nx_backtrace.nx

nx_backtrace.nx -- frame-pointer-based stack walker. Walks the saved-frame-pointer chain to recover return addresses for a backtrace. Used by: - Panic handler (print stack on assertion / SIGSEGV) - Sampling profilers - Memory-leak attribution (allocate-site tracking) Convention (RV64 ABI): - s0 / fp = current frame pointer - At each call, the prologue stores ra and previous-fp on the stack and sets fp = sp + frame_size. - The slot at [fp - 8] holds the previous frame's ra. - The slot at [fp - 16] holds the previous frame's saved-fp. Limitations: - Functions compiled WITHOUT a frame pointer (e.g. -fomit-frame-pointer leaf functions) are invisible to this walker. For full coverage we need DWARF .eh_frame / nx_dwarf_cfi-driven unwinding -- that belongs in a future nx_unwind module. - We bound the walk at 64 frames to avoid runaway loops on corrupted stacks. API: n = nx_bt_capture(start_fp, frames, max) -> count of frames captured nx_bt_print(io, frames, n) -> print to fd ra = nx_bt_self_fp() -> read fp register nx_bt_self_fp is a stub today (NishiLang doesn't expose inline asm yet); callers pass in a frame pointer they captured via the host runtime. Once asm-block syntax lands the stub becomes a real read of x8.