diff --git a/lldb/source/Plugins/TypeSystem/Clang/TypeSystemClang.cpp b/lldb/source/Plugins/TypeSystem/Clang/TypeSystemClang.cpp
index 21c265ede0bc5..12cabff7c36e4 100644
--- a/lldb/source/Plugins/TypeSystem/Clang/TypeSystemClang.cpp
+++ b/lldb/source/Plugins/TypeSystem/Clang/TypeSystemClang.cpp
@@ -849,8 +849,20 @@ lldb::BasicType TypeSystemClang::GetBasicTypeEnumeration(llvm::StringRef name) {
       {"unsigned long long int", eBasicTypeUnsignedLongLong},
 
       // "int128"
+      //
+      // The following two lines are here only
+      // for the sake of backward-compatibility.
+      // Neither "__int128_t", nor "__uint128_t" are basic-types.
+      // They are typedefs.
       {"__int128_t", eBasicTypeInt128},
       {"__uint128_t", eBasicTypeUnsignedInt128},
+      // In order to be consistent with:
+      // - gcc's C programming language extension related to 128-bit integers
+      //   https://gcc.gnu.org/onlinedocs/gcc/_005f_005fint128.html
+      // - the "BuiltinType::getName" method in LLVM
+      // the following two lines must be present:
+      {"__int128", eBasicTypeInt128},
+      {"unsigned __int128", eBasicTypeUnsignedInt128},
 
       // "bool"
       {"bool", eBasicTypeBool},
diff --git a/lldb/test/API/python_api/sbtype_basic_type/TestSBTypeBasicType.py b/lldb/test/API/python_api/sbtype_basic_type/TestSBTypeBasicType.py
index 535d8b900673e..f8594dfc6b78d 100644
--- a/lldb/test/API/python_api/sbtype_basic_type/TestSBTypeBasicType.py
+++ b/lldb/test/API/python_api/sbtype_basic_type/TestSBTypeBasicType.py
@@ -28,3 +28,11 @@ def test(self):
         self.assertEqual(b.GetType().GetBasicType(), int_basic_type)
         self.assertEqual(c.GetType().GetBasicType(), int_basic_type)
         self.assertEqual(d.GetType().GetBasicType(), int_basic_type)
+
+        # Check the size of the chosen basic types.
+        self.assertEqual(self.target().FindFirstType("__int128").size, 16)
+        self.assertEqual(self.target().FindFirstType("unsigned __int128").size, 16)
+
+        # Check the size of the chosen aliases of basic types.
+        self.assertEqual(self.target().FindFirstType("__int128_t").size, 16)
+        self.assertEqual(self.target().FindFirstType("__uint128_t").size, 16)
diff --git a/lldb/unittests/Symbol/TestTypeSystemClang.cpp b/lldb/unittests/Symbol/TestTypeSystemClang.cpp
index f673cceae00dd..1981e912fa4fa 100644
--- a/lldb/unittests/Symbol/TestTypeSystemClang.cpp
+++ b/lldb/unittests/Symbol/TestTypeSystemClang.cpp
@@ -162,6 +162,9 @@ TEST_F(TestTypeSystemClang, TestGetBasicTypeFromName) {
   EXPECT_EQ(GetBasicQualType(eBasicTypeInt128), GetBasicQualType("__int128_t"));
   EXPECT_EQ(GetBasicQualType(eBasicTypeUnsignedInt128),
             GetBasicQualType("__uint128_t"));
+  EXPECT_EQ(GetBasicQualType(eBasicTypeInt128), GetBasicQualType("__int128"));
+  EXPECT_EQ(GetBasicQualType(eBasicTypeUnsignedInt128),
+            GetBasicQualType("unsigned __int128"));
   EXPECT_EQ(GetBasicQualType(eBasicTypeVoid), GetBasicQualType("void"));
   EXPECT_EQ(GetBasicQualType(eBasicTypeBool), GetBasicQualType("bool"));
   EXPECT_EQ(GetBasicQualType(eBasicTypeFloat), GetBasicQualType("float"));