[analyzer] Implemented RangeSet::Factory::unite function to handle intersections and adjacency

[ASDenysPetrov]

Summary

Handle intersected and adjacent ranges uniting them into a single one.
Example:
intersection [0, 10] U [5, 20] = [0, 20]
adjacency [0, 10] U [11, 20] = [0, 20]

[vsavchenko]

Thanks for working on improvements of the solver and constraints! However, I have some tough questions about this patch.

What I really want to understand here is motivation. Why do we need to have add operation semantics like this in the first place? My guess is that "the user" will be in the following patch.
Additionally, I don't really like the idea of replacing something simple and fast (old add methods`) with something more complex unconditionally. Old users still don't need this additional logic. C++ has always been the language where we "pay for what we use".
So, with good motivation, I'd still prefer methods like add and addUnchecked or smith similar.

clang/include/clang/StaticAnalyzer/Core/PathSensitive/RangedConstraintManager.h

128

This most certainly can be done in O(N + M) the same way the intersection is done.

[ASDenysPetrov]

In D99797#2666358, @vsavchenko wrote:

Thanks for working on improvements of the solver and constraints! However, I have some tough questions about this patch.

What I really want to understand here is motivation. Why do we need to have add operation semantics like this in the first place? My guess is that "the user" will be in the following patch.
Additionally, I don't really like the idea of replacing something simple and fast (old add methods`) with something more complex unconditionally. Old users still don't need this additional logic. C++ has always been the language where we "pay for what we use".
So, with good motivation, I'd still prefer methods like add and addUnchecked or smith similar.

My motivation is that I'm currently working on some bigger improvement (symbolic integral cast) and stucked here of the lack of handling intersections. Would you mind of accepting this revision in case if I restore complexity to O(N+M)?

Or. We always can have both methods. The quick add and caring add. Actually you've said that :)

clang/include/clang/StaticAnalyzer/Core/PathSensitive/RangedConstraintManager.h

128

Actually yes, it can. And it was, when I played with different solutions. But the code was poor readable. So I decided make it easier to understand to bring to review. Of couse I can move further to improve it and retain readability. I'll do.

[ASDenysPetrov]

@vsavchenko
OK, what do you think of *adjacency* feature? I mean it simplifies such ranges [1,2][3,4][5,6] to [1,6]. Is it worth for implementation?

[ASDenysPetrov]

Updated. Restored complexity to O(N).

[ASDenysPetrov]

@vsavchenko FYI.

clang/lib/StaticAnalyzer/Core/RangeConstraintManager.cpp

114–115

Also optimized this particular case.

131–132

This allows to add a RangeSet of any type. E.g. RangeSet(uchar) + RangeSet(int) = valid, because of pin

I'm wondering whether we really need it here in practice?

clang/unittests/StaticAnalyzer/RangeSetTest.cpp

172

Fixed the misprint.

[vsavchenko]

In D99797#2666565, @ASDenysPetrov wrote:

@vsavchenko
OK, what do you think of *adjacency* feature? I mean it simplifies such ranges [1,2][3,4][5,6] to [1,6]. Is it worth for implementation?

I want to clarify my position here. It's not that I am opposed to this change in principle, but I want to understand the motivation (a small example will be sufficient) and I don't want to sacrifice a single bit of performance efficiency of this part of code.
Even for the O(N + M) solution, I'd still be standing strong on keeping the old functions as is (except for maybe renaming them). Range sets are small and asymptotics don't work that well when reasoning about the expected performance benefits and gains.
For this reason, whenever we can, we should have the simplest operation possible in terms of the number of instructions, ie the constant factor is very strong here.

clang/include/clang/StaticAnalyzer/Core/PathSensitive/RangedConstraintManager.h

128

Bring it here and we'll see what can be done on that front. *makeover time!*

146

nit: O(N + log(N)) == O(N)

[vsavchenko]

clang/lib/StaticAnalyzer/Core/RangeConstraintManager.cpp

131–132

We mix ranges of different types way more than one would expect.

[vsavchenko]

clang/lib/StaticAnalyzer/Core/RangeConstraintManager.cpp

116–117

This is REAL bad. The main benefit of the new RangeSet over the old one is the fact that common operations that consist of many basic operations are done on mutable containers, i.e. when RHS has 10 elements this code will create and copy a new array 10 times discarding 9 of them.

That's why every implementation method here operates on mutable ContainerType and then makes is persistent.

Additionally, merging add can be done with one iteration over two containers, but instead we do O(N) operation here M times, so it is not O(N + M), but O(N * M).

135

Is there a reason not to use range-based loop in this case?

143

This should be done outside of the loop, we assume that all the ranges are of the same type.

165

This is a problem here. This essentially doubles the work you did before. What can be done in one O(N) loop is done with two.

However, I don't really see a point in fixing this algorithm because the more generic RangeSet + RangeSet should be optimal O(N + M) and this one can be implemented as a special case.

[ASDenysPetrov]

@vsavchenko Many thanks for your feedback!
I will make a new separate function for checking intersections considering all your suggestions along with the old quick add versions. It'll be more optimized.

clang/lib/StaticAnalyzer/Core/RangeConstraintManager.cpp

135

You are right. Working on restoring O(N) I been played with iterators, then found another solution, but forgot to return range-based loop back. Thnx.

143

+1 I'll move it outside the loop.

[ASDenysPetrov]

Updated. Implemented four separate functions RangeSet::Factory::unite. Each function has the most optimized approach to handle intersections for the particular case.

@vsavchenko You are welcome to evaluate this changes.

I want to understand the motivation (a small example will be sufficient)

I'm afraid a snippet out of context wouldn't make much sense for you. Let me present you the next patch soon.

[vsavchenko]

Well, that is a nice exercise for "two pointer" problems, but can we please talk about the actual use case for it?

clang/lib/StaticAnalyzer/Core/RangeConstraintManager.cpp

139

I'd prefer merge

220

This algorithm is indeed O(N +M), good job!
But let me get picky again 😅

It looks like we do too many comparisons in this loop. As I said earlier, constant factor is the key here and while this iterator idea is good it is a tradeoff. There is a piece of knowledge that would've been obvious with regular iterators, now is a run-time unknown that we constantly need to check (isTo and isFrom).

What we are trying to achieve here is not harder than what we do in intersect and there it is way less comparisons. While iterating through sets we can keep a set of invariants, so we don't need to re-check something that we already know.

Here is a sketch for the algorithm:

{
  assert(First != FirstEnd && Second != SecondEnd);
  // We have && here because one of them reaches its end,
  // we should not check for it again and simply add the rest
  // of the other set.
  while (true) {
    // We need to find the beginning of the next range to add
    // First should be the iterator which To is a candidate to be
    // an and for the merged range.
    if (First.From() > Second.From()) {
      swap();
    }

    auto From = First.From();

    // That's it, we found our start, and we need to go through
    // other ranges and look for the end.
    // After this point, First.From() shouldn'y be accessed.

    while (true) {
      // We can compress all the checks into just one.
      // This essentially means that Second should not get merged with First.
      if (Second.From() > First.To() + 1) {
        break;
      }

      if (Second.From() > First.From()) {
        // First is maintained as a candidate for the end.
        swap();
      }

      // At this point we know that Second lives fully inside
      // of the new range and we can skip it.
      ++Second;

      // If we have nothing else in the second set...
      if (Second == SecondEnd) {
        // ...let's finish the current range first...
        Result.emplace_back(From, First.To());
        while (++First != FirstEnd) {
          // ...and copy the rest of the ranges
          Result.push_back(*First);
        }
        // The range is ready.
        return makePersistent(Result);
      }
    };

    // Second is outside of the range, and we can
    // safely add a new range.
    Result.emplace_back(From, First.To());
    ++First;

    // First set can be over at this point and we should...
    if (First == FirstEnd) {
      // ...copy the rest if the second set's ranges.
      while (Second != SecondEnd) {
        Result.push_back(*(Second++));
      }
      // Nothing left to do.
      return makePersistent(Result);
    }
  };

  // No way for us to get here!
  llvm_unreachable("...");
}

It's trickier in the way we end things than the intersection because we still need to add the rest of the other set.

282–296

I don't see any practical reasons to keep this version over the more generic RangeSet + RangeSet

324–338

Same here, it is just way more code to maintain.

[vsavchenko]

clang/unittests/StaticAnalyzer/RangeSetTest.cpp

474

I guess I also want to have more cases where ranges from RHS are in between ranges from LHS, also it should be a case with LHS being a set of ranges.

500

Also I'd like to see cases when there are ranges to merge from two sets but then one set has a bunch of other ranges that should be added as is. We can automatically think of two possibilities here: when these additional ranges are before and after the common part.
And I guess I want at least one check with two sets with a good amount of ranges in both covering all possible situations and overlappings.

[ASDenysPetrov]

@vsavchenko
Thank you for the proposed solution. It looks much easier to understand and maintain. Great! I will take it into account.

Well, that is a nice exercise for "two pointer" problems, but can we please talk about the actual use case for it?

I'm currently working on integral cast between ranges. Consider the range of int which is casted to char. You've got some ranges of int which obviously should be corespondently represented as some other ranges of char.
Some examples:

int [257, 259]  -> char [1, 3]
int [510, 513]  -> char [-2, 1]
int [42, 1000]  -> char [-128, 127]
int [257, 259] U [2049, 2051]  -> char [1,3] // Here we need `unite` logic to get the casted range because both original ranges lay on the same area after trancation.

[ASDenysPetrov]

Updated the patch due to comments. Added more tests. Simplified and improved solution.

clang/lib/StaticAnalyzer/Core/RangeConstraintManager.cpp

139

There are common namings of operations with sets and union is one of them. IMO this is most appropriate one.

[ASDenysPetrov]

Minor comment fix.

[ASDenysPetrov]

Minor performance improvement. Add more comments.

[vsavchenko]

clang/include/clang/StaticAnalyzer/Core/PathSensitive/RangedConstraintManager.h

148

LHS

251

nit: "...on the fact that..."

254

ContainerType is basically a mutable version of RangeSet, so there is only one reason to return it - you believe that the users might want to modify it after they called this unite. But as long as this unite is just a generalized version of user-facing unites, it can totally return RangeSet`.

clang/lib/StaticAnalyzer/Core/RangeConstraintManager.cpp

114

Let's reserve some place here. Because LHS and RHS don't have intersections, the result always has size(LHS) + size(RHS) elements

224

Oof, I don't know about this algorithm. I mean it does its job. But IMO it lacks a good description of what are the invariants and what are the different situations we are looking for.
Aaaand you kind of re-check certain conditions multiple times. One example here is the check for Min and Max. Those situations are super rare, but we check for them on every single iteration. std::min and std::max are additional comparisons. As I mentioned before, constant factor is the key here and less comparisons we do is way more important than doing binary search at some point.
Just make a benchmark if you don't believe me (with google-benchmark, for example). The version with less comparisons will dominate one with more on RangeSet under 20 (and they'll be even smaller in practice).

[ASDenysPetrov]

@vsavchenko Thanka for the suggestions! I'll take them into account and update the patch.

clang/include/clang/StaticAnalyzer/Core/PathSensitive/RangedConstraintManager.h

254

I'm going to use raw ContainerType in further patches. So this is exactly what I want.

clang/lib/StaticAnalyzer/Core/RangeConstraintManager.cpp

224

I'll investigate the whole algorithm once more and reduce comparisons.

[vsavchenko]

clang/include/clang/StaticAnalyzer/Core/PathSensitive/RangedConstraintManager.h

254

Oh, I see. OK then :)

[ASDenysPetrov]

Reworked the solution. Returned to Implemented two versions of the same algorithm. Most optimized (but more verbose) and generalized one (but less optimized).
Added a bit more tests. @vsavchenko , please, look.

[ASDenysPetrov]

Minor improvements in unit tests.

[ASDenysPetrov]

More minor improvements in unit tests.

[vsavchenko]

To be honest, I don't think that it solves the problem I mentioned before. The fact that conditions and branching are part of operator++ now, doesn't cancel them. I noticed that you made the first loop, so we don't need to check for Min in the main loop, and this is the right direction. But this approach has the same problem as one of the previous methods - you check isFrom, this is the price of abstraction. This is something that we won't be doing when working outside of such abstractions. Also, when one of the range sets is shorter, you will keep checking if the other iterator is end on every iteration when you run out of the ranges in shorter set.

I'm sorry for being so hard on you about this patch. 😔

clang/lib/StaticAnalyzer/Core/RangeConstraintManager.cpp

259

nit: it has different ticks than you use in all other places

[ASDenysPetrov]

@vsavchenko
Thanks for your suggestions! I really appreciate it! I'll do my best on this algorithm.
BTW, I've just presented the motivation you looked D103094 and D103096.

clang/lib/StaticAnalyzer/Core/RangeConstraintManager.cpp

259

I'm sorry. I didn't get it. What do you mean?

[vsavchenko]

clang/lib/StaticAnalyzer/Core/RangeConstraintManager.cpp

259

This is a super duper tiny thing: you usually write ) or (, but here wrote '('.

[ASDenysPetrov]

clang/lib/StaticAnalyzer/Core/RangeConstraintManager.cpp

259

Oh! :-) Sharp eye!

[ASDenysPetrov]

@vsavchenko
Reworked the algorithm.

I hope this is a final version. Honestly, I also have the most optimized version but it has twice more similar(but different) code and gotos. I decided not to present it. Let it be less micro-optimized but much readable version.

[ASDenysPetrov]

Fixed the issue. Added more unit tests.

[vsavchenko]

I think this iteration is much better, it requires way more description as it has now. You didn't actually describe anywhere how this algorithm actually works.

clang/lib/StaticAnalyzer/Core/RangeConstraintManager.cpp

212

nit: "is adjacent"

[vsavchenko]

clang/lib/StaticAnalyzer/Core/RangeConstraintManager.cpp

202–203

At this point, we know for a fact that the next range we are going to add to our result set starts with I1->From(). No need to check F for null or anything

214

Why break? At this point we know that what we peaked into is actually a part of another range (in terms of the end result).
And this range is over, you just need to add it to the final result!

216–220

You actually don't need it, the moment you swap two iterators and find out that I1->From() <= I2->From(), that's it I1->From() is the beginning of the new range. No need to carry extra flag here.

229

nit
Additionally, you didn't tell your readers WHY you are even swapping iterators in the first place and what relationship they have. You need to communicate in terms of invariants.

246

This goto is always finishing with a return, so we can refactor goto end1 into something like return copyIntoResult(I1, E1); and goto end2 into return copyIntoResult(I2, E2);.
As I mentioned before, optional F should be removed. On every path we should know deterministically what is the beginning and what is the end of the range that we are trying to add.

[ASDenysPetrov]

Updated. Removed F as flag. Replaced goto with closure. Detailed comments and fixed typos.

@vsavchenko made changes according your suggestions.

[ASDenysPetrov]

Rebased

[ASDenysPetrov]

How about this patch and the entire stack?

[ASDenysPetrov]

Rebased. Review, please.

[ASDenysPetrov]

Gentle ping.

[martong]

@ASDenysPetrov Nice work! I really appreciate the hard work you guys (with @vsavchenko) had done here. I really like that you have created visible test cases (though the last ones are a bit cryptic for me). It is going to take some more time to finish my review.

clang/lib/StaticAnalyzer/Core/RangeConstraintManager.cpp

179

This is called

[steakhal]

I still need to chew through the code but on a high level, I think it looks correct.
PS: the test coverage is outstanding!

unite-patch-line-coverage.zip51 KBDownload

clang/lib/StaticAnalyzer/Core/RangeConstraintManager.cpp

149

Why do you take APSInts by value? Generally, we take them by reference.

clang/unittests/StaticAnalyzer/RangeSetTest.cpp

81–84

Shouldn't you use sizeof(BaseType) * CHAR_BIT instead?

[ASDenysPetrov]

In D99797#3059203, @steakhal wrote:

PS: the test coverage is outstanding!

Thank you for this analysis.

clang/lib/StaticAnalyzer/Core/RangeConstraintManager.cpp

149

I want to send a message to the caller that he can pass an arbitrary APSInt without warrying about making it permanent (aka stored by the Factory). But we can revise this contract and carry this responsibility to a caller.

clang/unittests/StaticAnalyzer/RangeSetTest.cpp

81–84

Agree. It's better to avoid magic numbers. I'll fix.

[martong]

clang/lib/StaticAnalyzer/Core/RangeConstraintManager.cpp

149

Why do you take APSInts by value? Generally, we take them by reference.

Actually, it is specific to BasicValueFactory to cache the APSInts, however, it might not be the best practice. I doubt that somebody has ever measured the performance of passing APSInts by value, so my guess is the caching of APSInts might be an early optimization that might be more harmful than advantageous. On top of all this, we do the caching inconsistently, just consider the member functions of APSIntType, they all return by value.

Perhaps (totally independently from this patch of course), it might be worth to have a measurement/comparison with removed cache and pass by value.

[steakhal]

clang/lib/StaticAnalyzer/Core/RangeConstraintManager.cpp

149

Okay, then we shall leave this as-is now.

clang/unittests/StaticAnalyzer/RangeSetTest.cpp

81–84

It's not only that but just imagine testing a clang on a special hardware where they have let's say 9 bit bytes, for parity or something similar stuff.
The test would suddenly break.
Although I suspect there would be many more things to break TBH xD

[martong]

I think, the visual comments that we have in intersect makes the code of intersect a lot easier to follow. Could you please add similar visual comments here? Also, I find First and Second in intersect way more better naming than I1 and I2, could you please update?

clang/lib/StaticAnalyzer/Core/RangeConstraintManager.cpp

169

I think, the naming conventions we have in intersect are easier to follow. Could we call I1 as First and I2 as Second?

181

188

It is not clear what is the complicated condition that you'd like to avoid. Could you please elaborate?

189–201

I'd like to see similar visual comments that we have in intersect.

204–205

Let's reuse as much as possible from intersect, both code and comments.

207–208

We could reuse SwapIterators from intersect. Of course for that we need to make a real function out of the lambda.

211

212

Let's be consistent with intersect. Also, you could introduce the variable here, and it is not needed to declare that at L176.

214

242

[ASDenysPetrov]

@martong
Thanks for your inlines. I'll update the patch.

clang/unittests/StaticAnalyzer/RangeSetTest.cpp

81–84

I am always skeptical about using`CHAR_BIT`, beacuse it represents bit number in char. And what if it would be 16 for instance (aka 2 bytes). But my intention is to get an amount of bits for a particular type. And I want something to represent a number of bits in a byte as a fundamental unit, but not something that depends on a char size on a particular platform.
I would better introduce something like constexpr size_t BITS_IN_BYTE = 8;.

[ASDenysPetrov]

@martong @steakhal
Updated according to your comments. Thank you!

[steakhal]

clang/lib/StaticAnalyzer/Core/RangeConstraintManager.cpp

200–204

Might be First->To() == Second->To(). In this case the comment is not completely accurate.

clang/unittests/StaticAnalyzer/RangeSetTest.cpp

81–84

basic.memobj 6.7.1/22:

The number of bits in a byte is reported by the macro CHAR_­BIT in the header <climits>.

[ASDenysPetrov]

clang/lib/StaticAnalyzer/Core/RangeConstraintManager.cpp

200–204

I'll update.

clang/unittests/StaticAnalyzer/RangeSetTest.cpp

81–84

These legacy names... :) I'll update.

[ASDenysPetrov]

Fixed nits.

[ASDenysPetrov]

clang/unittests/StaticAnalyzer/RangeSetTest.cpp

81–84

BTW, the first time this definition appeared only in C++17 in a footnote. The committee is not yet confident enough about CHAR_BIT that it still resides there :)

[martong]

Thanks Denys for the update! Very good! However, I think maybe we could make the code a bit more simpler.

clang/lib/StaticAnalyzer/Core/RangeConstraintManager.cpp

229–233

I am not sure about this, but perhaps we could put this swapIterators call right at the beginning of the nested while loop (L243). That would eliminate the need to call it again at the end of the second while loop.

243

So, this loop is about to merge conjunct Ranges. The first time when we find disjoint Ranges then we break out. (Or we return once we reach the end of any of the RangeSets.)
This makes we wonder, if it would be possible to split this while loop into a lambda named mergeConjunctRanges ?

290–292

Should this be like in the code suggestion? You incremented First at L276.

293

?

[martong]

Gentle ping.

[ASDenysPetrov]

clang/lib/StaticAnalyzer/Core/RangeConstraintManager.cpp

229–233

I'm afraid, it is not the case. Every loop needs its own swapIterators. As you can see, swapIterators in the nested loop invokes not always because of break; in the middle. Otherwise, it would invokes each time. But I checked your suggestion. It doesn't work.

243

I'm not in favor of introducing another level of abstraction. It would divide the flow/comments and could reduce readability and performance which is crucial here.

[ASDenysPetrov]

Fixed comments.

[martong]

Thanks for the update, I am okay with the .cpp file, now I continue the review with the tests.

[martong]

Nice, assiduous work! The tests are awesome!
LGTM, with minor revisions. Please check out my suggestions about the tests' formatting and there are those disturbing (LHS, RHS) swaps in the comments.

I am going to continue with the next patch in the stack. I recognize that the handling of casts is very important and problems related to not handling them occurs even more frequently as other parts of the engine evolve (e.g. https://reviews.llvm.org/D113753#3167134)

clang/unittests/StaticAnalyzer/RangeSetTest.cpp

441

449

I think, either we should use {{X, X}} or X everywhere, but not mixed.

525–526

LHS and RHS is swapped here?

528–529

I think we could better format these more complex cases.

533–534

LHS and RHS is swapped?

536–537

541–542

LHS and RHS is swapped?

558–559

LHS and RHS is swapped here as well.

582

590–592

590–592

What do you think about this format? The result can be easily verified this way I think, but a bit ugly ...

[ASDenysPetrov]

@martong

Nice, assiduous work!

Many thanks for your time! Your work is not less assiduous!

(LHS, RHS) swaps

it doesn't really matter, as the operation is comutative, but, yes, it does matter to depict the particular test case. I'll revise twise LHS-RHS's.

I am going to continue with the next patch in the stack. I recognize that the handling of casts is very important and problems related to not handling them occurs even more frequently as other parts of the engine evolve (e.g. https://reviews.llvm.org/D113753#3167134)

Aha. I saw you patch. I'll join to review it.

clang/unittests/StaticAnalyzer/RangeSetTest.cpp

441

There are three overloads of checkUnite which correspond to three overloads of RangeSet::unite. I made it consistent to other check-functions (add e.g.).

449

This tests different oveloaded versions of RangeSet::unite.

528–529

clang-fromat acts on its own. But I agree, it looks the way better. I'll consider wrapping it into // clang-format on/off directives.

590–592

I think ASCII art does this job. Let code look as code :)

[ASDenysPetrov]

Fixed code formatting in the unit test file according to remarks. Ready to load.

clang/include/clang/StaticAnalyzer/Core/PathSensitive/RangedConstraintManager.h

public:
 
   class Factory {
   public:
     Factory(BasicValueFactory &BV) : ValueFactory(BV) {}
 
     /// Create a new set with all ranges from both LHS and RHS.
     /// Possible intersections are not checked here.
     ///
     /// Complexity: O(N + M)
     ///             where N = size(LHS), M = size(RHS)
     RangeSet add(RangeSet LHS, RangeSet RHS);
     /// Create a new set with all ranges from the original set plus the new one.
     /// Possible intersections are not checked here.
     ///
     /// Complexity: O(N)
     ///             where N = size(Original)
     RangeSet add(RangeSet Original, Range Element);
     /// Create a new set with all ranges from the original set plus the point.
     /// Possible intersections are not checked here.
     ///
     /// Complexity: O(N)
     ///             where N = size(Original)
     RangeSet add(RangeSet Original, const llvm::APSInt &Point);
+    /// Create a new set which is a union of two given ranges.
+    /// Possible intersections are not checked here.
+    ///
+    /// Complexity: O(N + M)
+    ///             where N = size(LHS), M = size(RHS)
+    RangeSet unite(RangeSet LHS, RangeSet RHS);
+    /// Create a new set by uniting given range set with the given range.
+    /// All intersections and adjacent ranges are handled here.
+    ///
+    /// Complexity: O(N)
+    ///             where N = size(Original)
+    RangeSet unite(RangeSet Original, Range Element);
+    /// Create a new set by uniting given range set with the given point.
+    /// All intersections and adjacent ranges are handled here.
+    ///
+    /// Complexity: O(N)
+    ///             where N = size(Original)
+    RangeSet unite(RangeSet Original, llvm::APSInt Point);
+    /// Create a new set by uniting given range set with the given range
+    /// between points. All intersections and adjacent ranges are handled here.
+    ///
+    /// Complexity: O(N)
+    ///             where N = size(Original)
+    RangeSet unite(RangeSet Original, llvm::APSInt From, llvm::APSInt To);
 
     RangeSet getEmptySet() { return &EmptySet; }
 
     /// Create a new set with just one range.
     /// @{
     RangeSet getRangeSet(Range Origin);
     RangeSet getRangeSet(const llvm::APSInt &From, const llvm::APSInt &To) {
       return getRangeSet(Range(From, To));
public:
 
   private:
     /// Return a persistent version of the given container.
     RangeSet makePersistent(ContainerType &&From);
     /// Construct a new persistent version of the given container.
     ContainerType *construct(ContainerType &&From);
 
     RangeSet intersect(const ContainerType &LHS, const ContainerType &RHS);
+    /// NOTE: This function relies on the fact that all values in the
+    /// containers are persistent (created via BasicValueFactory::getValue).
+    ContainerType unite(const ContainerType &LHS, const ContainerType &RHS);
 
     // Many operations include producing new APSInt values and that's why
     // we need this factory.
     BasicValueFactory &ValueFactory;
     // Allocator for all the created containers.
     // Containers might own their own memory and that's why it is specific
     // for the type, so it calls container destructors upon deletion.
     llvm::SpecificBumpPtrAllocator<ContainerType> Arena;
     // Usually we deal with the same ranges and range sets over and over.

clang/lib/StaticAnalyzer/Core/RangeConstraintManager.cpp

 };
 
 //===----------------------------------------------------------------------===//
 //                           RangeSet implementation
 //===----------------------------------------------------------------------===//
 
 RangeSet::ContainerType RangeSet::Factory::EmptySet{};
 
+RangeSet RangeSet::Factory::add(RangeSet LHS, RangeSet RHS) {
+  ContainerType Result;
+  Result.reserve(LHS.size() + RHS.size());
+  std::merge(LHS.begin(), LHS.end(), RHS.begin(), RHS.end(),
+             std::back_inserter(Result));
+  return makePersistent(std::move(Result));
+}
+
 RangeSet RangeSet::Factory::add(RangeSet Original, Range Element) {
   ContainerType Result;
   Result.reserve(Original.size() + 1);
 
   const_iterator Lower = llvm::lower_bound(Original, Element);
   Result.insert(Result.end(), Original.begin(), Lower);
   Result.push_back(Element);
   Result.insert(Result.end(), Lower, Original.end());
 
   return makePersistent(std::move(Result));
 }
 
 RangeSet RangeSet::Factory::add(RangeSet Original, const llvm::APSInt &Point) {
   return add(Original, Range(Point));
 }
 
+RangeSet RangeSet::Factory::unite(RangeSet LHS, RangeSet RHS) {
+  ContainerType Result = unite(*LHS.Impl, *RHS.Impl);
+  return makePersistent(std::move(Result));
+}
+
+RangeSet RangeSet::Factory::unite(RangeSet Original, Range R) {
+  ContainerType Result;
+  Result.push_back(R);
+  Result = unite(*Original.Impl, Result);
+  return makePersistent(std::move(Result));
+}
+
+RangeSet RangeSet::Factory::unite(RangeSet Original, llvm::APSInt Point) {
+  return unite(Original, Range(ValueFactory.getValue(Point)));
+}
+
+RangeSet RangeSet::Factory::unite(RangeSet Original, llvm::APSInt From,
+                                  llvm::APSInt To) {
+  return unite(Original,
+               Range(ValueFactory.getValue(From), ValueFactory.getValue(To)));
+}
+
+template <typename T>
+void swapIterators(T &First, T &FirstEnd, T &Second, T &SecondEnd) {
+  std::swap(First, Second);
+  std::swap(FirstEnd, SecondEnd);
+}
+
+RangeSet::ContainerType RangeSet::Factory::unite(const ContainerType &LHS,
+                                                 const ContainerType &RHS) {
+  if (LHS.empty())
+    return RHS;
+  if (RHS.empty())
+    return LHS;
+
+  using llvm::APSInt;
+  using iterator = ContainerType::const_iterator;
+
+  iterator First = LHS.begin();
+  iterator FirstEnd = LHS.end();
+  iterator Second = RHS.begin();
+  iterator SecondEnd = RHS.end();
+  APSIntType Ty = APSIntType(First->From());
+  const APSInt Min = Ty.getMinValue();
+
+  // Handle a corner case first when both range sets start from MIN.
+  // This helps to avoid complicated conditions below. Specifically, this
+  // particular check for `MIN` is not needed in the loop below every time
+  // when we do `Second->From() - One` operation.
+  if (Min == First->From() && Min == Second->From()) {
+    if (First->To() > Second->To()) {
+      //    [ First    ]--->
+      //    [ Second ]----->
+      // MIN^
+      // The Second range is entirely inside the First one.
+
+      // Check if Second is the last in its RangeSet.
+      if (++Second == SecondEnd)
+        //    [ First     ]--[ First + 1 ]--->
+        //    [ Second ]--------------------->
+        // MIN^
+        // The Union is equal to First's RangeSet.
+        return LHS;
+    } else {
+      // case 1: [ First ]----->
+      // case 2: [ First   ]--->
+      //         [ Second  ]--->
+      //      MIN^
+      // The First range is entirely inside or equal to the Second one.
+
+      // Check if First is the last in its RangeSet.
+      if (++First == FirstEnd)
+        //    [ First ]----------------------->
+        //    [ Second  ]--[ Second + 1 ]---->
+        // MIN^
+        // The Union is equal to Second's RangeSet.
+        return RHS;
+    }
+  }
+
+  const APSInt One = Ty.getValue(1);
+  ContainerType Result;
+
+  // This is called when there are no ranges left in one of the ranges.
+  // Append the rest of the ranges from another range set to the Result
+  // and return with that.
+  const auto AppendTheRest = [&Result](iterator I, iterator E) {
+    Result.append(I, E);
+    return Result;
+  };
+
+  while (true) {
+    // We want to keep the following invariant at all times:
+    // ---[ First ------>
+    // -----[ Second --->
+    if (First->From() > Second->From())
+      swapIterators(First, FirstEnd, Second, SecondEnd);
+
+    // The Union definitely starts with First->From().
+    // ----------[ First ------>
+    // ------------[ Second --->
+    // ----------[ Union ------>
+    // UnionStart^
+    const llvm::APSInt &UnionStart = First->From();
+
+    // Loop where the invariant holds.
+    while (true) {
+      // Skip all enclosed ranges.
+      // ---[                  First                     ]--->
+      // -----[ Second ]--[ Second + 1 ]--[ Second + N ]----->
+      while (First->To() >= Second->To()) {
+        // Check if Second is the last in its RangeSet.
+        if (++Second == SecondEnd) {
+          // Append the Union.
+          // ---[ Union      ]--->
+          // -----[ Second ]----->
+          // --------[ First ]--->
+          //         UnionEnd^
+          Result.emplace_back(UnionStart, First->To());
+          // ---[ Union ]----------------->
+          // --------------[ First + 1]--->
+          // Append all remaining ranges from the First's RangeSet.
+          return AppendTheRest(++First, FirstEnd);
+        }
+      }
+
+      // Check if First and Second are disjoint. It means that we find
+      // the end of the Union. Exit the loop and append the Union.
+      // ---[ First ]=------------->
+      // ------------=[ Second ]--->
+      // ----MinusOne^
+      if (First->To() < Second->From() - One)
+        break;
+
+      // First is entirely inside the Union. Go next.
+      // ---[ Union ----------->
+      // ---- [ First ]-------->
+      // -------[ Second ]----->
+      // Check if First is the last in its RangeSet.
+      if (++First == FirstEnd) {
+        // Append the Union.
+        // ---[ Union       ]--->
+        // -----[ First ]------->
+        // --------[ Second ]--->
+        //          UnionEnd^
+        Result.emplace_back(UnionStart, Second->To());
+        // ---[ Union ]------------------>
+        // --------------[ Second + 1]--->
+        // Append all remaining ranges from the Second's RangeSet.
+        return AppendTheRest(++Second, SecondEnd);
+      }
+
+      // We know that we are at one of the two cases:
+      // case 1: --[ First ]--------->
+      // case 2: ----[ First ]------->
+      // --------[ Second ]---------->
+      // In both cases First starts after Second->From().
+      // Make sure that the loop invariant holds.
+      swapIterators(First, FirstEnd, Second, SecondEnd);
+    }
+
+    // Here First and Second are disjoint.
+    // Append the Union.
+    // ---[ Union    ]--------------->
+    // -----------------[ Second ]--->
+    // ------[ First ]--------------->
+    //       UnionEnd^
+    Result.emplace_back(UnionStart, First->To());
+
+    // Check if First is the last in its RangeSet.
+    if (++First == FirstEnd)
+      // ---[ Union ]--------------->
+      // --------------[ Second ]--->
+      // Append all remaining ranges from the Second's RangeSet.
+      return AppendTheRest(Second, SecondEnd);
+  }
+
+  llvm_unreachable("Normally, we should not reach here");
+}
+
 RangeSet RangeSet::Factory::getRangeSet(Range From) {
   ContainerType Result;
   Result.push_back(From);
   return makePersistent(std::move(Result));
 }
 
 RangeSet RangeSet::Factory::makePersistent(ContainerType &&From) {
   llvm::FoldingSetNodeID ID;
   void *InsertPos;
 
   From.Profile(ID);
   ContainerType *Result = Cache.FindNodeOrInsertPos(ID, InsertPos);
 
   if (!Result) {
     // It is cheaper to fully construct the resulting range on stack
     // and move it to the freshly allocated buffer if we don't have
     // a set like this already.
     Result = construct(std::move(From));
     Cache.InsertNode(Result, InsertPos);
   }
 
   return Result;
 }
 
 RangeSet::ContainerType *RangeSet::Factory::construct(ContainerType &&From) {
   void *Buffer = Arena.Allocate();
   return new (Buffer) ContainerType(std::move(From));
 }
 
-RangeSet RangeSet::Factory::add(RangeSet LHS, RangeSet RHS) {
-  ContainerType Result;
-  std::merge(LHS.begin(), LHS.end(), RHS.begin(), RHS.end(),
-             std::back_inserter(Result));
-  return makePersistent(std::move(Result));
-}
-
 const llvm::APSInt &RangeSet::getMinValue() const {
   assert(!isEmpty());
   return begin()->From();
 }
 
 const llvm::APSInt &RangeSet::getMaxValue() const {
   assert(!isEmpty());
   return std::prev(end())->To();

 RangeSet RangeSet::Factory::intersect(const RangeSet::ContainerType &LHS,
                                       const RangeSet::ContainerType &RHS) {
   ContainerType Result;
   Result.reserve(std::max(LHS.size(), RHS.size()));
 
   const_iterator First = LHS.begin(), Second = RHS.begin(),
                  FirstEnd = LHS.end(), SecondEnd = RHS.end();
 
-  const auto SwapIterators = [&First, &FirstEnd, &Second, &SecondEnd]() {
-    std::swap(First, Second);
-    std::swap(FirstEnd, SecondEnd);
-  };
-
   // If we ran out of ranges in one set, but not in the other,
   // it means that those elements are definitely not in the
   // intersection.
   while (First != FirstEnd && Second != SecondEnd) {
     // We want to keep the following invariant at all times:
     //
     //    ----[ First ---------------------->
     //    --------[ Second ----------------->
     if (Second->From() < First->From())
-      SwapIterators();
+      swapIterators(First, FirstEnd, Second, SecondEnd);
 
     // Loop where the invariant holds:
     do {
       // Check for the following situation:
       //
       //    ----[ First ]--------------------->
       //    ---------------[ Second ]--------->
       //
do {
       const llvm::APSInt &IntersectionStart = Second->From();
 
       // It is important to know which of the two ranges' ends
       // is greater.  That "longer" range might have some other
       // intersections, while the "shorter" range might not.
       if (Second->To() > First->To()) {
         // Here we make a decision to keep First as the "longer"
         // range.
-        SwapIterators();
+        swapIterators(First, FirstEnd, Second, SecondEnd);
       }
 
       // At this point, we have the following situation:
       //
       //    ---- First      ]-------------------->
       //    ---- Second ]--[  Second+1 ---------->
       //
       // We don't know the relationship between First->From and

clang/unittests/StaticAnalyzer/RangeSetTest.cpp

 LLVM_ATTRIBUTE_UNUSED static std::string toString(const llvm::APSInt &Point) {
   return toString(Point, 10);
 }
 // We need it here for better fail diagnostics from gtest.
 LLVM_ATTRIBUTE_UNUSED static std::ostream &operator<<(std::ostream &OS,
                                                       const RangeSet &Set) {
   return OS << toString(Set);
 }
+// We need it here for better fail diagnostics from gtest.
+LLVM_ATTRIBUTE_UNUSED static std::ostream &operator<<(std::ostream &OS,
+                                                      const Range &R) {
+  return OS << toString(R);
+}
 
 } // namespace ento
 } // namespace clang
 
 namespace {
 
+template <typename T> struct TestValues {
+  static constexpr T MIN = std::numeric_limits<T>::min();
+  static constexpr T MAX = std::numeric_limits<T>::max();
+  // MID is a value in the middle of the range
+  // which unary minus does not affect on,
+  // e.g. int8/int32(0), uint8(128), uint32(2147483648).
+  static constexpr T MID =
+      std::is_signed<T>::value ? 0 : ~(static_cast<T>(-1) / static_cast<T>(2));
+  static constexpr T A = MID - (MAX - MID) / 3 * 2;
+  static constexpr T B = MID - (MAX - MID) / 3;
+  static constexpr T C = -B;
+  static constexpr T D = -A;
+
+  static_assert(MIN < A && A < B && B < MID && MID < C && C < D && D < MAX,
+                "Values shall be in an ascending order");
+};
+
 template <typename BaseType> class RangeSetTest : public testing::Test {
 public:
   // Init block
   std::unique_ptr<ASTUnit> AST = tooling::buildASTFromCode("struct foo;");
   ASTContext &Context = AST->getASTContext();
   llvm::BumpPtrAllocator Arena;
   BasicValueFactory BVF{Context, Arena};
   RangeSet::Factory F{BVF};
   // End init block
 
   using Self = RangeSetTest<BaseType>;
   using RawRange = std::pair<BaseType, BaseType>;
   using RawRangeSet = std::initializer_list<RawRange>;
 
-  static constexpr BaseType getMin() {
-    return std::numeric_limits<BaseType>::min();
-  }
-  static constexpr BaseType getMax() {
-    return std::numeric_limits<BaseType>::max();
-  }
-  static constexpr BaseType getMid() {
-    return isSigned() ? 0 : ~(fromInt(-1) / fromInt(2));
-  }
-
-  static constexpr bool isSigned() { return std::is_signed<BaseType>::value; }
-  static constexpr BaseType fromInt(int X) { return static_cast<BaseType>(X); }
-
-  static llvm::APSInt Base;
   const llvm::APSInt &from(BaseType X) {
-    llvm::APSInt Dummy = Base;
-    Dummy = X;
-    return BVF.getValue(Dummy);
+    static llvm::APSInt Base{sizeof(BaseType) * CHAR_BIT,
+                             std::is_unsigned<BaseType>::value};
+    Base = X;
+    return BVF.getValue(Base);
   }
 
   Range from(const RawRange &Init) {
     return Range(from(Init.first), from(Init.second));
   }
 
   RangeSet from(const RawRangeSet &Init) {
     RangeSet RangeSet = F.getEmptySet();
public:
   }
 
   void checkAdd(RawRangeSet RawLHS, RawRange RawRHS, RawRangeSet RawExpected) {
     wrap(&Self::checkAddImpl<Range>, RawLHS, RawRHS, RawExpected);
   }
 
   void checkAdd(RawRangeSet RawLHS, RawRangeSet RawRHS,
                 RawRangeSet RawExpected) {
-    wrap(&Self::checkAddImpl<RangeSet>, RawRHS, RawLHS, RawExpected);
+    wrap(&Self::checkAddImpl<RangeSet>, RawLHS, RawRHS, RawExpected);
   }
 
   void checkAdd(RawRangeSet RawLHS, BaseType RawRHS, RawRangeSet RawExpected) {
     wrap(&Self::checkAddImpl<const llvm::APSInt &>, RawLHS, RawRHS,
          RawExpected);
   }
 
+  template <class RHSType>
+  void checkUniteImpl(RangeSet LHS, RHSType RHS, RangeSet Expected) {
+    RangeSet Result = F.unite(LHS, RHS);
+    EXPECT_EQ(Result, Expected)
+        << "while uniting " << toString(LHS) << " and " << toString(RHS);
+  }
+
+  void checkUnite(RawRangeSet RawLHS, RawRange RawRHS,
+                  RawRangeSet RawExpected) {
+    wrap(&Self::checkUniteImpl<Range>, RawLHS, RawRHS, RawExpected);
+  }
+
+  void checkUnite(RawRangeSet RawLHS, RawRangeSet RawRHS,
+                  RawRangeSet RawExpected) {
+    wrap(&Self::checkUniteImpl<RangeSet>, RawLHS, RawRHS, RawExpected);
+  }
+
+  void checkUnite(RawRangeSet RawLHS, BaseType RawRHS,
+                  RawRangeSet RawExpected) {
+    wrap(&Self::checkUniteImpl<const llvm::APSInt &>, RawLHS, RawRHS,
+         RawExpected);
+  }
+
   void checkDeleteImpl(const llvm::APSInt &Point, RangeSet From,
                        RangeSet Expected) {
     RangeSet Result = F.deletePoint(From, Point);
     EXPECT_EQ(Result, Expected)
         << "while deleting " << toString(Point) << " from " << toString(From);
   }
 
   void checkDelete(BaseType Point, RawRangeSet RawFrom,
                    RawRangeSet RawExpected) {
     wrap(&Self::checkDeleteImpl, Point, RawFrom, RawExpected);
   }
 };
 
 } // namespace
 
-template <typename BaseType>
-llvm::APSInt RangeSetTest<BaseType>::Base{sizeof(BaseType) * 8, !isSigned()};
-
 using IntTypes = ::testing::Types<int8_t, uint8_t, int16_t, uint16_t, int32_t,
                                   uint32_t, int64_t, uint64_t>;
 TYPED_TEST_SUITE(RangeSetTest, IntTypes, );
 
 TYPED_TEST(RangeSetTest, RangeSetNegateTest) {
-  // Use next values of the range {MIN, A, B, MID, C, D, MAX}.
-
-  constexpr TypeParam MIN = TestFixture::getMin();
-  constexpr TypeParam MAX = TestFixture::getMax();
-  // MID is a value in the middle of the range
-  // which unary minus does not affect on,
-  // e.g. int8/int32(0), uint8(128), uint32(2147483648).
-  constexpr TypeParam MID = TestFixture::getMid();
-  constexpr TypeParam A = MID - TestFixture::fromInt(42 + 42);
-  constexpr TypeParam B = MID - TestFixture::fromInt(42);
-  constexpr TypeParam C = -B;
-  constexpr TypeParam D = -A;
-
-  static_assert(MIN < A && A < B && B < MID && MID < C && C < D && D < MAX,
-                "Values shall be in an ascending order");
+  using TV = TestValues<TypeParam>;
+  constexpr auto MIN = TV::MIN;
+  constexpr auto MAX = TV::MAX;
+  constexpr auto MID = TV::MID;
+  constexpr auto A = TV::A;
+  constexpr auto B = TV::B;
+  constexpr auto C = TV::C;
+  constexpr auto D = TV::D;
 
   this->checkNegate({{MIN, A}}, {{MIN, MIN}, {D, MAX}});
   this->checkNegate({{MIN, C}}, {{MIN, MIN}, {B, MAX}});
   this->checkNegate({{MIN, MID}}, {{MIN, MIN}, {MID, MAX}});
   this->checkNegate({{MIN, MAX}}, {{MIN, MAX}});
   this->checkNegate({{A, D}}, {{A, D}});
   this->checkNegate({{A, B}}, {{C, D}});
   this->checkNegate({{MIN, A}, {D, MAX}}, {{MIN, A}, {D, MAX}});
TYPED_TEST(RangeSetTest, RangeSetPointIntersectTest) {
   // Check that intersection with itself produces the same set.
   this->checkIntersect({{42, 42}}, 42, {{42, 42}});
   // Check more general cases.
   this->checkIntersect({{0, 10}, {20, 30}, {30, 40}, {50, 60}}, 42, {});
   this->checkIntersect({{0, 10}, {20, 30}, {30, 60}}, 42, {{42, 42}});
 }
 
 TYPED_TEST(RangeSetTest, RangeSetRangeIntersectTest) {
-  constexpr TypeParam MIN = TestFixture::getMin();
-  constexpr TypeParam MAX = TestFixture::getMax();
+  using TV = TestValues<TypeParam>;
+  constexpr auto MIN = TV::MIN;
+  constexpr auto MAX = TV::MAX;
 
   // Check that we can correctly intersect empty sets.
   this->checkIntersect({}, 10, 20, {});
   this->checkIntersect({}, 20, 10, {});
   // Check that intersection with itself produces the same set.
   this->checkIntersect({{10, 20}}, 10, 20, {{10, 20}});
   this->checkIntersect({{MIN, 10}, {20, MAX}}, 20, 10, {{MIN, 10}, {20, MAX}});
   // Check non-overlapping range intersections.
TYPED_TEST(RangeSetTest, RangeSetContainsTest) {
   this->checkContains({{5, 10}}, 10, true);
   //   * when the range has the point somewhere in the middle
   this->checkContains({{0, 25}}, 10, true);
   // Check similar cases, but with larger sets.
   this->checkContains({{0, 5}, {10, 10}, {15, 20}}, 10, true);
   this->checkContains({{0, 5}, {10, 12}, {15, 20}}, 10, true);
   this->checkContains({{0, 5}, {5, 7}, {8, 10}, {12, 41}}, 10, true);
 
-  constexpr TypeParam MIN = TestFixture::getMin();
-  constexpr TypeParam MAX = TestFixture::getMax();
-  constexpr TypeParam MID = TestFixture::getMid();
+  using TV = TestValues<TypeParam>;
+  constexpr auto MIN = TV::MIN;
+  constexpr auto MAX = TV::MAX;
+  constexpr auto MID = TV::MID;
+
   this->checkContains({{MIN, MAX}}, 0, true);
   this->checkContains({{MIN, MAX}}, MID, true);
   this->checkContains({{MIN, MAX}}, -10, true);
   this->checkContains({{MIN, MAX}}, 10, true);
 }
 
 TYPED_TEST(RangeSetTest, RangeSetAddTest) {
   // Check adding single points
TYPED_TEST(RangeSetTest, RangeSetAddTest) {
   this->checkAdd({{0, 5}, {30, 40}}, {{10, 20}}, {{0, 5}, {10, 20}, {30, 40}});
   this->checkAdd({{0, 5}, {30, 40}}, {{10, 20}, {50, 60}},
                  {{0, 5}, {10, 20}, {30, 40}, {50, 60}});
   this->checkAdd({{10, 20}, {50, 60}}, {{0, 5}, {30, 40}, {70, 80}},
                  {{0, 5}, {10, 20}, {30, 40}, {50, 60}, {70, 80}});
 }
 
 TYPED_TEST(RangeSetTest, RangeSetDeletePointTest) {
-  constexpr TypeParam MIN = TestFixture::getMin();
-  constexpr TypeParam MAX = TestFixture::getMax();
-  constexpr TypeParam MID = TestFixture::getMid();
+  using TV = TestValues<TypeParam>;
+  constexpr auto MIN = TV::MIN;
+  constexpr auto MAX = TV::MAX;
+  constexpr auto MID = TV::MID;
 
   this->checkDelete(MID, {{MIN, MAX}}, {{MIN, MID - 1}, {MID + 1, MAX}});
   // Check that delete works with an empty set.
   this->checkDelete(10, {}, {});
   // Check that delete can remove entire ranges.
   this->checkDelete(10, {{10, 10}}, {});
   this->checkDelete(10, {{0, 5}, {10, 10}, {20, 30}}, {{0, 5}, {20, 30}});
   // Check that delete can split existing ranges into two.
   this->checkDelete(10, {{0, 5}, {7, 15}, {20, 30}},
                     {{0, 5}, {7, 9}, {11, 15}, {20, 30}});
   // Check that delete of the point not from the range set works as expected.
   this->checkDelete(10, {{0, 5}, {20, 30}}, {{0, 5}, {20, 30}});
 }
+
+TYPED_TEST(RangeSetTest, RangeSetUniteTest) {
+  using TV = TestValues<TypeParam>;
+  constexpr auto MIN = TV::MIN;
+  constexpr auto MAX = TV::MAX;
+  constexpr auto MID = TV::MID;
+  constexpr auto A = TV::A;
+  constexpr auto B = TV::B;
+  constexpr auto C = TV::C;
+  constexpr auto D = TV::D;
+
+  // LHS and RHS is empty.
+  // RHS =>
+  // LHS =>                     =
+  //        ___________________   ___________________
+  this->checkUnite({}, {}, {});
+
+  // RHS is empty.
+  // RHS =>
+  // LHS =>        _____        =        _____
+  //        ______/_____\______   ______/_____\______
+  this->checkUnite({{A, B}}, {}, {{A, B}});
+  this->checkUnite({{A, B}, {C, D}}, {}, {{A, B}, {C, D}});
+  this->checkUnite({{MIN, MIN}}, {}, {{MIN, MIN}});
+  this->checkUnite({{MAX, MAX}}, {}, {{MAX, MAX}});
+  this->checkUnite({{MIN, MIN}, {MAX, MAX}}, {}, {{MIN, MIN}, {MAX, MAX}});
+
+  // LHS is empty.
+  // RHS =>         ___
+  // LHS =>        /   \        =        _____
+  //        ______/_____\______   ______/_____\______
+  this->checkUnite({}, B, {{B, B}});
+  this->checkUnite({}, {B, C}, {{B, C}});
+  this->checkUnite({}, {{MIN, B}, {C, MAX}}, {{MIN, B}, {C, MAX}});
+  this->checkUnite({}, {{MIN, MIN}}, {{MIN, MIN}});
+  this->checkUnite({}, {{MAX, MAX}}, {{MAX, MAX}});
+  this->checkUnite({}, {{MIN, MIN}, {MAX, MAX}}, {{MIN, MIN}, {MAX, MAX}});
+
+  // RHS is detached from LHS.
+  // RHS =>             ___
+  // LHS =>    ___     /   \    =    ___     _____
+  //        __/___\___/_____\__   __/___\___/_____\__
+  this->checkUnite({{A, C}}, D, {{A, C}, {D, D}});
+  this->checkUnite({{MID, C}, {D, MAX}}, A, {{A, A}, {MID, C}, {D, MAX}});
+  this->checkUnite({{A, B}}, {MID, D}, {{A, B}, {MID, D}});
+  this->checkUnite({{MIN, A}, {D, MAX}}, {B, C}, {{MIN, A}, {B, C}, {D, MAX}});
+  this->checkUnite({{B, MID}, {D, MAX}}, {{MIN, A}, {C, C}},
+                   {{MIN, A}, {B, MID}, {C, C}, {D, MAX}});
+  this->checkUnite({{MIN, A}, {C, C}}, {{B, MID}, {D, MAX}},
+                   {{MIN, A}, {B, MID}, {C, C}, {D, MAX}});
+  this->checkUnite({{A, B}}, {{MAX, MAX}}, {{A, B}, {MAX, MAX}});
+  this->checkUnite({{MIN, MIN}}, {A, B}, {{MIN, MIN}, {A, B}});
+  this->checkUnite({{MIN, MIN}}, {MAX, MAX}, {{MIN, MIN}, {MAX, MAX}});
+
+  // RHS is inside LHS.
+  // RHS =>         ___
+  // LHS =>     ___/___\___     =     ___________
+  //        ___/__/_____\__\___   ___/___________\___
+  this->checkUnite({{A, C}}, MID, {{A, C}});
+  this->checkUnite({{A, D}}, {B, C}, {{A, D}});
+  this->checkUnite({{MIN, MAX}}, {B, C}, {{MIN, MAX}});
+
+  // RHS wraps LHS.
+  // RHS =>      _________
+  // LHS =>     /  _____  \     =     ___________
+  //        ___/__/_____\__\___   ___/___________\___
+  this->checkUnite({{MID, MID}}, {A, D}, {{A, D}});
+  this->checkUnite({{B, C}}, {A, D}, {{A, D}});
+  this->checkUnite({{A, B}}, {MIN, MAX}, {{MIN, MAX}});
+
+  // RHS equals to LHS.
+  // RHS =>      _________
+  // LHS =>     /_________\     =     ___________
+  //        ___/___________\___   ___/___________\___
+  this->checkUnite({{MIN, MIN}}, MIN, {{MIN, MIN}});
+  this->checkUnite({{A, B}}, {A, B}, {{A, B}});
+  this->checkUnite({{MAX, MAX}}, {{MAX, MAX}}, {{MAX, MAX}});
+  this->checkUnite({{MIN, MIN}}, {{MIN, MIN}}, {{MIN, MIN}});
+  this->checkUnite({{MIN, MIN}, {MAX, MAX}}, {{MIN, MIN}, {MAX, MAX}},
+                   {{MIN, MIN}, {MAX, MAX}});
+
+  // RHS edge is MIN and attached and inside LHS.
+  // RHS =>   _____
+  // LHS =>  /_____\_____     =  ___________
+  //        /_______\____\___   /___________\___
+  this->checkUnite({{MIN, A}}, {MIN, B}, {{MIN, B}});
+
+  // RHS edge is MIN and attached and outsude LHS.
+  // RHS =>   __________
+  // LHS =>  /______    \     =  ___________
+  //        /_______\____\___   /___________\___
+  this->checkUnite({{MIN, B}}, {MIN, A}, {{MIN, B}});
+
+  // RHS intersects right of LHS.
+  // RHS =>         ______
+  // LHS =>     ___/____  \     =     ___________
+  //        ___/__/_____\__\___   ___/___________\___
+  this->checkUnite({{A, C}}, C, {{A, C}});
+  this->checkUnite({{A, C}}, {B, D}, {{A, D}});
+
+  // RHS intersects left of LHS.
+  // RHS =>      ______
+  // LHS =>     /  ____\___     =     ___________
+  //        ___/__/_____\__\___   ___/___________\___
+  this->checkUnite({{B, D}}, B, {{B, D}});
+  this->checkUnite({{B, D}}, {A, C}, {{A, D}});
+  this->checkUnite({{MID, MAX}}, {MIN, MID}, {{MIN, MAX}});
+
+  // RHS adjacent to LHS on right.
+  // RHS =>            _____
+  // LHS =>   ______  /     \   =   _______________
+  //        _/______\/_______\_   _/_______________\_
+  this->checkUnite({{A, B - 1}}, B, {{A, B}});
+  this->checkUnite({{A, C}}, {C + 1, D}, {{A, D}});
+  this->checkUnite({{MIN, MID}}, {MID + 1, MAX}, {{MIN, MAX}});
+
+  // RHS adjacent to LHS on left.
+  // RHS =>    _____
+  // LHS =>   /     \  ______   =   _______________
+  //        _/_______\/______\_   _/_______________\_
+  this->checkUnite({{B + 1, C}}, B, {{B, C}});
+  this->checkUnite({{B, D}}, {A, B - 1}, {{A, D}});
+
+  // RHS adjacent to LHS in between.
+  // RHS =>         ___
+  // LHS =>   ___  /   \  ___   =   _______________
+  //        _/___\/_____\/___\_   _/_______________\_
+  this->checkUnite({{A, MID - 1}, {MID + 1, D}}, MID, {{A, D}});
+  this->checkUnite({{MIN, A}, {D, MAX}}, {A + 1, D - 1}, {{MIN, MAX}});
+
+  // RHS adjacent to LHS on the outside.
+  // RHS =>    __         __
+  // LHS =>   /  \  ___  /  \   =   _______________
+  //        _/____\/___\/____\_   _/_______________\_
+  this->checkUnite({{C, C}}, {{A, C - 1}, {C + 1, D}}, {{A, D}});
+  this->checkUnite({{B, MID}}, {{A, B - 1}, {MID + 1, D}}, {{A, D}});
+
+  // RHS wraps two subranges of LHS.
+  // RHS =>     ___________
+  // LHS =>    / ___   ___ \    =    _____________
+  //        __/_/___\_/___\_\__   __/_____________\__
+  this->checkUnite({{B, B}, {MID, MID}, {C, C}}, {{A, D}}, {{A, D}});
+  this->checkUnite({{A, B}, {MID, C}}, {{MIN, D}}, {{MIN, D}});
+
+  // RHS intersects two subranges of LHS.
+  // RHS =>      _________
+  // LHS =>   __/__      _\__   =   _______________
+  //        _/_/___\____/__\_\_   _/_______________\_
+  this->checkUnite({{MIN, B}, {C, MAX}}, {{A, D}}, {{MIN, MAX}});
+  this->checkUnite({{A, MID}, {C, MAX}}, {{B, D}}, {{A, MAX}});
+
+  // Multiple intersections.
+
+  // clang-format off
+  // RHS =>
+  // LHS =>   /\   /\            =   __   __
+  //        _/__\_/__\_/\_/\_/\_   _/__\_/__\_/\_/\_/\_
+  this->checkUnite({{MID, C}, {C + 2, D - 2}, {D, MAX}},
+                   {{MIN, A}, {A + 2, B}},
+                   {{MIN, A}, {A + 2, B}, {MID, C}, {C + 2, D - 2}, {D, MAX}});
+  this->checkUnite({{B, B}, {C, C}, {MAX, MAX}},
+                   {{MIN, MIN}, {A, A}},
+                   {{MIN, MIN}, {A, A}, {B, B}, {C, C}, {MAX, MAX}});
+
+  // RHS =>
+  // LHS =>             /\   /\   =            __   __
+  //        _/\_/\_/\__/__\_/__\_   _/\_/\_/\_/__\_/__\_
+  this->checkUnite({{MIN, A}, {A + 2, B}, {MID, C}},
+                   {{C + 2, D - 2}, {D, MAX}},
+                   {{MIN, A}, {A + 2, B}, {MID, C}, {C + 2, D - 2}, {D, MAX}});
+  this->checkUnite({{MIN, MIN}, {A, A}, {B, B}},
+                   {{C, C}, {MAX, MAX}},
+                   {{MIN, MIN}, {A, A}, {B, B}, {C, C}, {MAX, MAX}});
+
+  // RHS =>
+  // LHS =>   _   /\   _   /\   _   /\  =
+  //        _/_\_/__\_/_\_/__\_/_\_/__\_
+  //
+  // RSLT =>  _   __   _   __   _   __
+  //        _/_\_/__\_/_\_/__\_/_\_/__\_
+  this->checkUnite({{MIN, A}, {B + 2, MID}, {C + 2, D}},
+                   {{A + 2, B}, {MID + 2, C}, {D + 2, MAX}},
+                   {{MIN, A}, {A + 2, B}, {B + 2, MID}, {MID + 2, C}, {C + 2, D}, {D + 2, MAX}});
+  this->checkUnite({{MIN, MIN}, {B, B}, {D, D}},
+                   {{A, A}, {C, C}, {MAX, MAX}},
+                   {{MIN, MIN}, {A, A}, {B, B}, {C, C}, {D, D}, {MAX, MAX}});
+
+  // RHS =>
+  // LHS =>   /\   _   /\   _   /\   _  =
+  //        _/__\_/_\_/__\_/_\_/__\_/_\_
+  //
+  // RSLT =>  __   _   __   _   __   _
+  //        _/__\_/_\_/__\_/_\_/__\_/_\_
+  this->checkUnite({{A + 2, B}, {MID + 2, C}, {D + 2, MAX}},
+                   {{MIN, A}, {B + 2, MID}, {C + 2, D}},
+                   {{MIN, A}, {A + 2, B}, {B + 2, MID}, {MID + 2, C}, {C + 2, D}, {D + 2, MAX}});
+  this->checkUnite({{A, A}, {C, C}, {MAX, MAX}},
+                   {{MIN, MIN}, {B, B}, {D, D}},
+                   {{MIN, MIN}, {A, A}, {B, B}, {C, C}, {D, D}, {MAX, MAX}});
+
+  // RHS =>    _     __       _
+  // LHS =>   /_\   /_ \  _  / \   =   ___   ____________
+  //        _/___\_/__\_\/_\/___\_   _/___\_/____________\_
+  this->checkUnite({{MIN, A}, {B, C}, {D, MAX}},
+                   {{MIN, A}, {B, C - 2}, {C + 1, D - 1}},
+                   {{MIN, A}, {B, MAX}});
+  this->checkUnite({{A, A}, {B, MID}, {D, D}},
+                   {{A, A}, {B, B}, {MID + 1, D - 1}},
+                   {{A, A}, {B, D}});
+
+  // RHS =>            ___      ___
+  // LHS =>      /\  _/_  \_   / _ \   /\  =
+  //        _/\_/__\//__\ /\\_/_/_\_\_/__\_
+  //
+  // RSLT =>     ___________   _____   __
+  //        _/\_/___________\_/_____\_/__\_
+  this->checkUnite({{MIN, MIN}, {B, MID}, {MID + 1, C}, {C + 4, D - 1}},
+                   {{A, B - 1}, {B + 1, C - 1}, {C + 2, D}, {MAX - 1, MAX}},
+                   {{MIN, MIN}, {A, C}, {C + 2, D}, {MAX - 1, MAX}});
+  // clang-format on
+}