MoltenVK Runtime User Guide

Copyright (c) 2015-2021 The Brenwill Workshop Ltd.

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 macOS, iOS, or tvOS.

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 a layered implementation of Vulkan 1.1 graphics and compute functionality, that is built on Apple's Metal graphics and compute framework on macOS, iOS, and tvOS. MoltenVK allows you to use Vulkan graphics and compute functionality to develop modern, cross-platform, high-performance graphical games and applications, and to run them across many platforms, including macOS, iOS, tvOS, Simulators, and Mac Catalyst on macOS 11.0+.

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.

To provide Vulkan capability to the macOS, iOS, and tvOS 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

Installation of MoltenVK in your application is straightforward and easy!

Depending on your build and deployment needs, you can link MoltenVK to your application either as a universal XCFramework or as a dynamic library (.dylib). Distributing an app containing a dynamic library via the iOS App Store or tvOS App Store can require specialized bundling. If you are unsure about which linking and deployment option you need, or on iOS or tvOS, unless you have specific needs for dynamic libraries, follow the steps for linking MoltenVK as an XCFramework, as it is the simpler option, and encompasses the largest set of supported platforms.

Install MoltenVK as a Universal XCFramework

To link MoltenVK to your application as an XCFramework, follow these steps:

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

  2. Open 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.

    2. 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, or if using IOSurfaces on tvOS, open the tvOS Deployment Target (aka TVOS_DEPLOYMENT_TARGET) setting, and ensure it is set to a value of tvOS 11.0 or greater.

  3. Open the Build Phases tab and open the Link Binary With Libraries list.

    1. Drag MoltenVK/MoltenVK.xcframework to the Link Binary With Libraries list.

    2. If your application does not use use C++, click the + button, and add libc++.tbd by selecting it from the list of system frameworks. This is needed because MoltenVK uses C++ system libraries internally.

    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 add the following items by selecting them from the list of system frameworks:

      • libc++.tbd (if not already done in Step 2)
      • Metal.framework
      • Foundation.framework.
      • QuartzCore.framework
      • IOKit.framework (macOS)
      • UIKit.framework (iOS or tvOS)
      • IOSurface.framework (macOS, or iOS if IPHONEOS_DEPLOYMENT_TARGET is at least iOS 11.0, or tvOS if TVOS_DEPLOYMENT_TARGET is at least tvOS 11.0)

Install MoltenVK as a Dynamic Library

To link MoltenVK to your application as a dynamic library (.dylib), follow these steps:

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

  2. Open 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.

    2. In the Library Search Paths (aka LIBRARY_SEARCH_PATHS) setting, add an entry that points to one of the following folders:

      • MoltenVK/dylib/macOS (macOS)
      • MoltenVK/dylib/iOS (iOS)
      • MoltenVK/dylib/tvOS (tvOS)

    3. In the Runpath Search Paths (aka 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 to one of these values:

      • @executable_path/../Frameworks (macOS)
      • @executable_path/Frameworks (iOS or tvOS)

      The libMoltenVK.dylib library is internally configured to be located at @rpath/libMoltenVK.dylib.

    4. 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, or if using IOSurfaces on tvOS, open the tvOS Deployment Target (aka TVOS_DEPLOYMENT_TARGET) setting, and ensure it is set to a value of tvOS 11.0 or greater.

  3. Open the Build Phases tab and open the Link Binary With Libraries list.

    1. Drag one of the following files to the Link Binary With Libraries list:
    • MoltenVK/dylib/macOS/libMoltenVK.dylib (macOS)
    • MoltenVK/dylib/iOS/libMoltenVK.dylib (iOS)
    • MoltenVK/dylib/tvOS/libMoltenVK.dylib (tvOS)

    1. If your application does not use use C++, click the + button, and add libc++.tbd by selecting it from the list of system frameworks. This is needed because MoltenVK uses C++ system libraries internally.

    2. 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 add the following items by selecting them from the list of system frameworks:

      • libc++.tbd (if not already done in Step 2)
      • Metal.framework
      • Foundation.framework.
      • QuartzCore.framework
      • IOKit.framework (macOS)
      • UIKit.framework (iOS or tvOS)
      • IOSurface.framework (macOS, or iOS if IPHONEOS_DEPLOYMENT_TARGET is at least iOS 11.0, or tvOS if TVOS_DEPLOYMENT_TARGET is at least tvOS 11.0)

  4. 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 Frameworks (and it should match the Runpath Search Paths (aka LD_RUNPATH_SEARCH_PATHS) build setting you added above).

      3. Drag one of the following files to the Copy Files list in this new build phase: - MoltenVK/dylib/macOS/libMoltenVK.dylib (macOS) - MoltenVK/dylib/iOS/libMoltenVK.dylib (iOS) - MoltenVK/dylib/tvOS/libMoltenVK.dylib (tvOS)

    • Alternately, you may create your own installation mechanism to install one of the following files into a standard macOS, iOS, or tvOS system library folder on the user's device:

      • MoltenVK/dylib/macOS/libMoltenVK.dylib (macOS)
      • MoltenVK/dylib/iOS/libMoltenVK.dylib (iOS)
      • MoltenVK/dylib/tvOS/libMoltenVK.dylib (tvOS)

Build and Runtime Requirements

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 on iOS or tvOS, any app that uses MoltenVK must be built with a minimum iOS Deployment Target (aka IPHONEOS_DEPLOYMENT_TARGET ) build setting of iOS 11.0 or greater, or a minimum tvOS Deployment Target (aka TVOS_DEPLOYMENT_TARGET ) build setting of tvOS 11.0 or greater.

Once built, your app integrating the MoltenVK libraries can be run on macOS, iOS or tvOS devices that support Metal, or on the Xcode iOS Simulator or tvOS Simulator.

  • At runtime, MoltenVK requires at least macOS 10.11, iOS 9, or tvOS 9 (or iOS 11 or tvOS 11 if using IOSurfaces).
  • Information on macOS devices that are compatible with Metal can be found in this article.
  • Information on iOS devices that are compatible with Metal can be found in this article.

When a Metal app is running from Xcode, the default Scheme settings may 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.

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. 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:

#include <vulkan/vulkan.h>

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

  • VK_KHR_16bit_storage
  • VK_KHR_8bit_storage
  • VK_KHR_bind_memory2
  • VK_KHR_create_renderpass2
  • VK_KHR_dedicated_allocation
  • VK_KHR_depth_stencil_resolve
  • VK_KHR_descriptor_update_template
  • VK_KHR_device_group
  • VK_KHR_device_group_creation
  • VK_KHR_driver_properties
  • VK_KHR_get_memory_requirements2
  • VK_KHR_get_physical_device_properties2
  • VK_KHR_get_surface_capabilities2
  • VK_KHR_imageless_framebuffer
  • VK_KHR_image_format_list
  • VK_KHR_maintenance1
  • VK_KHR_maintenance2
  • VK_KHR_maintenance3
  • VK_KHR_multiview
  • VK_KHR_portability_subset
  • VK_KHR_push_descriptor
  • VK_KHR_relaxed_block_layout
  • VK_KHR_sampler_mirror_clamp_to_edge (requires a Mac GPU or Apple family 7 GPU)
  • VK_KHR_sampler_ycbcr_conversion
  • VK_KHR_shader_draw_parameters
  • VK_KHR_shader_float16_int8
  • VK_KHR_shader_subgroup_extended_types (requires Metal 2.1 on Mac or Metal 2.2 and Apple family 4 on iOS)
  • VK_KHR_storage_buffer_storage_class
  • VK_KHR_surface
  • VK_KHR_swapchain
  • VK_KHR_swapchain_mutable_format
  • VK_KHR_timeline_semaphore
  • VK_KHR_uniform_buffer_standard_layout
  • VK_KHR_variable_pointers
  • VK_EXT_debug_marker
  • VK_EXT_debug_report
  • VK_EXT_debug_utils
  • VK_EXT_descriptor_indexing (initial release limited to Metal Tier 1: 96/128 textures, 16 samplers)
  • VK_EXT_fragment_shader_interlock (requires Metal 2.0 and Raster Order Groups)
  • VK_EXT_host_query_reset
  • VK_EXT_image_robustness
  • VK_EXT_inline_uniform_block
  • VK_EXT_memory_budget (requires Metal 2.0)
  • VK_EXT_metal_surface
  • VK_EXT_post_depth_coverage (iOS and macOS, requires family 4 (A11) or better Apple GPU)
  • VK_EXT_private_data
  • VK_EXT_robustness2
  • VK_EXT_scalar_block_layout
  • VK_EXT_shader_stencil_export (requires Mac GPU family 2 or iOS GPU family 5)
  • VK_EXT_shader_viewport_index_layer
  • VK_EXT_subgroup_size_control (requires Metal 2.1 on Mac or Metal 2.2 and Apple family 4 on iOS)
  • VK_EXT_swapchain_colorspace
  • VK_EXT_vertex_attribute_divisor
  • VK_EXT_texel_buffer_alignment (requires Metal 2.0)
  • VK_EXT_texture_compression_astc_hdr (iOS and macOS, requires family 6 (A13) or better Apple GPU)
  • VK_MVK_ios_surface (iOS) (Obsolete. Use VK_EXT_metal_surface instead.)
  • VK_MVK_macos_surface (macOS) (Obsolete. Use VK_EXT_metal_surface instead.)
  • VK_MVK_moltenvk
  • VK_AMD_gpu_shader_half_float
  • VK_AMD_negative_viewport_height
  • VK_AMD_shader_image_load_store_lod (requires Apple GPU)
  • VK_AMD_shader_trinary_minmax (requires Metal 2.1)
  • VK_IMG_format_pvrtc (requires Apple GPU)
  • VK_INTEL_shader_integer_functions2
  • VK_NV_glsl_shader

In order to visibly display your content on macOS, iOS, or tvOS, 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.

MoltenVK VK_MVK_moltenvk Extension

The VK_MVK_moltenvk Vulkan extension provides functionality beyond standard Vulkan functionality, to support configuration options and behaviour that is specific to the MoltenVK implementation of Vulkan functionality. 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 macOS, iOS, or tvOS. Use this header file in place of the vulkan.h header file, where access to a MoltenVK platform surface extension is required.

    The 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: Except for vkGetMoltenVKConfigurationMVK() and vkSetMoltenVKConfigurationMVK(), the functions in vk_mvk_moltenvk.h are not supported by the Vulkan SDK Loader and Layers framework. The opaque Vulkan objects used by the functions in vk_mvk_moltenvk.h (VkPhysicalDevice, VkShaderModule, 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.

Configuring MoltenVK

The 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:

  • Runtime API via the vkGetMoltenVKConfigurationMVK()/vkSetMoltenVKConfigurationMVK() functions.
  • Application runtime environment variables.
  • Build settings at MoltenVK build time.

To change some of the MoltenVK configuration settings at runtime using a programmatic API, use the vkGetMoltenVKConfigurationMVK() and vkSetMoltenVKConfigurationMVK() functions to retrieve, modify, and set a copy of the MVKConfiguration structure.

The initial value of each of the configuration settings can be 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 and corresponding environment variables in the vk_mvk_moltenvk.h file for more info about configuring and optimizing MoltenVK at runtime or build time.

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 uses Runtime Shader Conversion to automatically convert your SPIR-V shaders to their MSL equivalents, during loading your SPIR-V shaders, using the standard Vulkan vkCreateShaderModule() function.

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 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 some 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.

  • 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 required 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.

Swapchains

Metal supports a very small number (3) of concurrent swapchain images. In addition, Metal can sometimes hold onto these images during surface presentation.

MoltenVK supports using either 2 or 3 swapchain images. For best performance, it is recommended that you use 3 swapchain images (triple-buffering), to ensure that at least one swapchain image will be available when you need to render to it.

Using 3 swapchain images is particularly important when rendering to a full-screen surface, because in that situation, Metal uses its Direct to Display feature, and avoids compositing the swapchain image onto a separate composition surface before displaying it. Although Direct to Display can improve performance throughput, it also means that Metal may hold onto each swapchain image a little longer than when using an internal compositor, which increases the risk that a swapchain image will not be a vailable when you request it, resulting in frame delays and visual stuttering.

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.

  • On macOS versions prior to macOS 10.15.6, native host-coherent image device memory is not available. Because of this, changes made to VkImage VK_MEMORY_PROPERTY_HOST_COHERENT_BIT device memory by the CPU or GPU will not be available to the GPU or CPU, respectively, until the memory is flushed or unmapped by the application. Applications using vkMapMemory() with VkImage VK_MEMORY_PROPERTY_HOST_COHERENT_BIT device memory on macOS versions prior to macOS 10.15.6 must call either vkUnmapMemory(), or vkFlushMappedMemoryRanges() / vkInvalidateMappedMemoryRanges() to ensure memory changes are coherent between the CPU and GPU. This limitation does not apply to VKImage device memory on macOS starting with macOS 10.15.6, does not apply to VKImage device memory on any version of iOS or tvOS, and does not apply to VKBuffer device memory on any platform.

  • 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.

  • Pipeline statistics query pool using VK_QUERY_TYPE_PIPELINE_STATISTICS is not supported.

  • Application-controlled memory allocations using VkAllocationCallbacks are ignored.

  • Since MoltenVK is an implementation of Vulkan functionality, 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 Vulkan SDK. Refer to the Vulkan SDK Getting Started document for more info.