Cross compilation discussion

[Andrew Haley]

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.

>> 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?

-- 
Andrew Haley  (he/him)
Java Platform Lead Engineer
Red Hat UK Ltd. <https://www.redhat.com>
https://keybase.io/andrewhaley
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