LLVM was originally written to be a replacement for the existing code generator in the GCC stack, and many of the GCC front ends have been modified to work with it, resulting in the now-defunct LLVM-GCC suite. The modifications generally involve a GIMPLE-to-LLVM IR step so that LLVM optimizers and codegen can be used instead of GCC's GIMPLE system. Apple was a significant user of LLVM-GCC through Xcode 4.x (2013). This use of the GCC frontend was considered mostly a temporary measure, but with the advent of Clang and advantages of LLVM and Clang's modern and modular codebase (as well as compilation speed), is mostly obsolete.