[ValueLattice] Update markConstantRange to return false equal ranges.

Currently we always return true, when markConstantRange is used on an
object already containing a constant range. If NewR is equal to the
existing constant range however, nothing changes and we should return
false.

I also went ahead and added a clarifying comment and improved the
assertion.

Reviewers: efriedma, davide, nikic

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D73240

llvm/include/llvm/Analysis/ValueLattice.h

@@ -220,14 +220,21 @@ class ValueLatticeElement {
     return true;
   }
 
+  /// Mark the object as constant range with \p NewR. If the object is already a
+  /// constant range, nothing changes if the existing range is equal to \p
+  /// NewR. Otherwise \p NewR must be a superset of the existing range or the
+  /// object must be undefined.
   bool markConstantRange(ConstantRange NewR) {
     if (isConstantRange()) {
+      if (getConstantRange() == NewR)
+        return false;
+
       if (NewR.isEmptySet())
-        markOverdefined();
-      else {
-        assert(NewR.contains(getConstantRange()) && "Existing range must be a subset of NewR");
-        Range = std::move(NewR);
-      }
+        return markOverdefined();
+
+      assert(NewR.contains(getConstantRange()) &&
+             "Existing range must be a subset of NewR");
+      Range = std::move(NewR);
       return true;
     }
 

llvm/unittests/Analysis/ValueLatticeTest.cpp

@@ -43,6 +43,23 @@ TEST_F(ValueLatticeTest, ValueLatticeGetters) {
   EXPECT_TRUE(ValueLatticeElement::getNot(C2).isNotConstant());
 }
 
+TEST_F(ValueLatticeTest, MarkConstantRange) {
+  auto LV1 =
+      ValueLatticeElement::getRange({APInt(32, 10, true), APInt(32, 20, true)});
+
+  // Test markConstantRange() with an equal range.
+  EXPECT_FALSE(
+      LV1.markConstantRange({APInt(32, 10, true), APInt(32, 20, true)}));
+
+  // Test markConstantRange() with supersets of existing range.
+  EXPECT_TRUE(LV1.markConstantRange({APInt(32, 5, true), APInt(32, 20, true)}));
+  EXPECT_EQ(LV1.getConstantRange().getLower().getLimitedValue(), 5U);
+  EXPECT_EQ(LV1.getConstantRange().getUpper().getLimitedValue(), 20U);
+  EXPECT_TRUE(LV1.markConstantRange({APInt(32, 5, true), APInt(32, 23, true)}));
+  EXPECT_EQ(LV1.getConstantRange().getLower().getLimitedValue(), 5U);
+  EXPECT_EQ(LV1.getConstantRange().getUpper().getLimitedValue(), 23U);
+}
+
 TEST_F(ValueLatticeTest, MergeIn) {
   auto I32Ty = IntegerType::get(Context, 32);
   auto *C1 = ConstantInt::get(I32Ty, 1);