Re: LibJIT / Lightning interoperability issue

[Paulo César Pereira de Andrade]

Em sáb., 26 de dez. de 2020 às 22:58, Daniel Schwen <lists@schwen.de> escreveu:
>
> Hello list,

  Hi,

> I noticed that both the LibJIT as well as Lightning libraries export
> the jit_memcpy, jit_memmove, jit_realloc, and jit_free functions,
> causing crashes when linking to both.
>
> It looks like the functions are only declared in jit_private.h and not
> part of the public API, so I have tried renaming them and this fixes
> the problem:
>
> https://github.com/dschwen/lightning/commit/7bfc7f1e1857790a072ebe441b8fbc506739a873
>
> Another option would be to control symbol export in the build system, which 
> I've done in this commit:
>
> https://github.com/dschwen/lightning/commit/e9ee173fc92b90deb22fa6d5abf5d31505972b0f

  I wonder if somebody experimented building lightning with a compiler
other than gcc or clang. Also, I do not know how symbol visibility would
work on windows (cygwin/mingwin) when built with gcc or gcc based
compiler. That would be a good reason to have it a bit more complicated,
and have the preprocessor define to know if building lightning.

> I know this is an easy to dismiss issue, but I'm working on a generic
> mathematical expression parsing library with JIT support, and I'd like to
> offer as many options as possible for JIT backends, and am frequently 
> (actually
> always) building and linking against multiple JIT libs. It just seems like 
> good
> software practice not to export unneeded symbols, as they all occupy one 
> single
> global namespace and the more symbols you export the slower the link times 
> get.

  The easiest approach would be something like:

#if (__GNUC__ >= 4)
#  define PUBLIC        __attribute__ ((visibility("default")))
#  define HIDDEN        __attribute__ ((visibility("hidden")))
#else
#  define PUBLIC        /**/
#  define HIDDEN        /**/
#endif

then, likely just tag prototypes with PUBLIC and leave everything else
with default visibility, that would be passed based on some configure.ac
check. Could add a specific file, or just mostly/fully copy&paste the text
in m4/visibility.m4

  As a note, looking at Xorg sources, it uses a macro checking for
__GNUC__ >= 4, and not defined __CYGWIN__ or __MINGWIN__.
It also adds the proper preprocessor for suncc, and inlines a simpler
version of gl_VISIBILITY.

  Compiling with default hidden visibility is a very good optimization,
as it allows the compiler to generate better or more optimized code,
and more inlining of common patterns, what lightning has a lot; where
it was added mostly to make the code easier to read.

> Cheers,
> Daniel

Thanks,
Paulo