Remove redundant calls of toString()

[Remi Forax]

On 04/28/2014 05:43 PM, Claes Redestad wrote:
> On 04/28/2014 08:57 AM, David Holmes wrote:
>> On 28/04/2014 1:05 PM, Otávio Gonçalves de Santana wrote:
>>> In my opinion not, because Objects.requireNonNull is more readable than
>>> just string.toString. This way is more understandable which field is
>>> required and doesn't impact on performance.
>>
>> An invocation of requireNonNull is potentially more expensive than 
>> the implicit null check that happens with foo.toString().
>>
>> David
>> ----- 
>
> My thought was that these two would be inlined to the exact same 
> thing, so I did a quick test to see what happens when you do 
> foo.toString() versus Objects.requireNonNull(foo) on a set of randomly 
> generated String[]'s with different amounts of null elements(0p: no 
> null entries, 1p: 1% null entries etc):
>
> Benchmark                                     Mode Samples Mean   Mean 
> error    Units
> s.m.ThrowAwayBenchmark.nullToString0p        thrpt 6 356653.044     
> 3573.707   ops/ms
> s.m.ThrowAwayBenchmark.nullToString1p thrpt         6 353128.903     
> 2764.102   ops/ms
> s.m.ThrowAwayBenchmark.nullToString10p thrpt         6 297956.571     
> 9580.251   ops/ms
> s.m.ThrowAwayBenchmark.nullToString50p thrpt         6 158172.036     
> 1893.096   ops/ms
> s.m.ThrowAwayBenchmark.nullToString100p thrpt         6 18194.614      
> 472.091   ops/ms
> s.m.ThrowAwayBenchmark.requireNonNull0p thrpt         6 357855.126     
> 2979.090   ops/ms
> s.m.ThrowAwayBenchmark.requireNonNull1p thrpt         6 67601.134     
> 7004.689   ops/ms
> s.m.ThrowAwayBenchmark.requireNonNull10p thrpt         6 8150.595      
> 538.970   ops/ms
> s.m.ThrowAwayBenchmark.requireNonNull50p thrpt         6 1604.919      
> 220.903   ops/ms
> s.m.ThrowAwayBenchmark.requireNonNull100p thrpt         6 
> 820.626       60.752   ops/ms
>
> Yikes! As long as the value is never null they're inlined nicely and 
> neither have the upper hand performance-wise, but as soon as you get 
> some null values, Objects.requireNonNull degenerates much faster than 
> its foo.toString counterpart. I think this is a JIT issue - optimizing 
> exception-paths might not be the highest priority, but 
> Objects.requireNonNull is used pretty extensively in the JDK and my 
> expectation would be that it shouldn't degrade performance when things 
> actually are null now and then.
>
> /Claes

This is a know issue, I think it's not related to the way the JIT handle 
exception path but what is called 'profile pollution'.
Hotspot JITs have two ways to do a null check, either do nothing (yes 
nothing) and let the system do a fault and come back from dead using a 
signal handler, this solution is named implicit null check or by doing 
an explicit null check, i.e a conditional jump.
Implicit null check is faster but if the receiver is null, it cost you 
an harm, so the VM profiles receiver to remember if the receiver of each 
call can be null or not. The problem is that when you call 
foo.toString(), the profile information is associated with the 
instruction that does foo.toString() while if the call is 
Objects.requireNonNull(foo), the profile associated with the nullcheck 
is stored inside the method requireNonNull, so if one call to 
requireNonNull in the entire program throw a NPE, the profile inside 
requireNonNull now register that it may fail, so for the VM all calls to 
requireNonNull may fail. So currently you should not use requireNonNull 
is the value is not required to be null*

So the problem is that Hotspot blindly trusts the recorded profile 
compared to the profile that can come from the arguments of a call.
The good news is that recently some patches were included in the jdk9 
tree to fix or at least mitigate that issue.

cheers,
Rémi
* read that last sentence again, it seems very logical, no ?