diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -60,6 +60,14 @@
 Non-comprehensive list of changes in this release
 -------------------------------------------------
 
+- Maximum _ExtInt size was decreased from 16,777,215 bits to 8,388,608 bits.
+  Motivation for this was discussed in PR51829.
+- Configuration file syntax extended with ``<CFGDIR>`` token. This expands to
+  the base path of the current config file. See :ref:`configuration-files` for
+  details.
+
+- The builtin ``__builtin_bswap128`` was added.
+
 New Compiler Flags
 ------------------
 
diff --git a/clang/include/clang/Basic/Builtins.def b/clang/include/clang/Basic/Builtins.def
--- a/clang/include/clang/Basic/Builtins.def
+++ b/clang/include/clang/Basic/Builtins.def
@@ -513,6 +513,7 @@
 BUILTIN(__builtin_bswap16, "UsUs", "nc")
 BUILTIN(__builtin_bswap32, "UZiUZi", "nc")
 BUILTIN(__builtin_bswap64, "UWiUWi", "nc")
+BUILTIN(__builtin_bswap128, "ULLLiULLLi", "nc")
 
 BUILTIN(__builtin_bitreverse8, "UcUc", "nc")
 BUILTIN(__builtin_bitreverse16, "UsUs", "nc")
diff --git a/clang/lib/AST/ASTContext.cpp b/clang/lib/AST/ASTContext.cpp
--- a/clang/lib/AST/ASTContext.cpp
+++ b/clang/lib/AST/ASTContext.cpp
@@ -10839,8 +10839,8 @@
 /// to be an Integer Constant Expression.
 static QualType DecodeTypeFromStr(const char *&Str, const ASTContext &Context,
                                   ASTContext::GetBuiltinTypeError &Error,
-                                  bool &RequiresICE,
-                                  bool AllowTypeModifiers) {
+                                  bool &RequiresICE, bool AllowTypeModifiers,
+                                  bool AllowInt128 = false) {
   // Modifiers.
   int HowLong = 0;
   bool Signed = false, Unsigned = false;
@@ -10983,9 +10983,13 @@
       Type = Context.ShortTy;
     break;
   case 'i':
-    if (HowLong == 3)
+    if (HowLong == 3) {
+      if (!AllowInt128 && !Context.getTargetInfo().hasInt128Type()) {
+        Error = ASTContext::GE_Missing_type;
+        return {};
+      }
       Type = Unsigned ? Context.UnsignedInt128Ty : Context.Int128Ty;
-    else if (HowLong == 2)
+    } else if (HowLong == 2)
       Type = Unsigned ? Context.UnsignedLongLongTy : Context.LongLongTy;
     else if (HowLong == 1)
       Type = Unsigned ? Context.UnsignedLongTy : Context.LongTy;
@@ -11064,8 +11068,8 @@
     assert(End != Str && "Missing vector size");
     Str = End;
 
-    QualType ElementType = DecodeTypeFromStr(Str, Context, Error,
-                                             RequiresICE, false);
+    QualType ElementType =
+        DecodeTypeFromStr(Str, Context, Error, RequiresICE, false, true);
     assert(!RequiresICE && "Can't require vector ICE");
 
     // TODO: No way to make AltiVec vectors in builtins yet.
diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -11784,7 +11784,8 @@
 
   case Builtin::BI__builtin_bswap16:
   case Builtin::BI__builtin_bswap32:
-  case Builtin::BI__builtin_bswap64: {
+  case Builtin::BI__builtin_bswap64:
+  case Builtin::BI__builtin_bswap128: {
     APSInt Val;
     if (!EvaluateInteger(E->getArg(0), Val, Info))
       return false;
diff --git a/clang/lib/CodeGen/CGBuiltin.cpp b/clang/lib/CodeGen/CGBuiltin.cpp
--- a/clang/lib/CodeGen/CGBuiltin.cpp
+++ b/clang/lib/CodeGen/CGBuiltin.cpp
@@ -2929,7 +2929,8 @@
   }
   case Builtin::BI__builtin_bswap16:
   case Builtin::BI__builtin_bswap32:
-  case Builtin::BI__builtin_bswap64: {
+  case Builtin::BI__builtin_bswap64:
+  case Builtin::BI__builtin_bswap128: {
     return RValue::get(emitUnaryBuiltin(*this, E, Intrinsic::bswap));
   }
   case Builtin::BI__builtin_bitreverse8:
diff --git a/clang/test/CodeGen/builtin-bswap128.c b/clang/test/CodeGen/builtin-bswap128.c
new file mode 100644
--- /dev/null
+++ b/clang/test/CodeGen/builtin-bswap128.c
@@ -0,0 +1,12 @@
+// RUN: %clang_cc1 -emit-llvm -o - %s | FileCheck %s
+
+// CHECK-LABEL: @test(
+// CHECK-NEXT: entry:
+// CHECK-NEXT:    [[R1:%.*]] = alloca i128, align 16
+// CHECK-NEXT:    [[R2:%.*]] = alloca i128, align 16
+// CHECK-NEXT:    store i128 1329227995784915872903807060280344576, i128* [[R1:%.*]], align 16
+// CHECK-NEXT:    store i128 2658455991569831745807614120560689152, i128* [[R2:%.*]], align 16
+void test() {
+  __int128 r1 = __builtin_bswap128(1);
+  __int128 r2 = __builtin_bswap128((__int128)2);
+}
diff --git a/clang/test/CodeGen/builtins.cpp b/clang/test/CodeGen/builtins.cpp
--- a/clang/test/CodeGen/builtins.cpp
+++ b/clang/test/CodeGen/builtins.cpp
@@ -20,6 +20,10 @@
 decltype(__builtin_bswap32(0)) bswap32 = 42;
 extern uint64_t bswap64;
 decltype(__builtin_bswap64(0)) bswap64 = 42;
+#ifdef __SIZEOF_INT128__
+extern __uint128_t bswap128;
+decltype(__builtin_bswap128(0)) bswap128 = 42;
+#endif
 
 #ifdef __clang__
 extern uint8_t bitrev8;
diff --git a/clang/test/Sema/builtin-bswap128.c b/clang/test/Sema/builtin-bswap128.c
new file mode 100644
--- /dev/null
+++ b/clang/test/Sema/builtin-bswap128.c
@@ -0,0 +1,5 @@
+// RUN: %clang_cc1 -triple i386-mingw32 -fsyntax-only -verify %s
+
+void test() {
+  __builtin_bswap128(1); // expected-error {{use of unknown builtin '__builtin_bswap128'}}
+}
diff --git a/clang/test/Sema/constant-builtins-2.c b/clang/test/Sema/constant-builtins-2.c
--- a/clang/test/Sema/constant-builtins-2.c
+++ b/clang/test/Sema/constant-builtins-2.c
@@ -216,6 +216,9 @@
 int h3 = __builtin_bswap16(0x1234) == 0x3412 ? 1 : f();
 int h4 = __builtin_bswap32(0x1234) == 0x34120000 ? 1 : f();
 int h5 = __builtin_bswap64(0x1234) == 0x3412000000000000 ? 1 : f();
+#if defined(__SIZEOF_INT128__)
+int h6 = __builtin_bswap128(0x1234) == (((__int128)0x3412) << 112) ? 1 : f();
+#endif
 extern long int bi0;
 extern __typeof__(__builtin_expect(0, 0)) bi0;