C unwinding with setjmp/longjmp

[CAD97]

setjmp/longjmp based exception handling isn't particularly performant when cleanup frames are abundant, but it's supported by C89. Translation needs thread_local or some polyfill, though, which is C11.

Thread local state:

#include <setjmp.h>
#include <threads.h>
extern thread_local jmp_buf thread_jmp_buf;

unwind continue is trivial; nothing changes:

_1 = f() -> [return: bb1, unwind continue];

L1 = f();
goto bb1;

Cleanup adds to the jmp_buf stack:

_1 = f() -> [return: bb1, unwind: bb2];

jmp_buf tmp_jmp_buf; // scratch space

tmp_jmp_buf = thread_jmp_buf;
if (setjmp(thread_jmp_buf)) {
    thread_jmp_buf = tmp_jmp_buf;
    goto bb2;
}
L1 = f();
thread_jmp_buf = tmp_jmp_buf;
goto bb1;

Resuming an unwind is then a call to longjmp:

resume;

longjmp(thread_jmp_buf, 1);

[FractalFir]

That sound pretty doable... I will try & see if I can manage to get this working.

[FractalFir]

I have a WIP implementation done(unwinding seems to work), will just need to adjust my libunwind emulation to work with this setjump / longjump.