RFR: 8269957: facilitate alternate impls of NameTable and Name [v5]

[Jonathan Gibbons]

On Fri, 25 Aug 2023 23:02:53 GMT, Archie Cobbs <acobbs at openjdk.org> wrote:

>> The `Name.Table` class is used by the compiler to hold unique instances of strings as `Name` objects.
>> 
>> In theory the `Name` superclass supports alternate implementations beyond the two existing implementations (`SharedNameTable` and `UnsharedNameTable`), but its current design presumes that strings are stored as UTF-8 byte arrays, which discourages other approaches.
>> 
>> The goal of this PR is to refactor things to allow for more flexibility in alternate `Name` implementations.
>> 
>> As a simple test case of this idea, it should be relatively simple to implement a `Name.Table` that stores `String`s in a hash table. This patch includes such an example in the new class `StringNameTable`, which can be enabled via the `-XDuseStringTable=true` command line flag.  A simple performance test with this class enabled ([JavacNameTable.java.txt](https://github.com/openjdk/jdk/files/11602852/JavacNameTable.java.txt)) shows a 17% speedup.
>> 
>> Changes:
>> * Remove all byte-oriented methods from the `Name` and `Name.Table` API's, except for those that import/export Modified UTF-8.
>> * Change the semantics of `Name.subName()` so the offset is a character offset, not a byte offset.
>> * Consolidate the common UTF-8 machinery of `SharedNameTable` and `UnsharedNameTable` into a new common superclass `Utf8NameTable`.
>> * Rename `Name.lastIndexOf()` -> `Name.lastIndexOfAscii()` to more accurately reflect its expected behavior.
>> * Add new `StringNameTable` implementation.
>
> Archie Cobbs has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - Fix copyright year.
>  - Fix 8000 table size constant which was meant to be 0x8000.

The code passes javadoc tests and :tier1 tests with StringNameTable enabled.

I'll approve, but recommend we get Vicente's approval, as a javac dev, as well.

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Marked as reviewed by jjg (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/13282#pullrequestreview-1596949991