why does exception.cc have "unwinding" code for sjlj?
Fri Feb 1 05:55:00 GMT 2002
We're baffled by this. Why does SJLJ_EXCEPTIONS code need the
call-site table? Something to do with
// FIXME2: _Unwind_GetIP is nonsensical for SJLJ, being a call-site
// index instead of a PC value. We could perhaps arrange for
// _Unwind_GetRegionStart to return context->fc->jbuf, which
// is the address of the handler label for __builtin_longjmp, but
// there is no solution for DONT_USE_BUILTIN_SETJMP.
Adam Megacz writes:
> I don't get it -- I thought the whole point of SJLJ was that you
> couldn't meaningfully "unwind" the stack to get a stack trace. Isn't
> that [part of] what makes DWARF2 better than SJLJ?
> #ifdef SJLJ_EXCEPTIONS
> // The given "IP" is an index into the call-site table, with two
> // exceptions -- -1 means no-action, and 0 means terminate. But
> // since we're using uleb128 values, we've not got random access
> // to the array.
> if ((int) ip <= 0)
> return _URC_CONTINUE_UNWIND;
> _Unwind_Word cs_lp, cs_action;
> p = read_uleb128 (p, &cs_lp);
> The main problem I'm running into is that some of the unwind code
> requires libstdc++ (specifically, #include <cstdlib>), but libstdc++
> has a lot of problems crosscompiling, and I'd rather just not build it
> than try to get my (ugly) patches to its configure.in accepted; I'd
> probably break more stuff than I would fix, anyways...
> Win32 never calls the _Unwind functions anyways; it just throws an
> exception out of the signal handler.
> Suggestions on how to deal with this?
> - a
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