RFR: JDK-8200178 Remove mapfiles for JDK native libraries

Magnus Ihse Bursie magnus.ihse.bursie at oracle.com
Wed Mar 28 22:14:13 UTC 2018


On 2018-03-28 23:53, Martin Buchholz wrote:
> I can't find any documentation for what JNIEXPORT and friends actually do.
> People including myself have been cargo-culting JNIEXPORT and JNICALL 
> for decades.
> Why aren't they in the JNI spec?
That surprises me. I'm quite certain that javah (or rather, java -h 
nowadays) generate header files with JNIEXPORT and JNICALL.

As you can see in the jni.h and jni_md.h files, JNIEXPORT equals 
__attribute__((visibility("default"))) for compilers that support it 
(gcc and friends), and __declspec(dllexport) for Windows. This means, 
that the symbol should be exported. (And it's ignored if you use 
mapfiles aka linker scripts.)

As for JNICALL, it's empty on most compilers, but evaluates to __stdcall 
on Windows. This defines the calling convention to use. This is required 
for JNI calls from Java. (Ask the JVM team why.) While it's not 
technically required for calling from one dll to another, it's good 
practice to use it all time to be consistent. In any way, it doesn't 
hurt us.

>
> ---
>
> It's fishy that the attribute externally_visible (which seems very 
> interesting!) is ARM specific.
>
>   #ifdef ARM
>     #define JNIEXPORT 
>  __attribute__((externally_visible,visibility("default")))
>     #define JNIIMPORT 
>  __attribute__((externally_visible,visibility("default")))

Yeah, this is broken on so many levels. :-(

The ARM here goes back to the old Oracle proprietary arm32 port. This 
used lto, link time optimization, to get an absolutely minimal runtime, 
at expense of a extremely long built time. (I think linking libjvm took 
like 20 minutes.) But when using lto, you also need to decorate your 
functions with the externally_visible attribute. So this was added to 
get hotspot to export the proper symbols (since they, too, used the 
jni.h file).

So, in short, we should:
1) have used a special, local jni.h file for the proprietary arm port, 
and/or
2) added the externally_visible attribute not based on platform, but on 
the existence of lto.

At this point in time, we're not building the old 32-bit arm port, and I 
doubt anyone does. And even if so, we could probably remove the lto 
part, and thus remove this from jni_md.h. If you want, please file a bug.

/Magnus

>   #else
>     #define JNIEXPORT  __attribute__((visibility("default")))
>     #define JNIIMPORT  __attribute__((visibility("default")))
>   #endif
>

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