADER_SEARCH_PATHS) setting, add an entry that points to the
If installing MoltenVK as a dynamic library in your application:
LIBRARY_SEARCH_PATHS) setting, add an entry that points to one of the following folders:
HEADER_SEARCH_PATHS) setting, add an entry that points to the
LD_RUNPATH_SEARCH_PATHS) setting, add an entry that matches where the dynamic library will be located in your runtime environment. If the dynamic library is to be embedded within your application, you would typically set this value to either
libMoltenVK.dyliblibrary is internally configured to be located at
With the Build Settings tab open, if using
IOSurfaces on iOS, open the iOS Deployment Target (aka
IPHONEOS_DEPLOYMENT_TARGET) setting, and ensure it is set to a value of
iOS 11.0 or greater.
On the Build Phases tab, open the Link Binary With Libraries list.
For macOS, drag one of the following files to the Link Binary With Libraries list:
For iOS, drag one of the following files to the Link Binary With Libraries list:
While in the Link Binary With Libraries list on the Build Phases tab, 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 items:
IOSurface.framework(macOS, or iOS if
IPHONEOS_DEPLOYMENT_TARGETis at least
If installing MoltenVK as a dynamic library in your application, arrange to install the
libMoltenVK.dylib file in your application environment:
To copy the
libMoltenVK.dylib file into your application or component library:
libMoltenVK.dylibfile. Typically this will be Executables.
Alternately, you may create your own installation mechanism to install either the
MoltenVK/iOS/dynamic/libMoltenVK.dylib file into a standard macOS or iOS system library folder on the user's device.
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:
The demo apps, found in the
Demos.xcworkspace, located in the
Demos folder, demonstrate each of the installation techniques discussed above:
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. MoltenVK 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:
In addition to the core Vulkan API, MoltenVK also supports the following Vulkan extensions:
VK_EXT_memory_budget(requires Metal 2.0)
VK_EXT_shader_stencil_export(requires Mac GPU family 2 or iOS GPU family 5)
VK_EXT_texel_buffer_alignment(requires Metal 2.0)
VK_MVK_ios_surface(iOS) (Obsolete. Use
VK_MVK_macos_surface(macOS) (Obsolete. Use
In order to visibly display your content on iOS or macOS, you must enable the
VK_EXT_metal_surface extension, and use the function defined in that extension to create a Vulkan rendering surface. You can enable the
VK_EXT_metal_surface extension by defining the
VK_USE_PLATFORM_METAL_EXT guard macro in your compiler build settings. See the description of the
mvk_vulkan.h file below for a convenient way to enable this extension automatically.
VK_MVK_moltenvk Vulkan extension provides functionality beyond the standard Vulkan API, to support configuration options 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
The following API header files are included in the MoltenVK package, each of which can be included in your application source code as follows:
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.
mvk_vulkan.h header file automatically enables the
VK_USE_PLATFORM_METAL_EXT build setting and
VK_EXT_metal_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.
Note: The functions in
vk_mvk_moltenvk.hare not supported by the Vulkan SDK Loader and Layers framework. The opaque Vulkan objects used by the functions in
VKImage, ...), must have been retrieved directly from MoltenVK, and not through the Vulkan SDK Loader and Layers framework. The Vulkan SDK Loader and Layers framework often changes these opaque objects, and passing them from a higher layer directly to MoltenVK will result in undefined behaviour.
VK_MVK_moltenvk Vulkan extension provides the ability to configure and optimize MoltenVK for your particular application runtime requirements.
There are three mechanisms for setting the values of the MoltenVK configuration parameters:
To change the MoltenVK configuration settings at runtime using a programmatic API, use the
vkSetMoltenVKConfigurationMVK() functions to retrieve, modify, and set a copy of the
The initial value of each of the configuration settings can established at runtime by a corresponding environment variable, or if the environment variable is not set, by a corresponding build setting at the time MoltenVK is compiled. The environment variable and build setting for each configuration parameter share the same name.
See the description of the
MVKConfiguration structure parameters in the
vk_mvk_moltenvk.h file for more info about configuring and optimizing MoltenVK at build time or runtime.
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
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.
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.
MoltenVKShaderConverter tool uses the same conversion technology as the Runtime Shader Conversion feature of MoltenVK.
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.
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 shader conversion, by turning on the
MVKConfiguration::debugMode configuration parameter, or setting the value of the
MVK_DEBUG runtime environment variable to
1. See the MoltenVK Configuration description above.
Enabling debug mode in MoltenVK includes shader conversion logging, which 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.
This section discusses various options for improving performance when using MoltenVK.
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
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.
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.
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.
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.
VkEvents are not supported.
Application-controlled memory allocations using
VkAllocationCallbacks are ignored.
Pipeline statistics query pool using
VK_QUERY_TYPE_PIPELINE_STATISTICS is not supported.
Image content in
PVRTC compressed formats must be loaded directly into a
VkImage using host-visible memory mapping. Loading via a staging buffer will result in malformed image content.