https://github.com/oriansj/stage0-posix-x86/blob/master/cc_x...
It's part of a chain of minimal open-source x86 bootstrap/boot chain compiler source codes, where larger compilers (more complex ones) are compiled from their own source further down the chain, from simpler earlier ones, all found in the following GitHub repository:
https://github.com/oriansj/stage0-posix-x86/tree/master
...which appears to be an x86 specific version of the following:
https://github.com/oriansj/stage0
Which is found along with these other excellent related repositories also by Jeremiah Orians, here: