[llvm] Add ConstantRangeList::unionWith() and ::intersectWith() (PR #96547)

[Arthur Eubanks via llvm-commits]

================
@@ -81,6 +81,86 @@ void ConstantRangeList::insert(const ConstantRange &NewRange) {
   }
 }
 
+ConstantRangeList
+ConstantRangeList::unionWith(const ConstantRangeList &CRL) const {
+  assert(getBitWidth() == CRL.getBitWidth() &&
+         "ConstantRangeList types don't agree!");
+  // Handle common cases.
+  if (empty())
+    return CRL;
+  if (CRL.empty())
+    return *this;
+
+  ConstantRangeList Result;
+  size_t i = 0, j = 0;
+  ConstantRange PreviousRange(getBitWidth(), false);
+  if (Ranges[i].getLower().slt(CRL.Ranges[j].getLower())) {
+    PreviousRange = Ranges[i++];
+  } else {
+    PreviousRange = CRL.Ranges[j++];
+  }
+  auto UnionAndUpdateRange = [&PreviousRange,
+                              &Result](const ConstantRange &CR) {
+    assert(!CR.isSignWrappedSet() && "Upper wrapped ranges are not supported");
+    if (PreviousRange.getUpper().slt(CR.getLower())) {
+      Result.Ranges.push_back(PreviousRange);
+      PreviousRange = CR;
+    } else {
+      PreviousRange = ConstantRange(
+          PreviousRange.getLower(),
+          APIntOps::smax(PreviousRange.getUpper(), CR.getUpper()));
+    }
+  };
+  while (i < size() || j < CRL.size()) {
+    if (j == CRL.size() ||
+        (i < size() && Ranges[i].getLower().slt(CRL.Ranges[j].getLower()))) {
+      // Merge PreviousRange with this.
+      UnionAndUpdateRange(Ranges[i++]);
+    } else {
+      // Merge PreviousRange with CRL.
+      UnionAndUpdateRange(CRL.Ranges[j++]);
+    }
+  }
+  Result.Ranges.push_back(PreviousRange);
+  return Result;
+}
+
+ConstantRangeList
+ConstantRangeList::intersectWith(const ConstantRangeList &CRL) const {
+  assert(getBitWidth() == CRL.getBitWidth() &&
+         "ConstantRangeList types don't agree!");
+
+  // Handle common cases.
+  if (empty())
+    return *this;
+  if (CRL.empty())
+    return CRL;
+
+  ConstantRangeList Result;
+  size_t i = 0, j = 0;
+  while (i < size() && j < CRL.size()) {
+    auto &Range = this->Ranges[i];
+    auto &OtherRange = CRL.Ranges[j];
+    assert(!Range.isSignWrappedSet() && !OtherRange.isSignWrappedSet() &&
+           "Upper wrapped ranges are not supported");
+
+    APInt Start = Range.getLower().slt(OtherRange.getLower())
----------------
aeubanks wrote:

could you add some comments here (and also in union)? it seems like there are some details that would be good to have written down

https://github.com/llvm/llvm-project/pull/96547