RFR: 8304759: Add BitMap iterators [v2]

[Axel Boldt-Christmas]

On Wed, 29 Mar 2023 09:22:33 GMT, Axel Boldt-Christmas <aboldtch at openjdk.org> wrote:

>> Kim Barrett has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   better names for step functions
>
> src/hotspot/share/utilities/bitMap.hpp line 420:
> 
>> 418:   // Returns the index of the last set bit in the remaining iteration range.
>> 419:   // precondition: !is_empty()
>> 420:   idx_t last() const;
> 
> The comments for `first()` and `last()` are a little misleading. The invariant that the `first()`/`last()` is a set bit is only true for `first()` in the forward iterator and for `last()` in the reverse iterator. 
> 
> It seems more like a contract that if you use `first()` then you must have created `IteratorImpl(const BitMap* map, idx_t beg, idx_t end)` with `beg` being a set bit. And if you use `last()` then you must have created it with `end` being a set bit.

Or `beg == end`, that is an `is_empty()` Iterator

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PR Review Comment: https://git.openjdk.org/jdk/pull/13184#discussion_r1151653215