RFR: 8294062: Improve parsing performance of j.l.c.MethodTypeDesc

[Paul Sandoz]

On Tue, 20 Sep 2022 13:08:20 GMT, Adam Sotona <asotona at openjdk.org> wrote:

> Parsing performance of `j.l.c.MethodTypeDescriptor::ofDescriptor` can be significantly improved and add performance boost to intensive users.
> 
> Two main reasons have been identified and fixed in this patch:
> 
> First glitch is late insertion of return type to the first position of `j.u.ArrayList`, causing all previously parsed parameter types to shift using `System::arraycopy`.
> 
> Proposed patch inserts a placeholder for the return type first and replaces it with return type later.
> 
> Second performance degradation is in `MethodTypeDescriptor::skipOverFieldSignature`, where each identified class descriptor is exactly located by `String::indexOf`, extracted by `String::substring`, and passed for for the purpose of verification to `ConstantUtils::verifyUnqualifiedClassName`.
> 
> Proposed patch inlines the unqualified class name verification into `ConstantUtils::skipOverFieldSignature` parsing loop and so eliminates calls of `String::indexOf` and `String::substring`. Package private method `ConstantUtils::verifyUnqualifiedClassName` has no other use within the package, so it is removed.
> 
> Please review proposed performance improving patch.
> 
> Thanks,
> Adam

src/java.base/share/classes/java/lang/constant/ConstantUtils.java line 217:

> 215:                             }
> 216:                             case '/' -> {
> 217:                                 if (!legal) return 0;

IIUC that works to reject `//` and trailing `/`, but does not reject leading `/`? which the previous code rejected.

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PR: https://git.openjdk.org/jdk/pull/10358