RFR: 8293116: Incremental JDK build could be sped up [v3]

[Magnus Ihse Bursie]

On Thu, 8 Sep 2022 13:54:02 GMT, Erik Joelsson <erikj at openjdk.org> wrote:

>> I looked up what kind of dependencies we are passing in through `DEPEND`. None, it turns out. It is not used. So that is not a worry. We might consider removing it. Sending in exceptional dependencies like that is never a good design decision. If it ever happens again that this would be the only acceptable solution, we can re-add the `DEPEND` argument.
>> 
>> But that basically mean that Jan's patch might be okay, after all. `DEPENDS` does not exist, and I want to remove it. `EXTRA_DEPS` is only modified if we change the `depend` javac plugin. I am willing to sacrifice automatic recompilation of all Java classes if that is changed.
>
> Ok I was wrong about what EXTRA_DEPS was used for, I read the code too hastily. However, the dependency declaration for the pubapi files from other modules is added in make/CompileJavaModules.gmk like this:
> 
> 
> $($(MODULE)_COMPILE_TARGET): $(foreach d, $(call FindDepsForModule, $(MODULE)), \
>     $(call SetupJavaCompilationApiTarget, $d, \
>         $(if $($d_BIN), $($d_BIN), $(JDK_OUTPUTDIR)/modules/$d)))
> 
> 
> So this still doesn't change my main point. The pubapi files only appear on the dependency declaration for `$($(MODULE)_COMPILE_TARGET)`. This means that with the current patch, we will not recompile the full module when a parent module has a change to the pubapi. The Depend plugin needs to see the pubapi files in `$$?` for that to happen.
> 
> One could also make the case for using DEPEND when adding such a dependency instead of referencing `$($(MODULE)_COMPILE_TARGET)` in explicit rules. It would be more inline with more recent design patterns.

@erikj79 I agree that setting DEPEND would be better. I hate those weird "injection attacks" where we fake arguments to functions, they make the code hopeless to understand (and I'd like to forbid them in `NamedParamsMacroTemplate`, but I can't just now since we still use it in so many places).

But can we really do that? I don't think we have that dependency information available when setting up the module java compilation. (In any case, that is a separate cleanup)

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PR: https://git.openjdk.org/jdk/pull/10104