Request for reviews (S): 6826736: CMS: core dump with -XX:+UseCompressedOops

Y.S.Ramakrishna at Sun.COM Y.S.Ramakrishna at Sun.COM
Wed Jul 22 14:12:55 PDT 2009

Never mind. I realized that the old code already
filters null-holding locations, so my fears about new
behaviours this introduces are ill-founded.

Sorry for the noise.
-- ramki

On 07/22/09 14:02, Y.S.Ramakrishna at Sun.COM wrote:
> Hi Vladimir -- i am fine with the fix. I worry just a little bit
> about the decision to filter/suppress these "null-equivalent" oop locations
> by the oop-map code for some future use case where we want to
> apply a closure to all oop-holding locations irrespective of
> whether the value is null; i do not however have a ready use case
> in mind at the moment. Would it be beneficial, for the future and/or the
> unwary, to add appropriate documentation about this filtering
> in the header file where the all_do() and oops_do() methods
> are declared?
> Rest looks good; thanks!
> -- ramki
> On 07/22/09 12:51, Paul Hohensee wrote:
>> You've got 4 uses of this expression
>> Universe::narrow_oop_base() == (address)(void*)(*foo)
>> You might write a new method in Universe to factor it out, vis
>> bool is_narrow_oop_base(void* addr) {
>>  return narrow_oop_base() == (address)addr);
>> }
>> Paul
>> Vladimir Kozlov wrote:
>>> Fixed 6826736: CMS: core dump with -XX:+UseCompressedOops
>>> Problem:
>>> Compiled code may produce decoded oop = narrow_oop_base
>>> when a narrow oop implicit null check is used.
>>> The narrow_oop_base could be NULL or be the address
>>> of the page below heap depending on the used mode of
>>> compressed oops. Such decoded oop could live across
>>> a safepoint and as result referenced in oopMap and debug info.
>>> The implicit NULL exception code checks for such oop value
>>> but deoptimization code and GC does not. And this cause
>>> the problem.
>>> Solution:
>>> Fix deoptimization code and GC to check for such oops values.
>>> During deoptimization convert them to NULL.
>>> And ignore them for GC (like NULL values are ignored).
>>> Added the regression test which shows the problem.
>>> Reviewed by:
>>> Fix verified (y/n): y, regression test
>>> Other testing:
>>> JPRT, CTW with/without UseCompressedOops and +/- ScavengeALot

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