RFR: 8237363: Remove automatic is in heap verification in OopIterateClosure

[Stefan Karlsson]

On 2020-01-22 12:02, Thomas Schatzl wrote:
> Hi,
> 
> On 17.01.20 14:31, Stefan Karlsson wrote:
>> Hi all,
>>
>> Please review this patch to remove the automatic "is in heap" 
>> verification from OopIterateClosure.
>>
>> https://cr.openjdk.java.net/~stefank/8237363/webrev.01/
>> https://bugs.openjdk.java.net/browse/JDK-8237363
>>
>> OopIterateClosure provides some automatic verification that loaded 
>> objects are inside the heap. Closures can opt out from this by 
>> overriding should_verify_oops().
>>
>> I propose that we move this verification, and the way to turn it off, 
>> and instead let the implementations of the closures decide the kind of 
>> verification that is appropriate. I want to do this to de-clutter the 
>> closure APIs a bit.
>>
> 
> While the change is correct, I am not really convinced it is a good idea 
> to trade verification in one place to the same verification in many place.

An alternative would be to simply remove the verification altogether. As 
I said, we almost always check the result of the object address.

> 
> The closure API does not seem to be particularly "cluttered up" by this 
> particular API to me.

It's a slippery slope. Previously, we had a lot of GC specific functions 
in these interfaces. I've been cleaning this over the years, and this is 
one of the last non-essential parts of that interface that implementors 
need to consider.

With my removal people don't have to think about this anymore.

  It is true that other code typically has many
> other asserts that would fail anyway, but it would be an additional 
> safety net when writing new closures.

It's a safety net that works for G1, but almost always is incorrectly 
trips in the assert with ZGC.

> 
> This is not a hard no for this change, but is there something else you 
> are planning to do in this area where this code would be in the way?

No.

StefanK

> 
>> I've gone through all OopIterateClosures that don't override 
>> should_verify_oops() and added calls to 
>> assert_oop_field_points_to_object_in_heap[_or_null] where the closures 
>> didn't have equivalent checks.
>>
>> A lot of the places didn't explicitly check that the object is within 
>> the heap but they would check for other things like:
>> - Is the corresponding bit index within the range
>> - Is the heap region index within range
>> - Is the object in the reserved heap range (weaker than is_in)
>>
>> I've added asserts to those places. If you think I should remove some 
>> of them, please let me now.
>>
>> Tested with tier1-3
>>
> 
> Thanks,
>    Thomas
>