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Metal Tips and Techniques

This chapter discusses tips and techniques that can improve app performance or developer productivity.

Creating Libraries During the App Build Process

Compiling shader language source files and building a library (.metallib file) during the app build process achieves better app performance than compiling shader source code at runtime. You can build a library within Xcode or by using command line utilities.

Using Xcode to Build a Library

Any shader source files that are in your project are automatically used to generate the default library, which you can access from Metal framework code with the  method of .

Using Command Line Utilities to Build a Library

 shows the command line utilities that form the compiler toolchain for Metal shader source code. When you include .metal files in your project, Xcode invokes these tools to build a library file that you can access in your app at run time.

To compile shader source into a library without using Xcode:

1. Use metal to compile each .metal file into a single .air file, which stores an intermediate representation of shader language code.

2. Use metal-ar to archive several .air files together into a single .metalar file. metal-ar is similar to the Unix utility ar.

3. Use metallib to build a Metal .metallib library file from the archive .metalar file.

To access the resulting library in framework code, call the  method of .

Xcode Scheme Settings and Performance

When a Metal app is running from Xcode, the default scheme settings reduce performance. Xcode detects whether the Metal API is used in the source code and automatically enables the GPU Frame Capture and Metal API Validation settings, as seen in Figure 8-2. When GPU Frame Capture is enabled, the debug layer is activated. When Metal API Validation is enabled, each call is validated, which affects performance further. For both settings, CPU performance is more affected than GPU performance. Unless you disable these settings, app performance may noticeably improve when the app is run outside of Xcode.

Debugging

Use the tips in the following sections to gain more useful diagnostic information when debugging and profiling your Metal app.

File Extension for Metal Shading Language Source Files

For Metal shading language source code file names, you must use the .metal file name extension to ensure that the development tools (Xcode and the GPU frame debugger) recognize the source files when debugging or profiling.

Performing Frame Capture with Xcode

To perform frame capture in Xcode, enable debug and call the  method of  to inform Xcode. The  and  methods of  similarly inform Xcode about frame capture, so call  only if those methods are not present.

The Label Property

Many Metal framework objects—such as command buffers, pipeline states, and resources—support a  property. You can use this property to assign a name for each object that is meaningful in the context of your application’s design. These labels appear in the Xcode Frame Capture debugging interface, allowing you to more easily identify objects.

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