Cross compilation discussion

[Magnus Ihse Bursie]

On 2023-12-11 10:24, Andrew Haley wrote:
> On 12/8/23 20:36, erik.joelsson at oracle.com wrote:
>>
>> On 12/8/23 08:29, Magnus Ihse Bursie wrote:
>>>
>>>> OK. So you are doing what I'd usually call cross compiling, sort 
>>>> of, but
>>>> you don't want to treat it as a cross-compiled build. It's an unusual
>>>> case, in that it is cross compiled, but it is also expected to be
>>>> able to
>>>> run on the build system.
>>>
>>> The main reason for the build to know if we're doing a
>>> cross-compilation or not has been if we need a special compiler to
>>> create build tools. For me, that is in essence what the
>>> cross-compilation flag is saying.
> >
>> I think the issue now is that automatically figuring out if a separate
>> compiler is needed or not can be hard. We do it in some cases, but not
>> in others, and sometimes we get it wrong (as in the linux-aarch64 issue
>> Mikael recently fixed, where the given tuple didn't exactly match what
>> config.guess found on the build machine). I think it would be reasonable
>> to have a configure parameter to control this explicitly.
>
> I think so, or to be able to override what the build system deduces from
> other build parameters.

Like everything else in configure, we should try to make automatic 
deduction of sane default values, but allow the user to override 
completely if necessary.

>
>>> So maybe we should have like "cross-compiling-need-build-compiler" and
>>> "cross-complining-can-use-target-compiler"?
>>>
>> Not sure of the naming here, but in essence, that's where we are making
>> implicit assumptions today that are hard to control.
>>
>> If you specify --with-openjdk-target and the tuple is different than the
>> build tuple, then we can assume the default is to need a build compiler,
>> but there are cases where that is still not strictly needed. If
>> --with-build-devkit is supplied, then that is also a good indication as
>> we are given an explicit build compiler. Just supplying a sysroot/devkit
>> should probably not be assumed as needing a build compiler, but paired
>> with a suitable option, this could support the cross compile to EL8 from
>> EL7. I could for instance imagine setting --with-build-sysroot=/ to help
>> nudge configure into defining a separate build compiler, but maybe
>> that's too clunky?
>
> Maybe, or at least fails the principle of least surprise. I think I may
> be missing something important here: what is the downside of treating a
> build as a cross compilation? Is it simply that more work has to be done,
> or something else?


The real downside is if we decide that we need a build jdk, then we need 
to build the JDK twice. Apart from that, I think it is mostly that it 
complicates the configuration.

I'm trying to imagine what would happen if we say we are doing a 
cross-compile but set the sysroot to /, and set the build toolchain and 
the target toolchain to the same native tools. I don't think there would 
be any other negative effects.

But maybe I'm forgetting something...

/Magnus