[APSInt] Fix bug in APSInt mentioned in #59515

Also provide a `tryExtValue()` API like APInt did in D139683

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D140059

llvm/include/llvm/ADT/APSInt.h

@@ -85,12 +85,26 @@ class [[nodiscard]] APSInt : public APInt {
   }
   using APInt::toString;
 
+  /// If this int is representable using an int64_t.
+  bool isRepresentableByInt64() const {
+    // For unsigned values with 64 active bits, they technically fit into a
+    // int64_t, but the user may get negative numbers and has to manually cast
+    // them to unsigned. Let's not bet the user has the sanity to do that and
+    // not give them a vague value at the first place.
+    return isSigned() ? isSignedIntN(64) : isIntN(63);
+  }
+
   /// Get the correctly-extended \c int64_t value.
   int64_t getExtValue() const {
-    assert(getMinSignedBits() <= 64 && "Too many bits for int64_t");
+    assert(isRepresentableByInt64() && "Too many bits for int64_t");
     return isSigned() ? getSExtValue() : getZExtValue();
   }
 
+  std::optional<int64_t> tryExtValue() const {
+    return isRepresentableByInt64() ? std::optional<int64_t>(getExtValue())
+                                    : std::nullopt;
+  }
+
   APSInt trunc(uint32_t width) const {
     return APSInt(APInt::trunc(width), IsUnsigned);
   }

llvm/unittests/ADT/APSIntTest.cpp

@@ -62,6 +62,13 @@ TEST(APSIntTest, getUnsigned) {
   EXPECT_EQ(UINT64_C(0) - 7, APSInt::getUnsigned(-7).getZExtValue());
 }
 
+TEST(APSIntTest, isRepresentableByInt64) {
+  ASSERT_TRUE(APSInt(APInt(3, 7), true).isRepresentableByInt64());
+  ASSERT_TRUE(APSInt(APInt(128, 7), true).isRepresentableByInt64());
+  ASSERT_TRUE(APSInt(APInt(128, 7), false).isRepresentableByInt64());
+  ASSERT_TRUE(APSInt(APInt(64, -1), false).isRepresentableByInt64());
+  ASSERT_FALSE(APSInt(APInt(64, (uint64_t)-1), true).isRepresentableByInt64());
+}
 TEST(APSIntTest, getExtValue) {
   EXPECT_TRUE(APSInt(APInt(3, 7), true).isUnsigned());
   EXPECT_TRUE(APSInt(APInt(3, 7), false).isSigned());
@@ -76,6 +83,16 @@ TEST(APSIntTest, getExtValue) {
   EXPECT_EQ(9, APSInt(APInt(4, -7), true).getExtValue());
   EXPECT_EQ(-7, APSInt(APInt(4, -7), false).getExtValue());
 }
+TEST(APSIntTest, tryExtValue) {
+  ASSERT_EQ(-7, APSInt(APInt(64, -7), false).tryExtValue().value_or(42));
+  ASSERT_EQ(42, APSInt(APInt(128, -7), false).tryExtValue().value_or(42));
+  ASSERT_EQ(-1,
+            APSInt(APInt::getAllOnes(128), false).tryExtValue().value_or(42));
+  ASSERT_EQ(42, APSInt(APInt(64, -7), true).tryExtValue().value_or(42));
+  ASSERT_EQ(1, APSInt(APInt(128, 1), true).tryExtValue().value_or(42));
+  ASSERT_EQ(42,
+            APSInt(APInt::getAllOnes(128), true).tryExtValue().value_or(42));
+}
 
 TEST(APSIntTest, compareValues) {
   auto U = [](uint64_t V) { return APSInt::getUnsigned(V); };