RFR: 8329764: G1: Handle null references during verification first

[Thomas Schatzl]

On Fri, 5 Apr 2024 11:09:17 GMT, Thomas Schatzl <tschatzl at openjdk.org> wrote:

> Hi all,
> 
>   please review this change that suggests to move the null reference check in object iteration during heap verification first.
> 
> The reason is as stated, since null references are fairly common (not only in that test mentioned in https://bugs.openjdk.org/browse/JDK-8329314), it may make sense to put it first. Also, null references never fail anyway (and verification failure is supposed to be uncommon).
> 
> Improves total runtime of that test case from 3s (~3.9s cpu time) to 2.2s (~3.1s cpu time).
> 
> If you think it makes the code too ugly, I will retract it.
> 
> Testing: gha, local testing.

Fwiw, removing the `assert(_containing_obj != nullptr` but keeping other code as is gives the following results: runtime 2.65s (cpu time 3.2s), so it may still be worth changing the code as suggested.

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PR Comment: https://git.openjdk.org/jdk/pull/18650#issuecomment-2039553997