RFR: 8199739: Use HeapAccess when loading oops from static fields in javaClasses.cpp

Erik Österlund erik.osterlund at oracle.com
Tue Mar 20 15:51:46 UTC 2018

Hi Roman,

This looks good to me. The unfortunate include problems in 
jvmciJavaClasses.hpp are pre-existing and should be cleaned up at some 


On 2018-03-20 16:13, Roman Kennke wrote:
> Am 20.03.2018 um 11:44 schrieb Erik Österlund:
>> Hi Roman,
>> On 2018-03-20 11:26, Roman Kennke wrote:
>>> Am 20.03.2018 um 11:07 schrieb Erik Österlund:
>>>> Hi Roman,
>>>> On 2018-03-19 21:11, Roman Kennke wrote:
>>>>> Am 19.03.2018 um 20:35 schrieb coleen.phillimore at oracle.com:
>>>>>> On 3/19/18 3:15 PM, Stefan Karlsson wrote:
>>>>>>> On 2018-03-19 20:00, coleen.phillimore at oracle.com wrote:
>>>>>>>> I like Roman's version with static_field_base() the best.  The
>>>>>>>> reason
>>>>>>>> I wanted to keep static_field_addr and not have static_oop_addr was
>>>>>>>> so there is one function to find static fields and this would work
>>>>>>>> with the jvmci classes and with loading/storing primitives also.  So
>>>>>>>> I like the consistent change that Roman has.
>>>>>>> That's OK with me. This RFE grew in scope of what I first
>>>>>>> intended, so
>>>>>>> I'm fine with Roman taking over this.
>>>>>>>> There's a subtlety that I haven't quite figured out here.
>>>>>>>> static_field_addr gets an address mirror+offset, so needs a load
>>>>>>>> barrier on this offset, then needs a load barrier on the offset of
>>>>>>>> the additional load (?)
>>>>>>> There are two barriers in this piece of code:
>>>>>>> 1) Shenandoah needs a barrier to be able to read fields out of the
>>>>>>> java mirror
>>>>>>> 2) ZGC and UseCompressedOops needs a barrier when loading oop fields
>>>>>>> in the java mirror.
>>>>>>> Is that what you are referring to?
>>>>>> I had to read this thread over again, and am still foggy, but it was
>>>>>> because your original change didn't work for shenandoah, ie Kim's last
>>>>>> response.
>>>>>> The brooks pointer has to be applied to get the mirror address as well
>>>>>> as reading fields out of the mirror, if I understand correctly.
>>>>>> OopHandle::resolve() which is what java_mirror() is not
>>>>>> accessorized but
>>>>>> should be for shenandoah.  I think.  I guess that was my question
>>>>>> before.
>>>>> The family of _at() functions in Access, those which accept oop+offset,
>>>>> do the chasing of the forwarding pointer in Shenandoah, then they apply
>>>>> the offset, load the memory field and return the value in the right
>>>>> type. They also do the load-barrier in ZGC (haven't checked, but that's
>>>>> just logical).
>>>>> There is also oop Access::resolve(oop) which is a bit of a hack. It has
>>>>> been introduced because of arraycopy and java <-> native bulk copy
>>>>> stuff
>>>>> that uses typeArrayOop::*_at_addr() family of methods. In those
>>>>> situations we still need to 1. chase the fwd ptr (for reads) or 2.
>>>>> maybe
>>>>> evacuate the object (for writes), where #2 is stronger than #1 (i.e. if
>>>>> we do #2, then we don't need to do #1). In order to keep things simple,
>>>>> we decided to make Access::resolve(oop) do #2, and have it cover all
>>>>> those cases, and put it in arrayOopDesc::base(). This does the right
>>>>> thing for all cases, but it is a bit broad, for example, it may lead to
>>>>> double-copying a potentially large array (resolve-copy src array from
>>>>> from-space to to-space, then copy it again to the dst array). For those
>>>>> reasons, it is advisable to think twice before using _at_addr() or
>>>>> in-fact Access::resolve() if there's a better/cleaner way to do it.
>>>> Are we certain that it is indeed only arraycopy that requires stable
>>>> accesses until the next thread transition?
>>>> I seem to recall that last time we discussed this, you thought that
>>>> there was more than arraycopy code that needed this. For example
>>>> printing and string encoding/decoding logic.
>>>> If we are going to make changes based on the assumption that we will be
>>>> able to get rid of the resolve() barrier, then we should be fairly
>>>> certain that we can indeed get rid of it. So have the other previously
>>>> discussed roadblocks other than arraycopy disappeared?
>>> No, I don't think that resolve() can go away. If you look at:
>>> http://mail.openjdk.java.net/pipermail/hotspot-gc-dev/2018-March/021464.html
>>> You'll see all kinds of uses of _at_addr() that cannot be covered by
>>> some sort of arraycopy, e.g. the string conversions stuff.
>>> The above patch proposes to split resolve() to resolve_for_read() and
>>> resolve_for_write(), and I don't think it is unreasonable to distinguish
>>> those. Besides being better for Shenandoah (reduced latency on read-only
>>> accesses), there are conceivable GC algorithms that require that
>>> distinction too, e.g. transactional memory based GC or copy-on-write
>>> based GCs. But let's probably continue this discussion in the thread
>>> mentioned above?
>> As I thought. The reason I bring it up in this thread is because as I
>> understand it, you are proposing to push this patch without renaming
>> static_field_base() to static_field_base_raw(), which is what we did
>> consistently everywhere else so far, with the motivation that you will
>> remove resolve() from the other ones soon, and get rid of base_raw().
>> And I feel like we should have that discussion first. Until that is
>> actually changed, static_field_base_raw() should be the name of that
>> method. If we decide to change the other code to do something else, then
>> we can revisit this then, but not yet.
> Ok, so I changed static_field_base() -> static_field_base_raw():
> Diff:
> http://cr.openjdk.java.net/~rkennke/JDK-8199739/webrev.01.diff/
> Full:
> http://cr.openjdk.java.net/~rkennke/JDK-8199739/webrev.01/
> Better?
> Thanks, Roman

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