RFR: 8261847: performace of java.lang.Record::toString should be improved [v4]

[Vicente Romero]

On Mon, 22 Nov 2021 15:53:53 GMT, Claes Redestad <redestad at openjdk.org> wrote:

> FWIW I did a few experiments trying to move the chunking to `SCF`. Most made things worse, but this is getting close: https://github.com/openjdk/jdk/compare/master...cl4es:scf_split?expand=1
> 
> The threshold for when the JIT fails to optimize seem to be pushed up a bit and I get a 4x gains when concatenating 100 Strings (though it takes a good while for the microbenchmark to stabilize). It also fails to make much of a difference when looking at 200 arguments (maybe 1.4x win). The experiment I have done so far are crude and aggregates the results into a String builder with no size estimation, so it adds a but of unnecessary allocation that could be improved. I think this needs way more work to be a good enhancement and that splitting up outside is going to remain better for now.
> 
> A "hidden" drawback with chunking is that you're likely going to be allocating more.
> 
> I also re-realized that it'll be pretty hard (or impossible) to get around having some MH slot limit: even though we chunk it up internally, the MH we return must have a type that matches the type given to the bootstrap method, and any operations on the tree that require some temporary arguments we need to pass around will require some argument slots. The current strategy doesn't use too many temporaries, so the real limit might be around 250, but it makes sense to give some room for alternative implementations that use more temporaries if that makes things more efficient.

Thanks for sharing this and for doing the experiment!

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PR: https://git.openjdk.java.net/jdk/pull/6403