jdk build system is limited

[Alexey Ivanov]

Hi Antoni,

This discussion belongs in build-dev mailing list.

-- 
Regards,
Alexey

On 2025-12-17 11:22, Antoni Jankowski wrote:
> Hi,
>
> I've recently studied the jdk21u build system and noticed that at its 
> core it is not meant to handle BUILDJDK and TARGETJDK being made for 
> two very different systems.
> Many of the FLAGS are shared between the two and in many cases that 
> may be problematic. A good example is the GENDEPS_FLAGS variable, 
> which is only present in one version, just in the 
> /make/autoconf/spec.gmk.in <http://spec.gmk.in>/, meaning that the 
> BUILDJDK gets the same GENDEPS_FLAGS as the TARGETJDK. Shouldn't this 
> variable be split into GENDEPS_FLAGS_TARGET and GENDEPS_FLAGS_BUILD to 
> ensure a correct generation of dependency files in all sorts of 
> cross-compilation scenarios?
>
> Same story is with the variables defined in 
> /make/autoconf/flags-cflags.m4, make/autoconf/flags-ldflags.m4 and 
> make/autoconf/flags-other.m4, /like
> * $2CFLAGS_JDKEXE
> * $2CXXFLAGS_JDKEXE
> * $2CFLAGS_JDKLIB
> as they all depend on the OPENJDK_TARGET_OS variable, which is the 
> same for both TARGETJDK and BUILDJDK. I suppose the root of the 
> problem is right there, where in /build-spec.gmk.in 
> <http://build-spec.gmk.in>/ the OPENJDK_TARGET_OS variable is not 
> overwritten with the value of OPENJDK_BUILD_OS, like it's done with 
> other OPENJDK_TARGET_* variables. I suppose no problems occur when the 
> compiler is shared between the target and the build platforms, even 
> less problems in case of linux or macOS in both cases being the 
> operating system, however in case of the target system differing 
> greatly from the build, the cross-compilation is close to impossible 
> with the current build system.
>
> I wonder if that is a design choice, or perhaps a limitation that was 
> not planned.
> Would you be open for changes that make it a bit easier to introduce a 
> new platform target to openjdk? I'd gladly post a pull request solving 
> the issues mentioned.
>
> Best regards,
> Antoni Jankowski
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