Is this actually smaller than TCC? It's kind of hard to tell since TCC is split into files and I don't know what's actually necessary. And that question includes earlier, less featureful versions of TCC. Note that the more-or-less-C compiler it's based off of is absolutely miniscule: https://bellard.org/otcc/
Also if you add up the binaries used via dynamic linking you are looking at which in the case of /lib/x86_64-linux-gnu/libc.so.6 is 16MB in size?
Did I mention it works on all x86+amd64 POSIX systems too?
It uses calloc, fopen, and fgetc, which can trivially forwarded to the same syscalls as stage0 with only a few bytes. It uses isalnum, isspace, isdigit, strcpy, and strstr, which are all trivial. Add all that up and maybe you need to penalize it by a kilobyte. I'm not sure exactly how it's using dlsym but it seems to also be worth few bytes. It's not using the other 16MB of libc.
Which makes for a pertinent question: can you compile GCC or Clang with this?
OTCC is <4KB. It's admittedly using tiny names to squeeze under a size limit, but even when you correct for that it's a lot smaller.
(Edit: At the time of submission the link went to https://github.com/oriansj/stage0/blob/master/Linux%20Bootst... with a description of "World's Smallest C Compiler")
So instead of arguing back and forth, it would be more helpful for others reading your comments if you provided full information about the conditions under which those sizes were achieved.
The one that's self-compiled, so it shouldn't vary much if at all.