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

[Claes Redestad]

Hi Dean,

to avoid escape analysis-eliminated allocations in the past @DontInline
has been sufficient. This means a simpler patch (no changes to native
code needed - added assertions notwithstanding) and passes your tests
with C2 (it'd concern me if Graal's EA sees through this trick, as it
might break some existing places where DontInline is used to this
effect):

     /**
      * The value needs to be physically located in the frame, so that it
      * can be found by a stack walk.
      */
     @Hidden
     @DontInline
     private static void ensureMaterializedForStackWalk(Object o) {}

Thanks!

/Claes

On 2018-12-15 01:59, 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".  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