Fwd: jdk build system is limited

[Antoni Jankowski]

---------- Forwarded message ---------
Od: Antoni Jankowski <antoni.s.jankowski at gmail.com>
Date: śr., 17 gru 2025, 12:22
Subject: jdk build system is limited
To: <members at openjdk.org>


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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