8304016: Add BitMap find_last suite of functions

[kimbarrett]

Please review this change that adds functions to BitMap for finding the last
set/clear bit in a range.

Testing:
mach5 tier1, including new gtesting for the new functions.

---

Progress

• Change must be properly reviewed (1 review required, with at least 1 Reviewer)
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• Commit message must refer to an issue

Issue

• JDK-8304016: Add BitMap find_last suite of functions

Reviewers

• Stefan Karlsson (@stefank - Reviewer) ⚠️ Review applies to b3d2aed1
• Axel Boldt-Christmas (@xmas92 - Committer) ⚠️ Review applies to b3d2aed1

Reviewing

Using git

Checkout this PR locally:
$ git fetch https://git.openjdk.org/jdk.git pull/12988/head:pull/12988
$ git checkout pull/12988

Update a local copy of the PR:
$ git checkout pull/12988
$ git pull https://git.openjdk.org/jdk.git pull/12988/head

Using Skara CLI tools

Checkout this PR locally:
$ git pr checkout 12988

View PR using the GUI difftool:
$ git pr show -t 12988

Using diff file

Download this PR as a diff file:
https://git.openjdk.org/jdk/pull/12988.diff

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[openjdk]

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[mlbridge]

Webrevs

• 02: Full - Incremental (fcfebb2a)
• 01: Full - Incremental (b3d2aed1)
• 00: Full (c3a75dd1)

[kimbarrett]

I've improved find_first/last_bit_impl since the original PR commit.

(1) In find_last_bit_impl, testing the last bit and the last word has been
improved, shaving off an instruction or two (depending on platform). The
previous version incorrectly expected shifts to set condition codes.

(2) The previous version of find_last_bit_impl expected the compiler to CSE
merge the nearly (and now actually, because of the change above) identical
code for handling found bits in cword. But gcc (at least) doen't seem to do
that, so we do it manually. This doesn't have any performance cost, just saves
20-30% of the (inline) code size.

(3) Applied to find_first_bit_impl a similar technique to (2).

The manual tail merging is a little bit indirect. It might be that it would
be easier to understand by being explicit and using a goto.

[stefank]

Thanks for creating this PR. As we have discussed offline, this is not my preferred code style for these functions, but I'm happy to see this functionality being upstreamed, so consider this reviewed.

[openjdk]

@kimbarrett This change now passes all automated pre-integration checks.

ℹ️ This project also has non-automated pre-integration requirements. Please see the file CONTRIBUTING.md for details.

After integration, the commit message for the final commit will be:

8304016: Add BitMap find_last suite of functions

Reviewed-by: stefank, aboldtch

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[xmas92]

The rewrite made the algorithm more readable, imo.

At some point I would like to type up the beg and end so it is clear that it is a range with [beg, end)

[kimbarrett]

Thanks for reviews @stefank and @xmas92

[kimbarrett]

/integrate

[openjdk]

Going to push as commit 2d0d057.

[openjdk]

@kimbarrett Pushed as commit 2d0d057.

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