extensions.h

Why this exists

C99 has served me well for a long time, but it's missing a lot of ergonomics that C++ standards take for granted, things like nullptr, real compile-time constants, typeof, checked arithmetic, and clean attribute syntax. Rather than reach for C++ wholesale, the goal here is to pull in the useful parts of C23 (and the GNU extensions C23 still doesn't cover) into a single, well-documented header, so the rest of the codebase can use clear, self-explanatory names (packed, noreturn, force_inline) instead of scattering raw __attribute__((...)) and __builtin_... calls everywhere.

In short: this header is the seam between "plain C" and "everything modern GCC actually offers," kept in one place so it's easy to see exactly what compiler magic the project depends on.

What it's for

This header is part of a custom RV64 based system[*] built from scratch, targeting the GNU toolchain exclusively (no Clang/MSVC portability attempted). It centralizes:

• Struct layout control (packed, align)
• Linker placement (section)
• Function behavior hints (noreturn, hot, cold, noinline, force_inline, noclone)
• Trap/interrupt handler generation for RISC-V's privilege modes (interrupt(int_machine | int_supervisor | int_rnmi))
• Optimizer hints (likely, unlikely, unreachable)

[*] "System" here refers to the complete Von Neumann architecture this project targets, not just the OS, but the CPU, GPU, and NIC design it will eventually run on.

Scope and assumptions

• Toolchain: GCC only.
• Architecture: RISC-V, starting from a base rv64i target. Specific extensions (M, A, F, D, C, etc.) are still being decided as the project grows.
• Environment: freestanding (-ffreestanding, no hosted libc), every macro here needs to work without any standard library support.

Status

Actively evolving alongside the OS itself. Expect additions as new architecture-specific needs come up (memory model, per-hart data, atomics, etc.) once the extension set is finalized.