RFR JDK-8022442: Fix unchecked warnings in HashMap
brent.christian at oracle.com
Wed Aug 7 18:38:05 UTC 2013
I did a before/after run with the changes using Doug Lea's MapCheck
microbenchmark . I tested java.util.HashMap with Object, String, and
It should be mentioned this was a quick check for any major performance
changes: 2 iterations, run by hand on my own (relatively quiet) Mac.
The changes are small. (A more serious run would be needed to
accurately measure such a small performance difference, ideally using
jmh, and a benchmark with more precise scoring.)
A brief summary of the scoring differences is below, by type of access
and data type. The benchmark scores are based on the length of time to
perform the operation, so a lower score is better, and a positive
percent change means the operation took longer with the warnings cleanup
MapCheck, java.util.HashMap, trials: 1000, size: 36864
%Change No Changes vs HashMap Warnings Cleanup
Object String Integer
Access Present 0.00% 0.00% 6.67%
Add Absent 0.00% 0.76% -1.27%
Modify Present 0.00% -1.16% 0.00%
Remove Present 0.00% 0.00% 0.00%
Search Absent 0.00% 2.00% 3.03%
Traverse entry 0.00% 0.00% 0.00%
Traverse key/val 0.00% 2.86% 0.00%
One might think the 6.6% increase for Access Present/Integer is cause
for concern, but I think this is more a reflection of the scores being
fairly coarse. Specifically, the two scores before the changes were 15
& 15, and with the changes were 15 & 17 (a mean of 16, 6.67% change from
I think the change is fine to go back.
For reference, my spreadsheet of results is at .
On 8/6/13 3:23 PM, Dan Smith wrote:
> On Aug 6, 2013, at 2:43 PM, Remi Forax <forax at univ-mlv.fr> wrote:
>> On 08/06/2013 11:11 PM, Dan Smith wrote:
>>> Please review this warnings cleanup.
>>> Bug: http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=8022442 (not yet visible)
>>> Webrev: http://cr.openjdk.java.net/~dlsmith/8022442/webrev.00/
>> Hi Dan,
>> I've seen that you have introduce a common super interface for entry and tree node,
>> I suppose that you check that there is no performance regression.
>> I wonder if an abstract class is not better than an interface because as far as I know
>> CHA implemented in hotspot doesn't work on interface
>> (but I may be wrong, there is perhaps a special optimization for arrays).
> To make sure I understand: your concern is that an aastore will be more expensive when assigning to a KeyValueData than to an Object (or even to SomeOtherClass)?
> For what it's worth, all assignments to table[i] are statically known to be safe. E.g.:
> Entry<K,V> next = (Entry<K,V>) e.next;
> table[i] = next;
> So surely a smart VM only does the check once?
> Here are some other things that might be concerns, but don't apply here:
> - interface method invocations: there are no methods in the interface to invoke
> - checkcast to an interface: all the casts are to concrete classes (Entry, TreeBin, TreeNode)
> (There are some unchecked casts from KeyValueData to KeyValueData with different type parameters, but I assume these don't cause any checkcasts.)
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