RFR (11u, XXL): Upstream/backport Shenandoah to JDK11u

[Gil Tene]

> On Jul 9, 2020, at 8:04 AM, Roman Kennke <rkennke at redhat.com> wrote:
> 
> 
>> Now, about that proof: I will spend the day looking if we can do it
>> by
>> comparing object files of builds. E.g. do builds (-shenandoahgc)
>> before/after a change is applied, run checksums over each object
>> file,
>> and compare those checksums. Let's see if there's any pitfalls there.
>> 
> 
> I digged a little into this. I built JDK11u with and without the patch,
> and without debug-symbols (important), and compared the resulting
> objects. I number of object files differ in their generated code, and
> they all look like:
> 
> mov $0x3e, %eax
> vs
> mov $0x44, %eax
> 
> In other words, constant numbers are different. I very strongly suspect
> that this is line numbers that are caught by __LINE__, for example in
> guarantee(). There are a number of other places where __LINE__ is used.
> Line numbers are not expected to be equal before/after the patch. I
> could probably hack around that -- but would that still be in the
> spirit of what we want to achieve? I have my doubts.

I agree that a difference purely around line number constants should not
be considered a "difference" for the purpose of the proof you are looking
for. The question is how to identify that…

Maybe instead of object files, we can go for differences in post-processed
sources (basically the actual input to compilation)? I have a feeling that we'll
run into issues there too, but at least there you can do some more textual
analysis…

> 
> Roman