Emitted functions report an exception by returning with the thread's
pending flag set, and every caller tests the flag after each call that
may throw. On Linux executables (glibc and musl), functions reached only
by direct calls now propagate exceptions by unwinding the native stack
instead:
- unwindAbiFunctions selects the may-throw functions that are not async
or generators, constructors, virtually called or overridden methods,
closure targets, the entry, library exports, module initializers or
callees of coroutine bodies.
- A native unwind costs about 2 µs a raise and 0.1 µs a frame, so functions that
exceptions are likely to cross keep the flag: those with a `throw` of
their own, those up to three calls above one, and recursive functions,
counting calls through methods, closures and callbacks.
- Their propagating exits call scr_unwind_raise (scr_unwind.c), a forced
unwind through _Unwind_ForcedUnwind with the exception still in the
active cell.
- Direct calls to them are `invoke`s without a pending check. The
landing pad runs the frame's usual unwind path: it releases the frame's
values and then enters the enclosing try handler or resumes the unwind.
- Remaining plain calls to them (wrappers, adapters, runtime-facing entry
points) become invokes whose landing pad continues on the pending flag,
or resumes when the wrapper is itself invoked. Every other call is
marked `nounwind`, so inlining a raising function at an invoke keeps its
calls plain.
- When emitted text breaks these rules (a raising function's address is
taken, or an invoked function returns with the flag pending), the
module is emitted again with those functions on the pending flag.
Emitted code does not declare scr_unwind_raise `noreturn`, so sanitized
builds do not run the sanitizer's no-return hook, which needs stack
bounds that fiber stacks do not report. Instead, scr_unwind_raise clears
the shadow of the frames it unwinds.
Executables link a static unwinder: libgcc_eh with the host toolchain and
the glibc runtime packs, and Zig's libunwind with Zig and the musl packs.
macOS, Windows, Android, WASI, libraries and debug builds keep the
pending flag everywhere and emit the same code as before.
The throw-chains runtime workload measures exceptions caught 2, 5 and 20
recursive calls above the throw, through direct calls and a callback,
and a parser that backtracks by throwing.
