Optimize binary search in MutableSparseIntSet.add

[liblit]

Replace the linear scan used to find the insertion point in MutableSparseIntSet.add with a binary search via the new IntSetUtil.binarySearchInsertionPoint helper, which delegates to java.util.Arrays.binarySearch.

This operation is the inner loop of points-to-set construction, so the change yields large gains on IntSet-heavy workloads:

• SlicerBenchmark.computeBackwardSliceInTestList: ~19% faster
• DataflowBenchmark.analyzeTestContextSensitive: ~11% faster
• DataflowBenchmark.analyzeTestContextInsensitive: ~8% faster

No measurable regression was observed on CallGraphBenchmark.

[codecov]

Codecov Report

❌ Patch coverage is 38.46154% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 50.81%. Comparing base (8a541c9) to head (24685fa).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
...main/java/com/ibm/wala/util/intset/IntSetUtil.java	33.33%	4 Missing and 4 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #2052      +/-   ##
============================================
- Coverage     50.81%   50.81%   -0.01%     
+ Complexity    12761    12760       -1     
============================================
  Files          1368     1368              
  Lines         83499    83500       +1     
  Branches      14414    14414              
============================================
- Hits          42431    42428       -3     
- Misses        36385    36387       +2     
- Partials       4683     4685       +2     

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[msridhar]

Looks good! I'm curious, are the performance improvement numbers given here in addition to those from #2051? Or were these numbers in comparison to the master branch without #2051?

[msridhar]

I'm also curious about the reduction in code coverage for IntSetUtil, though sadly codecov is not able to show me the differences right now...

[liblit]

are the performance improvement numbers given here in addition to those from #2051? Or were these numbers in comparison to the master branch without #2051?

Astute question! The latter, and that's a problem. I expected that this PR would show even better improvements when applied to the larger arrays introduced by #2051. However, I'm seeing evidence that the opposite is true. Please hold off on approving this PR until I can do a more thorough investigation. We may discover that each change is an improvement by itself, but not when taken together.