RFR: 8254807: Optimize startsWith() for String.substring()

[Xin Liu]

On Tue, 20 Oct 2020 08:26:54 GMT, Xin Liu <xliu at openjdk.org> wrote:

>> 8254807: Optimize startsWith() for String.substring()
>
> I am fixing 4 regressions. 2 of them are reproducible on my linux host. 
> both crash in loopopt. it seems that substring optimization end up some broken ideal graph.

I add a microbenchmark. currently, api-level

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PR: https://git.openjdk.java.net/jdk/pull/704