On Jul 9, 2014, at 8:04 PM, John Rose <john.r.r...@oracle.com> wrote:
> On Jul 9, 2014, at 3:14 AM, Paul Sandoz <paul.san...@oracle.com> wrote:
>
>>
>> I quickly verified the fold up does as you expect. Also, if i do the
>> following the null check goes away:
>>
>> ...
>>
>> void testLoop() {
>> for (int i = 0; i < 1000000; i++) {
>> testLoopOne(a);
>> testLoopOne(snull);
>> }
>> }
>
> Good observation. So rather than missing a null-case fold-up (good it's
> not!), the optimizer is speculating not-nullness (based on profile) and
> adding a test. (Either the test is being used for a downstream optimization,
> or else the test is not being detected as useless and removed—which would be
> bad!.)
>
I guess it's the pesky null check in the class cast causing issues:
public T cast(Object obj) {
if (obj != null && !isInstance(obj))
throw new ClassCastException(cannotCastMsg(obj));
return (T) obj;
}
>> I am probably obsessing too much over some micro/nano-benchmarks,
>
> (Hi, I'm John and I'm a micro-obsess-aholic.)
>
:-)
>> but i am wondering if this could cause some unwanted de-opt/recompilation
>> effects when all is good with no null values then suddenly a null triggers
>> de-optimization.
>
> Besides jumping after the micro-benchmark and chewing on the optimizer until
> the code shrinks, there are two other things we can do:
>
> 1. Mentally file the issue and watch real benchmarks for evidence of the
> problem. (This works pretty well, provided enough time and focus, and
> provided enough people have some consciousness of the optimizer's workings.)
>
> 2. Create a self-test and check it into the test base. It could be either a
> unit test of assertion. In this case, I don't see an easy way to do it, but
> creating clever permanent tests almost always pays off much better than
> cleverly pounding on the micro-benchmark of the moment.
>
How about the a variant of following?
public class NullCheckDroppingsTest {
volatile String svalue = "A";
volatile String snull = null;
String ssink;
public static void main(String[] args) {
NullCheckDroppingsTest t = new NullCheckDroppingsTest();
t.testLoop();
}
void testLoop() {
// Ensure testLoopOne is compiled and does not
// see a null value
for (int i = 0; i < 1000000; i++) {
testLoopOne(svalue);
// Uncomment the following and the call outside
// the loop should not result in any deoptimization
// testLoopOne(snull);
}
// Sleep just to create delay in the compilation log
try {
Thread.currentThread().sleep(1000);
} catch (Exception e) {}
// This should cause a de-optimisation
// if testLoopOne is compiled with a null-check
testLoopOne(snull);
}
void testLoopOne(String s) {
try {
ssink = String.class.cast(s);
} catch (Throwable t) {
throw new Error(t);
}
}
}
$ java -XX:-TieredCompilation -XX:+UnlockDiagnosticVMOptions
-XX:+PrintCompilation NullCheckDroppingsTest
64 1 java.lang.String::hashCode (55 bytes)
69 2 java.lang.String::indexOf (70 bytes)
81 3 ! NullCheckDroppingsTest::testLoopOne (27 bytes)
81 5 n java.lang.Class::isInstance (native)
81 4 java.lang.Class::cast (27 bytes)
82 6 % ! NullCheckDroppingsTest::testLoop @ 2 (45 bytes)
85 6 % ! NullCheckDroppingsTest::testLoop @ -2 (45 bytes)
made not entrant
1086 3 ! NullCheckDroppingsTest::testLoopOne (27 bytes) made
not entrant
The log can be searched to see of a deopt ("made not entrant") occurred or not.
Paul.
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