Mapfiles and static linking of standard libraries
david.holmes at oracle.com
Wed Feb 19 09:33:35 UTC 2014
Note that we (hotspot) do use a mapfile on OSX. This was needed to avoid
every symbol in libjvm being exported resulting in the wrong versions of
operator new/delete being used.
On 19/02/2014 7:22 PM, Magnus Ihse Bursie wrote:
> (Adding in build-dev since we're moving into the jdk build as well now.)
> On 2014-02-17 17:39, Volker Simonis wrote:
>> On Mon, Feb 17, 2014 at 4:08 PM, Dmitry Samersoff
>> <dmitry.samersoff at oracle.com> wrote:
>>> It was my bad - missed two related threads.
>>> We have two problems related to map files:
>>> 1. We have hand-written mapfiles and have to manage it manually.
>>> It's better to generate map file automatically or use visibility
>> I would strongly vote against automatically generating the map files
>> from sources by parsing for special patterns as proposed by Magnus
>> because I think this will just introduce another level of complexity
>> and another point of failure.
> My priorities is to prefer no map files if we can avoid it, but to
> prefer automatically generated over static, checked in, mapfiles if they
> cannot be avoided. So I'll try to join you in the fight to get rid of
> them altogether, but if that fails, I still want to generate them. :-)
> Having static map files are a source of complexity and point of failure
> in itself as well.
>> >From this discussion so far I learned the following:
>> - as long as Oracle insists on static linking libstdc++ and libgcc
>> there's no way of getting rid of the map files.
>> - using -fvisibility=hidden/__attribute__((visibility("default"))) is
>> still desirable because it has positive performance impacts on some
>> platforms and because it is the easiest and cleanest solution for the
>> future when we can finally get rid of the map files. Moreover it is
>> already used anyway.
> __attribute__((visibility("default"))) sounds very much like a gcc
> extension. Is there a similar construct for solaris studio? Otherwise we
> would still need mapfiles on solaris. Also, does
> __attribute__((visibility("default"))) work with clang? When I check the
> jdk build system, I notice that we do not use mapfiles on macosx. (This
> does not stop us from having a "mapfile-macosx" in a library... :-/)
> Also, it seems that the same goes for xlc on aix; we will process
> mapfiles but in the end ignore them. I have no idea what this means for
> the situation on the exported symbols on these platforms.
> And then we have the reorder files in jdk, where we extend the mapfiles
> with a reorder section which specify a specific order in which to store
> functions, based on a (believed, perceived, or once upon a time, real)
> performance boost.
> Oh my! When I carefully checked the jdk code, it seems that there's a
> bug in NativeCompilation, that makes all mapfiles ignored unless there
> is also a reorder file. That's only true for 7 libraries, out of the 40
> that has mapfiles. The same goes for launchers, we point to an incorrect
> directory containing the mapfiles.
> So either I'm mis-reading the code, or we have not actually used most of
> the mapfiles for possibly a very long time. I'm not sure why this has
> not been noticed.
>> - This solution would also establish something like a very low level
>> contract that the VM will only ever export JVM_*/JNI_* symbols.
>> Currently, the following symbols which don't start with JNI_ and JVM_
>> and are exported by the
>> hotspot/make/linux/makefiles/mapfile-vers-product map file from
>> libjvm.so on Linux:
>> # miscellaneous functions
>> - exported and used in the jdk/ (trough
>> - could be easily renamed to JVM_*
> Unfortunately, it's not that easy. While not nicely designed, those
> function that by tradition has been exported by libjvm.so can be
> considered part of the already-existing JVM/JDK contract.
>> For the shared libraries in the jdk/ repository, the situation is a
>> little different.
>> Only the following three libraries do statically link libstdc++/libgcc:
> Yeah, that's a story in itself, isn't it. I remember being upset about
> it when I realized this was the case, but then forgot about it. We
> shouldn't mix statically and dynamically linking the standard library,
> that's just dumb.
> * The benefits of static linking is that we do not depend on a specific
> version of the library being available on the customer's computer; we
> can run with our own well known, tested and trusted version. But if you
> still do dynamic linking, other libraries are susceptible to this problem.
> * The benefits of dynamic linking is that we get a smaller footprint,
> and do not need to keep track of security patches in the standard
> libraries. But if we ship some libraries with static linking, we get a
> unneccessary large footprint and we *still* need to keep track of
> security issues.
> I can see good points for both approaches (even though I would tend to
> prefer dynamic linking), but mixing them just gives us the bad parts
> from both worlds.
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