Why do we need both - export maps AND -fvisibility=hidden/__attribute__((visibility("default")))

[David Holmes]

Hi Volker,

On 5/02/2014 2:18 AM, Volker Simonis wrote:
> I know this is an old topic and actually I was involved myself in some
> of these discussions. Nevertheless, the topic regularly pops up again
> and every time this happens and I take a deeper look at the topic I
> have the feeling I still haven’t completely understood it.
>
> Current state:
>
> On Linux, we are currently compiling the HotSpot with the
> '-fvisibility=hidden' flag which effectively hides all symbols except
> the ones which are explicitely declared to have default visibility
> (with the macro JNIEXPORT which expands to
> __attribute__((visibility("default")))).
>
> This was introduced by "6588413: Use -fvisibility=hidden for gcc
> compiles" [1] in HS 21 and it is potentially a good change because it
> hides all symbols by default and it enables certain compiler
> optimizations on some platforms (see "How To Write Shared Libraries"
> [2]).
>
> However, independently of this change, we still use export maps (i.e.
> --version-script=mapfile_reorder) in addition to setting the explicit
> symbol visibility. While [2] recommends this as good practice, I don't
> see the real benefit of having both -
> -fvisibility=hidden/__attribute__((visibility("default"))) and export
> maps except if we were maintaining a versioned ABI. But as far as I
> can tell, that's not the case for HotSpot.
>
> So finally my question: is there any real reason (except the fact that
> "we always did it that way") why we would still need to use export
> maps on Linux where symbol hiding is done with
> -fvisibility=hidden/__attribute__((visibility("default"))) anyway?
> Dropping the maps would make the maintenance of exported symbols
> easier as we wouldn't have to keep the export map files and the
> JNIEXPORT functions in sync.

 From past discussions isn't this because we need the map files to 
expose dynamically generated symbols? (And so we must then also add the 
other visible symbols.) Does that make the visibility attributes 
redundant? I'm not sure, but they serve as a clear statement of intent 
anyway.

David

> Thank you and best regards,
> Volker
>
> [1] https://bugs.openjdk.java.net/browse/JDK-6588413
> [2] http://www.akkadia.org/drepper/dsohowto.pdf
>