Making the foreign liner a required API

[Jorn Vernee]

Hello Tyler,


First of all, thanks for trying this out!


To answer some of your questions:


 > Does this createLibffiBundle.sh get called programmatically during 
the build process (I can't seem to find where that would happen 
currently), or does each platform check in a bundle with a compiled 
version of the library to source control (and the build phase assumes 
it's present for those platforms that support it)?

I pointed to the createLibffiBundle.sh script as an example, so you 
could get a rough idea of the steps I took. The script takes a devkit as 
input, which is an artifact we use internally for our CI builds. These 
bundles (devkit and libffi) are not distributed.


The libffi library is either automatically detected by the build system 
if it is installed in a well-known system location, or it can be 
provided manually using the --with-libffi configure option (I've used 
the latter myself). I you'd like to know how the build works, I suggest 
taking a look at the ./make/autoconf/lib-ffi.m4 file, which is 
responsible for pulling the contents of the --with-libffi option into 
the build system. It basically sets up several make variables with 
compiler and linker options. You could grep through the rest of the 
./make folder for some of the LIBFFI_* variables that are 'exported' at 
the end of the lib-ffi.m4 file using AC_SUBST. For the fallback linker 
you should find the relevant usages in the 
./make/modules/java.base/lib.gmk file (near the end).


To incorporate the library (.dll/.so/.dylib) into the JDK, it is 
essentially just copied into the right folder of the built JDK image. 
Automatic copying can be turned on using the --enable-libffi-bundling 
configuration flag. In practice, I'm using `--with-libffi=<path to 
extracted bundle> --enable-libffi-bundling --enable-fallback-linker` to 
create a working JDK image.


 > At present I'm getting an error at build time after enabling the 
fallback linker on aix.

This is likely caused by an outdated version of libffi. The 
ffi_get_struct_offsets function was added in version 3.3, so that one or 
a later version should work. I've done all my testing with version 
3.4.2, so I recommend going with that.


Also, note that we decided to postpone making the linker API required 
for now. Given the deprecation of the 32-bit Windows x86 port in JDK 21, 
making the linker API required after that port is removed would avoid 
the need of having to amend the public API and implementation to allow 
selecting one of the 2 major ABIs on that platform (cdecl or stdcall).


HTH,
Jorn


On 09/06/2023 19:02, Tyler Steele wrote:
> Hello Jorn and fellow porters,
>
> I am following up with this message from Jorn from a few months ago 
> [1]. I am generally supportive of their proposal, but I am not certain 
> how to implement it. I have successfully built and bundled libffi on 
> AIX, but I didn't use the createLibffiBundle.sh script provided by the PR.
>
> What I'm feeling confused about is how libffi will be incorporated 
> into the jvm. Does this createLibffiBundle.sh get called 
> programmatically during the build process (I can't seem to find where 
> that would happen currently), or does each platform check in a bundle 
> with a compiled version of the library to source control (and the 
> build phase assumes it's present for those platforms that support it)? 
> At present I'm getting an error at build time after enabling the 
> fallback linker on aix.
>
> src/java.base/share/native/libfallbackLinker/fallbackLinker.c:66:10: 
> warning: implicit declaration of function 'ffi_get_struct_offsets' is 
> invalid in C99 [-Wimplicit-function-declaration]
>   return ffi_get_struct_offsets((ffi_abi) abi, jlong_to_ptr(type), 
> jlong_to_ptr(offsets));
>          ^
> Which is not totally surprising given that I haven't figured out how 
> to link it yet.
>
> Thanks in advance,
> Tyler
>
> [1] https://mail.openjdk.org/pipermail/porters-dev/2023-March/000753.html
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