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Easily speed up your Xcode builds

A lot of developers don’t heavily modify the projects they instantiate from Xcode’s templates. This is a shame, because not only is it a great way to learn Xcode in depth, it’s also a great way to ensure your projects build as fast as possible!

To that end, then, here are two simple tips that you can apply to your projects right now that should get them building faster.

Normalize your build settings!

Projects and targets in Xcode have configurations — collections of build settings that influence how a target is built. The Debug configuration, for example, will typically specify the compiler optimization build setting with as low a value as possible. The Release configuration, on the other hand, will typically specify a relatively high value for this build setting. This begs the question: The Debug and Release configuration of what?

When you create a new project from one of Xcode’s templates, the principal target in that project will typically have a number of build settings customized in its configurations. The project itself, on the other hand, won’t have very may customized build settings. For the one-target-per-project case this doesn’t matter much. However, if you create a project with multiple targets, you can wind up with a number of targets that specify the same (or even subtly different!) information.

Instead of leaving your project this way, you can normalize your build settings such that build settings you want to specify for every target in the project — for example, that compiler optimization should be off for the Debug configuration — are specified at the project level rather than at the target level. This takes advantage of the fact that build settings are inherited in Xcode; if a build setting isn’t customized in a target, the value specified in the project is used.

What does this buy you? It ensures that you have a single, consistent set of settings that are passed to the compilers, linkers, and other tools that are used to build your software. That way you won’t wind up with subtle bugs like code built with C++ RTTI turned on calling a plug-in built with C++ RTTI turned off. But just as importantly, it’s enables the next trick, which can have a significant impact on the speed of large, multi-target builds.

Share your precompiled prefix!

Xcode, like many other IDEs, supports prefix files — header files that are included implicitly by every one of your source files when they’re built. Normally, as I described above, these are specified in target configurations. The text in the prefix file that is copied out of Xcode’s templates even mentions that it’s for a specific target.

Don’t be fooled!

Prefix files get precompiled for performance, so they can simply be loaded by the compiler rather than recomputed for every file you build. Precompiling a prefix file takes quite a bit of time though, especially if you use multiple languages in your project (such as C, Objective-C, and C++) because each language needs a separate precompiled prefix due to differences in how they’ll treat “the same” code.

However, just because precompiled prefix files can’t be shared between languages doesn’t mean they can’t be shared between targets. In fact, for performance, Xcode automates this sharing for you — if you meet it halfway. The critical thing that you need to do is to ensure that your prefix files are:

  1. Named the same; and,
  2. Built using the same compiler-related build settings.

That’s where the normalization I talked about above comes in. You can even promote your prefix file-related build settings to your project instead of your targets, so you can be certain that they’re shared.

In fact, if they meet the criteria that Xcode uses to determine whether precompiled prefix files should be shared, even multiple projects will wind up sharing them!

The pause between builds of a target’s dependent targets to generate a new precompiled prefix file is like a pipeline stall: An unwelcome hiccup that holds everything else up until it’s over. If Xcode can precompile a single set of prefix files at the start of your build and then re-use them for the entire rest of your build, it will stream past as quickly as it possibly can with only the occasional pause for linking. For large projects with a lot of dependent targets, this can make a big difference.

Separate your preprocessor macros!

“But Chris,” you say, “I have target-specific preprocessor macros that I can’t get rid of!” That’s OK. As long as you don’t need them in your prefix files, you can set them in the special Preprocessor Macros Not Used in Precompiled Headers build setting. These will be passed to the compiler when your sources are compiled, just like the regular Preprocess Macros will, but they won’t be passed when precompiling a prefix file. So you can have your cake and eat it, too.

Of course, there are macros that you do want to set in your precompiled prefix headers, because they change the behavior. Macros like NDEBUG to turn off C assert or NS_BLOCK_ASSERTIONS to turn off Foundation’s NSAssert are important to specify for your precompiled prefix files. Fortunately these types of macros typically differ only by configuration, and also remain consistent across targets and projects, allowing you to specify them at the project level rather than the target level.

Just these three small changes have the potential to make a dramatic difference in how quickly Xcode builds your project:

  1. Normalizing your build settings, so common settings across all your targets are specified at the project level;
  2. Increasing sharing of your precompiled prefix files by naming them consistently and using consistent compiler-related build settings; and
  3. Specifying separate preprocessor macros for your prefix files than for your targets,

Try it out and see for yourself!

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