[ORC] Fix serialization / deserialization of default-constructed StringRef.

Avoids accessing the data field on zero-length strings. This is the StringRef
counterpart to the ArrayRef<char> fix in 67220c2.

rdar://97285294

Author: lhames

llvm/include/llvm/ExecutionEngine/Orc/Shared/SimplePackedSerialization.h

@@ -479,6 +479,8 @@ template <> class SPSSerializationTraits<SPSString, StringRef> {
   static bool serialize(SPSOutputBuffer &OB, StringRef S) {
     if (!SPSArgList<uint64_t>::serialize(OB, static_cast<uint64_t>(S.size())))
       return false;
+    if (S.empty()) // Empty StringRef may have null data, so bail out early.
+      return true;
     return OB.write(S.data(), S.size());
   }
 
@@ -490,7 +492,7 @@ template <> class SPSSerializationTraits<SPSString, StringRef> {
     Data = IB.data();
     if (!IB.skip(Size))
       return false;
-    S = StringRef(Data, Size);
+    S = StringRef(Size ? Data : nullptr, Size);
     return true;
   }
 };

llvm/unittests/ExecutionEngine/Orc/SimplePackedSerializationTest.cpp

@@ -208,3 +208,24 @@ TEST(SimplePackedSerializationTest, ArrayRefEmpty) {
   EXPECT_EQ(AIn.data(), nullptr);
   EXPECT_EQ(AIn.size(), 0U);
 }
+
+TEST(SimplePackedSerializationTest, StringRefEmpty) {
+  // Make sure that empty StringRefs serialize and deserialize as expected.
+  // Empty StringRefs should not succeed even when the data field is null, and
+  // should deserialize to a default-constructed StringRef, not a pointer into
+  // the stream.
+  constexpr unsigned BufferSize = sizeof(uint64_t);
+  char Buffer[BufferSize];
+  memset(Buffer, 0, BufferSize);
+
+  StringRef SROut;
+  SPSOutputBuffer OB(Buffer, BufferSize);
+  EXPECT_TRUE(SPSArgList<SPSSequence<char>>::serialize(OB, SROut));
+
+  StringRef SRIn;
+  SPSInputBuffer IB(Buffer, BufferSize);
+  EXPECT_TRUE(SPSArgList<SPSSequence<char>>::deserialize(IB, SRIn));
+
+  EXPECT_EQ(SRIn.data(), nullptr);
+  EXPECT_EQ(SRIn.size(), 0U);
+}