[ConstantFPRange] Implement ConstantFPRange::makeExactFCmpRegion (#111490)

Note: The current implementation doesn't return optimal result for `fcmp
one/une x, +/-inf` since we don't handle this case in
#110891. Maybe we can make it
optimal after seeing some real-world cases.

Author: dtcxzyw

llvm/lib/IR/ConstantFPRange.cpp

@@ -268,7 +268,10 @@ ConstantFPRange::makeSatisfyingFCmpRegion(FCmpInst::Predicate Pred,
 std::optional<ConstantFPRange>
 ConstantFPRange::makeExactFCmpRegion(FCmpInst::Predicate Pred,
                                      const APFloat &Other) {
-  return std::nullopt;
+  if ((Pred == FCmpInst::FCMP_UNE || Pred == FCmpInst::FCMP_ONE) &&
+      !Other.isNaN())
+    return std::nullopt;
+  return makeSatisfyingFCmpRegion(Pred, ConstantFPRange(Other));
 }
 
 bool ConstantFPRange::fcmp(FCmpInst::Predicate Pred,

llvm/unittests/IR/ConstantFPRangeTest.cpp

@@ -743,4 +743,28 @@ TEST_F(ConstantFPRangeTest, fcmp) {
   }
 }
 
+TEST_F(ConstantFPRangeTest, makeExactFCmpRegion) {
+  for (auto Pred : FCmpInst::predicates()) {
+    EnumerateValuesInConstantFPRange(
+        ConstantFPRange::getFull(APFloat::Float8E4M3()),
+        [Pred](const APFloat &V) {
+          std::optional<ConstantFPRange> Res =
+              ConstantFPRange::makeExactFCmpRegion(Pred, V);
+          ConstantFPRange Allowed =
+              ConstantFPRange::makeAllowedFCmpRegion(Pred, ConstantFPRange(V));
+          ConstantFPRange Satisfying =
+              ConstantFPRange::makeSatisfyingFCmpRegion(Pred,
+                                                        ConstantFPRange(V));
+          if (Allowed == Satisfying)
+            EXPECT_EQ(Res, Allowed) << "Wrong result for makeExactFCmpRegion("
+                                    << Pred << ", " << V << ").";
+          else
+            EXPECT_FALSE(Res.has_value())
+                << "Wrong result for makeExactFCmpRegion(" << Pred << ", " << V
+                << ").";
+        },
+        /*IgnoreNaNPayload=*/true);
+  }
+}
+
 } // anonymous namespace