@ekaitz_zarraga As you know, I always think hackers should follow their own curiosity.
So there's nothing wrong studying GCC. There's even nothing wrong in using or hacking GCC.

I just don't feel safe by DEPENDING on GCC.

In case you are going to write about TCC internals, please don't forget to send me a link, though.
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I don't remember much about target description macros, but started from here https://wiki.osdev.org/OS_Specific_Toolchain

I remember that learning about Spec Files was a turning point that raised my productivity in the port https://gcc.gnu.org/onlinedocs/gcc/Spec-Files.html