RFR: 8368468: Split out everything but configure results from spec.gmk [v2]

[Erik Joelsson]

On Wed, 24 Sep 2025 14:57:40 GMT, Magnus Ihse Bursie <ihse at openjdk.org> wrote:

>> Most of spec.gmk contains lines on the form:
>> 
>> VAR := @VAR@
>> 
>> which are setup using `AC_SUBST(VAR)` in the configure script. However, there are also some other stuff there, which are mostly put there since we had no other good place to do so.
>> 
>> I have now created a new file, `CommonVars.gmk`, which is automatically included, and moved everything but pure configure assignments from `spec.gmk` into this file.
>> 
>> That makes `spec.gmk` a great interface between configure and make -- the job of configure is to fill in `spec.gmk`, plain and simple.
>> 
>> This refactoring also points to additional future improvements, e.g. generating `spec.gmk` automatically from all `AC_SUBST` statements, and to clean up the new `CommonVars.gmk`. Right now I kept all lines in the same order as they appeared in `spec.gmk.template`, to minimize risk of unintentional changes (we have some non-`:=` assignments), and to facilitate comparison for this patch. It is also clear that we have room for improvement in how we do bootcycle-builds and build the build-jdk. My current solution is to keep the special spec files, even if they just set a single special flag, to minimize impact of this patch.
>
> Magnus Ihse Bursie has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fix comment and remove dead code

Marked as reviewed by erikj (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/27452#pullrequestreview-3264149118