RFR: 8155638: Resource allocated BitMaps are often cleared twice

[Stefan Karlsson]

Thanks for reviewing, Nils!

StefanK

On 2016-06-02 10:23, Nils Eliasson wrote:
> Hi,
>
> After some additional discussions with Stefan we concluded that it is 
> better to keep the non-reinitialize version. There is the future 
> possiblity of reusing the bitmap in every iteration in this loop, but 
> reinitalize doesn't do that at the moment. And more testing would be 
> required for such a change.
>
> Regards,
> Nils Eliasson
>
> On 2016-06-02 10:04, Nils Eliasson wrote:
>> Hi Stefan,
>>
>> I prefer the reinitialize alternative in c1_linearscan.cpp. Please 
>> use that version.
>>
>> Reviewed.
>>
>> Thanks for fixing this!
>> Nils
>>
>>
>> On 2016-05-06 12:46, Stefan Karlsson wrote:
>>> Hi all,
>>>
>>> Please review this patch to remove some redundant clearing of bitmaps.
>>>
>>> http://cr.openjdk.java.net/~stefank/8155638/webrev.01
>>> https://bugs.openjdk.java.net/browse/JDK-8155638
>>>
>>>
>>> It used to be the case that Resource allocated bitmaps were _not_ 
>>> cleared in the constructor, and that explicit calls to clear() were 
>>> needed. However, when G1 was open sourced all the Resource allocated 
>>> bitmaps were always cleared in the BitMap constructor:
>>>
>>> http://hg.openjdk.java.net/jdk9/hs/hotspot/diff/37f87013dfd8/src/share/vm/utilities/bitMap.cpp
>>>
>>>
>>> It's apparent that the lazy clearing of bitmaps was used to skip 
>>> clearing when it wasn't necessary.
>>>
>>> See, for example:
>>> http://cr.openjdk.java.net/~stefank/8155638/webrev.01/src/share/vm/c1/c1_IR.cpp.udiff.html
>>>
>>> where _requires_phi_function is cleared even when all bits are going 
>>> to be set to 1.
>>>
>>> and:
>>> http://cr.openjdk.java.net/~stefank/8155638/webrev.01/src/share/vm/compiler/methodLiveness.cpp.frames.html
>>>
>>> in MethodLiveness::BasicBlock::get_liveness_at
>>>
>>> 1010   ResourceBitMap g(_gen.size()); g.set_from(_gen);
>>> 1011   ResourceBitMap k(_kill.size()); k.set_from(_kill);
>>>
>>> where we first clear the bitmap, then copy all bits with set_from. 
>>> Before G1 was introduced, the unnecessary clearing was not done.
>>>
>>> I've created a new CR for these kind of issues:
>>>
>>> https://bugs.openjdk.java.net/browse/JDK-8156207 - Resource 
>>> allocated BitMaps are often cleared unnecessarily
>>>
>>>
>>> During the review of "8141501: Problems with BitMap buffer 
>>> management", I got the request do the following change:
>>>
>>> http://mail.openjdk.java.net/pipermail/hotspot-dev/2016-May/022969.html
>>>
>>> I tried it, but felt that the change didn't fit the surrounding 
>>> code, where shallow-copying was used for other bitmaps. It would be 
>>> great if reviewers of this patch could decide if I should change the 
>>> code as suggested.
>>>
>>>
>>> Thanks,
>>> StefanK
>>>
>>
>

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