Remove redundant calls of toString() (original) (raw)

Remi Forax forax at univ-mlv.fr
Wed Apr 30 11:08:46 UTC 2014

On 04/29/2014 11:20 AM, Claes Redestad wrote:

On 2014-04-29 09:31, Remi Forax wrote: 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* I guess I should have given more details. :-) I ran my micros on a number of forked VMs to ensure I don't get excessive run-to-run variations, which among other things avoids profile pollution. Also I sloppily collected very few samples and only did minimal warmup after I saw that the micros produced scores that were orders of magnitude apart and the values stabilized after less than two seconds: Flags: -f 3 -wi 4 -i 2 When I run the micros sequentially in the same VM I see some added degradation for the exceptional cases, but the 0p cases still inline to the optimal(?) 360k ops/ms, so while it seems profile pollution might play in when provoked, HotSpot seems to manage these simple micros well enough.

Ok, so you have to take a look to the generated assembly code :) https://wiki.openjdk.java.net/display/HotSpot/PrintAssembly

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.

I've now run tests on JDK8 FCS and a recent internal build of JDK9 and see the same characteristics. How recently are we talking here? :-)

I don't know, I have seen the patches but not dive into the source yet, it's on my todo list, maybe it's not even activated by default.

cheers, Rémi * read that last sentence again, it seems very logical, no ? While your reading of the method name and suggested approach makes sense right now, the javadoc for Objects.requireNonNull states "this method is designed primarily for doing parameter validation in methods and constructors". To me, "parameter validation" suggests that we're dealing with unknowns and that there's at least some calculated risk that the input might be null, so the intent is really to encourage explicit fail-fast designs, no?

The great Java Book is clear on that, you shall never try to recover from an exception that inherits from RuntimeException :) A runtime exception is an exception that developers throw at the head of their fellow developers to indicate that they should read the fucking manual.

I thus think it makes sense to try and ensure these methods have the same performance characteristics as when provoking an implicit null check by dereferencing an object.

as I said, I think it's time to take a look to the generated assembly code.

/Claes

regards, Rémi

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