RFR: 8284579: Improve VarHandle checks for interpreter [v2]

[ExE Boss]

On Fri, 8 Apr 2022 12:20:32 GMT, Claes Redestad <redestad at openjdk.org> wrote:

>> A few additional enhancements aiming to improve VH performance in the interpreter:
>> 
>> - Flatten `TypeAndInvokers`: adds a pointer to `VarHandle` (a small increase 40->48) but removes an object and an indirection on any instance actually used - and might avoid allocating the `MethodHandle[]` unnecessarily on some instances
>> - Have `checkExactAccessMode` return the directness of the `VarHandle` so that we can avoid some `isDirect` method calls.
>> 
>> Baseline, `-Xint`
>> 
>> Benchmark                                 Mode  Cnt    Score   Error  Units
>> VarHandleExact.exact_exactInvocation      avgt   30  478.324 ? 5.762  ns/op
>> VarHandleExact.generic_exactInvocation    avgt   30  392.114 ? 1.644  ns/op
>> VarHandleExact.generic_genericInvocation  avgt   30  822.484 ? 1.865  ns/op
>> 
>> 
>> Patched, `-Xint`
>> 
>> Benchmark                                 Mode  Cnt    Score   Error  Units
>> VarHandleExact.exact_exactInvocation      avgt   30  437.704 ? 5.320  ns/op
>> VarHandleExact.generic_exactInvocation    avgt   30  374.512 ? 3.154  ns/op
>> VarHandleExact.generic_genericInvocation  avgt   30  757.054 ? 1.237  ns/op
>> 
>> 
>> No significant performance difference in normal mode.
>
> Claes Redestad has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Simplified as suggested by @ExE-Boss

How would the performance change if the `isDirect` and `checkExactAccessMode` merger was reverted?

src/java.base/share/classes/java/lang/invoke/VarHandle.java line 2075:

> 2073: 
> 2074:     @DontInline
> 2075:     final void throwWrongMethodTypeException(VarHandle.AccessDescriptor ad) {

This can actually be `private` and `static`:
Suggestion:

    private static final void throwWrongMethodTypeException(VarHandle.AccessDescriptor ad) {

src/java.base/share/classes/java/lang/invoke/VarHandles.java line 719:

> 717: //                        MethodHandle.linkToStatic(<LINK_TO_STATIC_ARGS>);
> 718: //                    } else {
> 719: //                        MethodHandle mh = handle.getMethodHandle(ad.mode);

The `direct`‑ness check can be hoisted into an enclosing `if` statement:
Suggestion:

//                    if (direct) {
//                        if (handle.vform.methodType_table[ad.type] == ad.symbolicMethodTypeErased) {
//                            MethodHandle.linkToStatic(<LINK_TO_STATIC_ARGS>);
//                            return;
//                        } else if (handle.vform.getMethodType_V(ad.type) == ad.symbolicMethodTypeErased) {
//                            MethodHandle.linkToStatic(<LINK_TO_STATIC_ARGS>);
//                            return;
//                        }
//                    }
//                    MethodHandle mh = handle.getMethodHandle(ad.mode);


Also, any reason `GUARD_METHOD_TEMPLATE_V` uses `LINK_TO_STATIC_ARGS` instead of `LINK_TO_STATIC_ARGS_V`?

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PR: https://git.openjdk.java.net/jdk/pull/8160