RFR: 8317677: Specialize Vtablestubs::entry_for() for VtableBlob

[Vladimir Kozlov]

On Mon, 9 Oct 2023 10:59:18 GMT, Thomas Schatzl <tschatzl at openjdk.org> wrote:

> Hi all,
> 
>   please review this change that specializes the call to `Vtablestubs::entry_for` for vtable blob.
> 
> This kind of code blobs must always be in the `Vtablestubs` table (they are only created by the `Vtablestubs` class), so trying to find that stub there using it is unnecessary. For verification I added an assert that verifies it's found.
> 
> Given that `Vtablestubs::entry_point()` would take a lock this removal of the lock results in a large speedup (https://bugs.openjdk.org/browse/JDK-8317527) in MT situations and is an even better change than that mentioned CR - and also worth the extra code (I did not retest performance since the `Vtablestubs` table lookup should be fairly fast).
> 
> Fwiw, the first commit contains a different variant of this change, keeping the decision out of `Vtablestubs`, but I liked the current one better for it encapsulating that decision more. I am fine with reverting the second commit.
> 
> Testing: tier1-7, gha, stress testing
> 
> Thanks,
>   Thomas

Good.

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Marked as reviewed by kvn (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/16099#pullrequestreview-1674579825