12 RFR(M) 8214583: AccessController.getContext may return wrong value after JDK-8212605

[dean.long at oracle.com]

On 12/16/18 4:06 PM, David Holmes wrote:
> On 15/12/2018 10:59 am, dean.long at oracle.com wrote:
>> https://bugs.openjdk.java.net/browse/JDK-8214583
>> http://cr.openjdk.java.net/~dlong/8214583/webrev
>>
>> This change includes two new regression test that demonstrate the 
>> problem, and a fix that allows the tests
>> to pass.
>>
>> The problem happens when the JIT compiler's escape analysis 
>> eliminates the allocation of the AccessControlContext object passed 
>> to doPrivileged.  The compiler thinks this is safe because it does 
>> not see that the object "escapes".
>
> Then surely the compiler's notion of "escapes" needs to be updated!
>

The compiler can inline the callee method and see that the value doesn't 
escape.  This is a valid optimization in cases where the callee method 
is known.

dl

> David
> -----
>
>   However, getContext needs to be able to find
>> the object using a stack walk, so we need a way to tell the compiler 
>> that it does indeed escape.  To do this we pass the value to a native 
>> method that does nothing.
>>
>> Microbenchmark results:
>>
>> jdk12-b18:
>>
>> Benchmark                Mode  Cnt    Score   Error  Units
>> DoPrivileged.test        avgt   25  255.626 ± 6.446  ns/op
>> DoPrivileged.testInline  avgt   25  250.968 ± 4.975  ns/op
>>
>>
>> jdk12-b19:
>>
>> Benchmark                Mode  Cnt  Score    Error  Units
>> DoPrivileged.test        avgt   25  5.689 ±  0.001  ns/op
>> DoPrivileged.testInline  avgt   25  2.765 ±  0.001  ns/op
>>
>> this fix:
>>
>> Benchmark                Mode  Cnt  Score    Error  Units
>> DoPrivileged.test        avgt   25  5.020 ±  0.001  ns/op
>> DoPrivileged.testInline  avgt   25  2.774 ±  0.025  ns/op
>>
>>
>> dl