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

[Magnus Ihse Bursie]

On Thu, 8 Sep 2022 09:40:13 GMT, Magnus Ihse Bursie <ihse at openjdk.org> wrote:

>> To clarify my last post. The Main compile rule has this prereq declaration:
>> 
>> 
>> $$($1_COMPILE_TARGET): $$($1_SRCS) $$($1_FILELIST) $$($1_DEPENDS) \
>>         $$($1_VARDEPS_FILE) $$($1_EXTRA_DEPS) $$($1_JAVAC_SERVER_CONFIG)
>> 
>> The `$$($1_DEPENDS)` and `$$($1_EXTRA_DEPS)` may contain prereqs/dependencies that we probably should always recompile everything for. The EXTRA_DEPS is where we get the API changes from other modules. The DEPENDS variable is set by the caller and can be anything. Both of these are (deliberately) missing from the FILELIST generation rule, but I think MODFILES generation needs them. Because of this, I think the new MODFILES logic needs to move to the main compilation rule (like in your original patch) but the FILELIST rule should still be left alone.
>> 
>> We need to verify that this still gives us the expected speedup in the common case though. I'm not sure in what situations other dependencies may be included in `$?` here.
>
> Actually, this is not an entirely correct description. EXTRA_DEPS points to the `depend` javac plugin itself in buildtools. Once built, this is unlikely to change. Dependency on the API changes themselves happen on a higher level, in `CompileJavaModules.gmk`, where we add a dependency of the outputted `pubapi` file for modules we depend on. 
> 
> (I did have to look this up, though)

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.

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