In many cases, GCC and LLVM/Clang are the two alternatives. But in this case, I believe we are talking about IBM's XL compilers. These are proprietary compiler

In many cases, GCC and LLVM/Clang are the two alternatives. But in this case, I believe we are talking about IBM's XL compilers. These are proprietary compilers written by IBM. See https://en.wikipedia.org/wiki/IBM_XL_C/C%2B%2B_Compilers https://en.wikipedia.org/wiki/IBM_XL_C/C%2B%2B_Compilers IBM has been offering proprietary compilers for operating systems like AIX for quite a long time. Going back to the mid-90s in my personal experience, and probably going back further than that too.