RFR: 8155638: Resource allocated BitMaps are often cleared twice

[Nils Eliasson]

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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