Index: llvm/include/llvm/Support/Bitfields.h
===================================================================
--- /dev/null
+++ llvm/include/llvm/Support/Bitfields.h
@@ -0,0 +1,63 @@
+//===-- 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"
+
+/// Gets the width in bits of Type::Field, which must be a bitfield with an
+/// underlying type that's an unsigned integer.
+///
+/// clang constant-folds this nicely, but other compilers (e.g. gcc) are not
+/// quite as smart.  So don't rely on this being free.
+#define LLVM_BITFIELD_WIDTH(Type, Field)                                       \
+  [] {                                                                         \
+    static_assert(::std::is_unsigned<decltype(Type::Field)>::value,            \
+                  #Type "::" #Field " must be an unsigned integral type.");    \
+    Type T;                                                                    \
+    ::memset(&T, 0xff, sizeof(T));                                             \
+    uint64_t Bits = T.Field;                                                   \
+    assert(::llvm::isMask_64(Bits) && "Error reading " #Type "::" #Field);     \
+    return ::llvm::countTrailingOnes(Bits);                                    \
+  }()
+
+/// Does
+///
+///   DstContainer.DstField = Src;
+///
+/// asserts that Src fits inside Dst.  DstField 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(DstContainer, DstField, Src)              \
+  do {                                                                         \
+    auto V = (Src);                                                            \
+    assert(::llvm::isUIntN(                                                    \
+               LLVM_BITFIELD_WIDTH(decltype(DstContainer), DstField), V) &&    \
+           #Src " is too wide to fit inside " #DstContainer "::" #DstField);   \
+    DstContainer.DstField = V;                                                 \
+  } while (0)
+
+#endif
Index: llvm/unittests/Support/BitfieldsTest.cpp
===================================================================
--- /dev/null
+++ llvm/unittests/Support/BitfieldsTest.cpp
@@ -0,0 +1,65 @@
+//===- 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.h"
+
+namespace {
+
+TEST(BitfieldsTest, BitfieldWidth) {
+  struct S {
+    unsigned a : 1;
+    uint32_t b : 32;
+    uint64_t c : 64;
+    uint64_t d : 1;
+    uint64_t e : 10;
+  };
+  // Can't stick LLVM_BITFIELD_WIDTH macro into EXPECT_EQ -- the two macros'
+  // magics are at odds.
+  int V;
+  V = LLVM_BITFIELD_WIDTH(S, a);
+  EXPECT_EQ(1, V);
+  V = LLVM_BITFIELD_WIDTH(S, b);
+  EXPECT_EQ(32, V);
+  V = LLVM_BITFIELD_WIDTH(S, c);
+  EXPECT_EQ(64, V);
+  V = LLVM_BITFIELD_WIDTH(S, d);
+  EXPECT_EQ(1, V);
+  V = LLVM_BITFIELD_WIDTH(S, e);
+  EXPECT_EQ(10, V);
+}
+
+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);
+}
+
+} // 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