In short, LLVM let you do whatever you wanted in building a compiler — and that’s why Apple hired Chris Lattner in 2005.
With LLVM, Apple had the technical means to build a compiler that suited all of its needs, and it could merge this compiler with the rest of Xcode — truly merge it — without open sourcing Xcode and sharing its proprietary software with the rest of the world.
Lattner declined to be interviewed for this story, saying that Apple frowns on employees talking to the press without approval. But it’s no secret that LLVM is now at the heart of Apple’s development philosophy, largely replacing the old GCC compiler, which isn’t as flexible or as powerful as LLVM — and lacks a permissive license.
Thanks to Lattner’s online resume and other sources, we know that LLVM is “deeply integrated” with Xcode, and that it was used to build the latest versions of Mac OS X and iOS. What’s more, it’s used to compile graphics code, in real-time, on iPhones and iPads, perhaps hinting at where the company will take it in the future.
Seems like a smart hire, way back when.