In the very early days of iPhone, we could only support what we called a “fat binary,” which means it has a slice for every CPU type that it needs to run on. It really comes down to binary download size and what an app has to download. There are a lot of performance improvements in this new model.īut why did Apple make this change, and why do they push you so hard to use bitcode in your builds? The Problem: Fat Binaries The reason for splitting the compilation into two steps is to allow us to make improvements in how the bitcode changes to ASM without having to tokenize the C code every single time. Then, a separate program called the LLVM Backend takes the bitcode and turns it into ASM. In LLVM, the code first runs through a Clang Frontend that transforms it into bitcode, which is an intermediate representation of your source code. In GCC, the code gets directly compiled to ASM. The New Way To Compile iOS Apps - LLVMĪ few years ago, Apple decided to move to the LLVM (Low Level Virtual Machine) compiler. You upload the assembly to Apple and then your users download it and run your app. You take your code, run it through GCC, and get binary assembly (ASM) out the other side that an iPhone can run. The key point is that it uses a one-step compilation process.
It was a legacy Linux-based compiler for C or Objective-C. When the iPhone was first released, the compiler that built programs was called GCC, the GNU Compiler Collection. The Old Way To Compile iOS Apps - GCC Compilation We’ll clarify that, but before we dive into that side of things, we need to talk about how code compilation on iOS used to work. You might have heard about bitcode or LLVM (Low Level Virtual Machine), but you might not know what they mean or how they affect your application. If you see these errors in a build log and Apple rejects your App Store submission, it’s almost certainly a bitcode problem.
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