Provide API points for implementing linkers with non-standard calling conventions

[Владимир Козелков]

I think the main use of alternative linkers is to reflect the existing
ability of systems to run binaries from other platforms.

In my example, it was possible to use old binaries for 32-bin systems on
64-bit systems. But platforms are not limited to this. You were wrong when
you said about the unified calling convention on new architectures - just
look at the ARM64EC calling convention - it allows an application to have
both aarch64 and x86_64 binaries in the process!

Also... I'm confused by the existence of Wine on Linux - it provides a
platform for running binaries of the same architecture, but of a different
operating system (Windows). Unfortunately, I don't know if it has the
ability to have a process with mixed binaries and how this relates to Java,
but this is also an interesting example.

пт, 11 окт. 2024 г., 4:04 Maurizio Cimadamore <
maurizio.cimadamore at oracle.com>:

> Hi,
> as you noticed, while the Linker javadoc alludes at the fact that there
> might be other calling conventions supported in the future, at the
> moment there's no API to expose this. What we had in mind the last time
> we discussed this was not too dissimilar to what you propose here -
> basically just keep calling convention open, by using strings, and then
> allow the "nativeLinker" factory to accept a calling convention string.
>
> Another possibility would be to use linker options - e.g. have a
> CallingConvention linker option that can be passed to
> downcallHandle/upcallStub. This would allow to keep a single linker, but
> to support downcalls with different calling conventions. Both approaches
> are equally expressive, at least in terms of allowing to call functions
> using different argument shuffling. That said, on some platforms, like
> PowerPC support for instance different kind of endianness. So perhaps it
> would be good to have a way to ask for the "big endian" Linker, whose
> canonical layouts will be... big endian. That is, a Linker is about
> functions as much as it is about the definition of fundamental data
> types. So, perhaps when adding support for different Linker "flavors" it
> would be good to keep this in mind.
>
> The reason we left this out in 22 was that we wanted to learn more use
> cases where this was useful. For instance, while it's true that x86
> supported several calling conventions, modern systems seems to have
> evolved a bit, so that each major platform tend to gravitate towards one
> main set of calling convention, typically specified in that platform's
> ABI (e.g. SysV for Linux). It seems to me that even in your case, the
> main driver for selecting an alternate calling convention is x86 really.
> So I'm still not 100% sure that this is something worth pursuing. I
> would feel more at ease if we had more cases where this was useful.
>
> Cheers
> Maurizio
>
>
> On 10/10/2024 20:14, Владимир Козелков wrote:
> > Greetings,
> >
> > The documentation for the Linker.nativeLinker() method says: "It is
> > not currently possible to obtain a linker for a different combination
> > of OS and processor."
> >
> > This is indeed true for hotspot, but what if another implementation
> > could provide the ability to create a linker for a different calling
> > convention? Even if the implementation wanted to do this, it would
> > fail because the API does not provide any points through which this
> > could be done.
> >
> > As an example - android allows us to use binaries for arm in aarch64
> > and for x86 in x86_64 with JNI. In the current implementation, I have
> > to filter the output of SymbolLookup.loaderLookup() so that the user
> > does not get symbols with a different calling convention, although the
> > platform really allows to use them.
> >
> > Additionally, I would like to note that the x86 and x86_64 platforms
> > have several "native" calling conventions, such as cdecl (which is
> > actually used now), fastcall, vectorcall, etc. Even if a hotspot does
> > not allow these calling conventions, it would be useful to have at
> > least the potential to implement them.
> >
> > I can suggest a not very good and naive method for solving the problem
> > - it is inspired by target-triple from LLVM:
> >
> > interface Linker ... {
> >     static List<String> supportedConventions() {return ... ;}
> >     static String defaultConvention() {return ... ;}
> >     static boolean isSupportedConvention(String convention) {return ...
> ;}
> >     static Linker linkerForConvention(String  convention) {return ... ;}
> >     static Linker nativeLinker() {
> >         return linkerForConvention(defaultConvention());
> >     }
> > }
> >
> > For android aarch64 defaultConvention() will return something like
> > "aarch64-android-cdecl"
> >
> > Thanks for reading
>
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