[M3devel] FW: NT386 jmpbuf size?

[Olaf Wagner]

Quoting Jay <jayk123 at hotmail.com>:

> I BELIEVE:
>   It is statically knowable:
>     msvcrt _setjmp uses 8 words
>     msvcrt _setjmp3 uses 16
>     cygwin uses I think 13 -- their header is just wrong by a factor of 4
>    You can tell from a mix of disassembly and source -- disassembly   
> for msvcrt, disassembly and source for cygwin.
>
> It isn't runtime variables that make the difference.
> (well, the 16 case might sometimes shrink, it appears)
>
> What is less clear is if _setjmp or _setjmp3 should be used.
> It appears that any use of setjmp in C or C++ uses _setjmp3.
>
> The difference appears to be whether or not the generally expected   
> interaction with C __finally and C++ destructors and exception   
> handling occurs.
> "generally expected' but also "generally unknown".
>
> One might argue that if C/C++ can afford the extra 32 bytes, then so  
>  can Modula-3 BUT C/C++ have MUCH more efficient exception handling   
> mechanisms available and so setjmp is much less used there. They get  
>  by with around 3 words for a frame with exception handling on x86,   
> and with zero overheard everywhere else.
>
> The C/C++ behavior might also be affected by command line options.   
> I'd have to experiment more..
>
> To clarify, imagine Modula-3 gave a callback function to some C++   
> and that C++ has a local with a destructor, and the Modula-3 raises   
> an exception.
> Does the C++ destructor run or not?
> With _setjmp probably not, with _setjmp3 probably.

IMO it should.

> Similarly, imagine some C++ gives some Modula-3 a callback and the   
> C++ raises an exception.
> Do the Modula-3 FINALLY blocks run?

Dito.

> Similar dilemna really on all platforms.
> There relatively recently an ABI for C++ exception handling that   
> maybe Modula-3 should use.

I think we should start on the safe side and try to optimize later
if needed. So I'd vote for 16 bytes and use of _setjmp3.

Olaf
-- 
Olaf Wagner -- elego Software Solutions GmbH
                Gustav-Meyer-Allee 25 / Gebäude 12, 13355 Berlin, Germany
phone: +49 30 23 45 86 96  mobile: +49 177 2345 869  fax: +49 30 23 45 86 95
    http://www.elegosoft.com | Geschäftsführer: Olaf Wagner | Sitz: Berlin
Handelregister: Amtsgericht Charlottenburg HRB 77719 | USt-IdNr: DE163214194