Xcode 9.3中Swift的-Osize编译设置具体是什么？

Hey there! Great question about the new -Osize optimization introduced in Swift 4.1 (shipped with Xcode 9.3). Let me break down all the key details you need to know:

What is -Osize? #

-Osize is a Swift compiler optimization flag focused exclusively on reducing your app's binary size. Unlike the default -O (Fastest, Aggressive Optimizations) which prioritizes runtime speed, -Osize tells the compiler to make intentional tradeoffs that shrink the final executable—even if it means a tiny, often negligible, hit to performance in some cases.

How to enable it in Xcode 9.3+ #

Here's a straightforward step-by-step to turn it on:

• Open your Xcode project or workspace
• Select your app target in the Project Navigator
• Switch to the Build Settings tab
• Use the search bar to find Optimization Level
• Under the Release configuration (you’ll rarely want this for Debug builds), select Fastest, Smallest [-Osize]

Key considerations when using -Osize #

• Compatibility: This is a Swift 4.1+ exclusive feature, so it only appears in Xcode 9.3 and later—you won’t find this option in older versions like Xcode 9.2.
• Speed vs. Size Tradeoff: The compiler will take actions like limiting over-inlining of functions, optimizing data structure memory layouts, and stripping redundant code paths to cut down on size. For most everyday apps, the performance impact is unnoticeable, but if you have performance-critical code (e.g., real-time graphics, heavy calculations), you might want to stick with -O for those specific targets.
• Testing: Always thoroughly test your app in Release mode after enabling -Osize. While the Swift compiler is highly reliable, aggressive size optimizations can occasionally edge out edge-case logic—better to catch that before shipping!
• Expected Savings: Depending on your codebase, you can typically expect a 10-25% reduction in binary size. Apps with lots of repeated code, generic types, or unused functionality will see the biggest gains.

When should you use it? #

Opt for -Osize if:

• Your app is hitting App Store size limits
• You’re targeting devices with limited storage or memory
• Runtime performance isn’t your top priority (e.g., utility apps, content-focused apps)

Stick with -O if performance is critical—like games, AR apps, or apps with heavy computational workloads.

内容的提问来源于stack exchange，提问作者Anup Gupta