Debian bookworm live images now reproducible

Does anyone have any information as to how they modified their C code such that the complier output was deterministic? I thought one of the hardest problems with a effort like this was writing your C such that the compiler would output everything in the same order (same bytes)? And I am not just talking about time stamps etc.

This is something compilers themselves need to guarantee, e.g. GCC has -frandom-seed [0].

[0] https://gcc.gnu.org/onlinedocs/gcc/Developer-Options.html#in...