Index: llvm/include/llvm/Support/Bitfields.h
===================================================================
--- /dev/null
+++ llvm/include/llvm/Support/Bitfields.h
@@ -0,0 +1,46 @@
+//===-- llvm/Support/BitFields.h - Utilities for C++ bit-fields -*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// Utilities for wrangling C++ bit-fields.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_SUPPORT_BIT_FIELDS_H
+#define LLVM_SUPPORT_BIT_FIELDS_H
+
+#include "llvm/Support/MathExtras.h"
+#include <stdint.h>
+
+/// Given a bitfield Dst, does Dst = Src, and asserts that Src fits into Dst.
+///
+/// Dst must be a bitfield with an unsigned integral underlying type.
+///
+/// For example:
+///
+/// \code
+///   struct T {
+///     unsigned X : 2;
+///   };
+///
+///   T t;
+///   LLVM_CHECKED_BITFIELD_ASSIGN(t.X, 3);  // t.X = 3;
+///   LLVM_CHECKED_BITFIELD_ASSIGN(t.X, 42); // error: 42 doesn't fit in T::X.
+/// \endcode
+///
+#define LLVM_CHECKED_BITFIELD_ASSIGN(Dst, Src)                                 \
+  do {                                                                         \
+    uint64_t _LlvmCheckedBitfieldAssignVal = (Src);                            \
+    static_assert(std::is_unsigned<decltype(Dst)>::value,                      \
+                  #Dst " must be an unsigned integer type.");                  \
+    (Dst) = _LlvmCheckedBitfieldAssignVal;                                     \
+    assert((Dst) == _LlvmCheckedBitfieldAssignVal && #Src                      \
+           " is too wide to fit inside " #Dst);                                \
+  } while (0)
+
+#endif
Index: llvm/unittests/Support/BitfieldsTest.cpp
===================================================================
--- /dev/null
+++ llvm/unittests/Support/BitfieldsTest.cpp
@@ -0,0 +1,55 @@
+//===- unittests/Support/MathExtrasTest.cpp - math utils tests ------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Support/Bitfields.h"
+#include "gtest/gtest-death-test.h"
+#include "gtest/gtest.h"
+
+namespace {
+
+TEST(BitfieldsTest, BitfieldAssign) {
+  struct {
+    unsigned a : 1;
+    uint32_t b : 32;
+    uint64_t c : 64;
+  } S;
+
+  LLVM_CHECKED_BITFIELD_ASSIGN(S.a, 0);
+  EXPECT_EQ(0U, S.a);
+  LLVM_CHECKED_BITFIELD_ASSIGN(S.a, 1);
+  EXPECT_EQ(1U, S.a);
+
+  LLVM_CHECKED_BITFIELD_ASSIGN(S.b, std::numeric_limits<uint32_t>::max());
+  EXPECT_EQ(std::numeric_limits<uint32_t>::max(), S.b);
+  LLVM_CHECKED_BITFIELD_ASSIGN(S.b, 0);
+  EXPECT_EQ(0U, S.b);
+  LLVM_CHECKED_BITFIELD_ASSIGN(S.b, 42);
+  EXPECT_EQ(42U, S.b);
+
+  LLVM_CHECKED_BITFIELD_ASSIGN(S.c, std::numeric_limits<uint32_t>::max());
+  EXPECT_EQ(std::numeric_limits<uint32_t>::max(), S.c);
+  LLVM_CHECKED_BITFIELD_ASSIGN(S.c, std::numeric_limits<uint64_t>::max());
+  EXPECT_EQ(std::numeric_limits<uint64_t>::max(), S.c);
+  LLVM_CHECKED_BITFIELD_ASSIGN(S.c, 0);
+  EXPECT_EQ(0U, S.c);
+}
+
+TEST(BitfieldsDeathTest, BitfieldAssignOutOfRange) {
+  struct {
+    unsigned a : 1;
+    uint32_t b : 31;
+    uint64_t c : 63;
+  } S;
+
+  EXPECT_DEBUG_DEATH(LLVM_CHECKED_BITFIELD_ASSIGN(S.a, 2), "");
+  EXPECT_DEBUG_DEATH(LLVM_CHECKED_BITFIELD_ASSIGN(S.b, (1U << 31) + 1), "");
+  EXPECT_DEBUG_DEATH(LLVM_CHECKED_BITFIELD_ASSIGN(S.b, (1LLU << 63) + 1), "");
+}
+
+} // namespace
Index: llvm/unittests/Support/CMakeLists.txt
===================================================================
--- llvm/unittests/Support/CMakeLists.txt
+++ llvm/unittests/Support/CMakeLists.txt
@@ -6,6 +6,7 @@
   AlignOfTest.cpp
   AllocatorTest.cpp
   ArrayRecyclerTest.cpp
+  BitfieldsTest.cpp
   BlockFrequencyTest.cpp
   BranchProbabilityTest.cpp
   Casting.cpp