RFR: 8033792: AltHashing used jint for imprecise bit shifting

Coleen Phillimore coleen.phillimore at oracle.com
Mon Feb 10 11:34:41 PST 2014

This looks really good.  Thank you!

On 2/8/14 12:08 AM, Yumin Qi wrote:
> Hi, David and all
>   New webrev:
>   http://cr.openjdk.java.net/~minqi/8033792/webrev01
>   Changed _seed to be defined as 'juint' and changed related uses. 
> This can avoid loss of precision for conversion between 'jint' and 
> 'juint'.
>   Tested with -XX:+ExecuteInternalVMTests and  JPRT.
>    InstanceKlass::host_klass:
>   Add assert for the path which has to contain a none NULL valid 
> Klass* (we did not check for valid Klass*, only check if it is not a 
> NULL here) in debug mode.
> Thanks
> Yumin
> On 2/7/2014 9:54 AM, Yumin Qi wrote:
>> On 2/6/2014 11:47 PM, David Holmes wrote:
>>> Hi Yumin,
>>> On 7/02/2014 2:14 PM, Yumin Qi wrote:
>>>> Hi,
>>>>    Please review the change for 8033792.
>>>>    Summary:  AltHashing uses 'jint' type in the way as 'unsigned 
>>>> int' in
>>>> bit shifting, which is imprecise.  This could lead loss of precision
>>>> when converted between jint and 'unsigned int' during bit 
>>>> operation. Fix
>>>> by changing operation variable type from 'jint' to 'juint', before
>>>> return, cast it into type 'jint'.
>>>> http://cr.openjdk.java.net/~minqi/8033792/webrev00/
>>> I don't see how casting to unsigned int can lead to a loss of 
>>> precision here. Further you now assign h1 from a jint and you pass 
>>> it as a jint parameter to the Integer_rotate functions which would 
>>> seem to me to have more opportunity for conversion issues. Either 
>>> way this code is completely confused about the type of arithmetic it 
>>> is trying to do (and I would think a hash should be unsigned to 
>>> begin with ??).
>> I will double check _seed, all the work here is related to it. If it 
>> is defined as 'unsigned int' and all the operations on it with same 
>> type, all the problems (complains from internal tool) should he gone.
>>> You didn't mention the unrelated change in 
>>> src/share/vm/oops/instanceKlass.hpp
>> This is for the bug 8030129 which will not fix, but add an assert 
>> here to catch problem in debug binary. Sorry forget to mention this.
>> Thanks
>> Yumin
>>> Thanks,
>>> David
>>>> Thanks
>>>> Yumin

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