RFR: 8254354: Add an asExact() VarHandle combinator [v9]

[Paul Sandoz]

On Thu, 29 Oct 2020 18:14:05 GMT, Jorn Vernee <jvernee at openjdk.org> wrote:

>> Hi,
>> 
>> This patch adds an asExact() combinator to VarHandle, that will return a new VarHandle that performs exact type checks, similar to MethodHandle::invokeExact, to help developers catch inexact VarHandle usage, which can lead to performance degradation.
>> 
>> This is implemented using a boolean flag in VarForm. If the flag is set, the exact type of the invocation is checked against the exact type in the VarForm. If there is a mismatch, a WrongMethodTypeException is thrown.
>> 
>> Other than that, there is also an asGeneric() combinator added that does the inverse operation (thanks to Rémi for the suggestion). I've also added The `@Hidden` annotation to the VarHandleGuards methods, as well as a type-checking helper method called from the generic invocation lambda form, so that the stack trace we get points at the location where the VarHandle is being used.
>> 
>> Thanks,
>> Jorn
>> 
>> CSR link: https://bugs.openjdk.java.net/browse/JDK-8255375
>
> Jorn Vernee has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains 12 additional commits since the last revision:
> 
>  - Update Javadoc, and rename asExact and asGeneric to withInvokeExactBehaviour and withInvokeBehaviour
>  - Merge branch 'master' into Exact_VarHandle
>  - Use AccessType ordinal in guard checks instead of the AccessMode ordinal
>  - Update accessModeType to use the AccessType ordinal directly.
>  - Add benchmarks
>  - - Update javadoc
>    - Make isExact() public
>  - Fixes failing tests, and enable verifier on Exact test
>  - Fix whitespace
>  - Comment out VarHandleGuards generator code
>  - Makes exactness a property of a VarHandle, not a VarForm, since the latter are shared. Use handle.accessModeType to get the exact type of the VarHandle.
>  - ... and 2 more: https://git.openjdk.java.net/jdk/compare/08226775...3c707bc7

Looks good. There is just one difference between the spec and implementation. The spec states: 
> If this VarHandle already has invoke{-exact} behaviour this VarHandle is returned.

I prefer this behaviour, but feel free to update the spec if you like e.g. If ... already has XXX then a new VH with exactly the same behaviour as this VH is returned.

I just realized i used "behaviour", the english spelling. We should use the US spelling, "behavior", which is more commonly used throughout the JDK documentation.

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Marked as reviewed by psandoz (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/843