Index: include/llvm/Bitcode/BitstreamWriter.h
===================================================================
--- include/llvm/Bitcode/BitstreamWriter.h
+++ include/llvm/Bitcode/BitstreamWriter.h
@@ -102,18 +102,13 @@
   /// Backpatch a 32-bit word in the output at the given bit offset
   /// with the specified value.
   void BackpatchWord(uint64_t BitNo, unsigned NewWord) {
+    using namespace llvm::support;
     unsigned ByteNo = BitNo / 8;
-    if ((BitNo & 7) == 0) {
-      // Already 8-bit aligned
-      support::endian::write32le(&Out[ByteNo], NewWord);
-    } else {
-      uint64_t CurDWord = support::endian::read64le(&Out[ByteNo]);
-      unsigned StartBit = BitNo & 7;
-      // Currently expect to backpatch 0-value placeholders.
-      assert(((CurDWord >> StartBit) & 0xffffffff) == 0);
-      CurDWord |= NewWord << StartBit;
-      support::endian::write64le(&Out[ByteNo], CurDWord);
-    }
+    assert((!endian::readAtBitAlignment<uint32_t, little, unaligned>(
+               &Out[ByteNo], BitNo & 7)) &&
+           "Expected to be patching over 0-value placeholders");
+    endian::writeAtBitAlignment<uint32_t, little, unaligned>(
+        &Out[ByteNo], NewWord, BitNo & 7);
   }
 
   void Emit(uint32_t Val, unsigned NumBits) {
Index: include/llvm/Support/Endian.h
===================================================================
--- include/llvm/Support/Endian.h
+++ include/llvm/Support/Endian.h
@@ -77,6 +77,81 @@
          &value,
          sizeof(value_type));
 }
+
+/// Read a value of a particular endianness from memory, for a location
+/// that starts at the given bit offset within the first byte.
+template <typename value_type, endianness endian, std::size_t alignment>
+inline value_type readAtBitAlignment(const void *memory, uint64_t startBit) {
+  assert(startBit < 8);
+  if (startBit == 0)
+    return read<value_type, endian, alignment>(memory);
+  else {
+    // Read two values and compose the result from them.
+    value_type val[2];
+    memcpy(&val[0],
+           LLVM_ASSUME_ALIGNED(
+               memory, (detail::PickAlignment<value_type, alignment>::value)),
+           sizeof(value_type) * 2);
+    val[0] = byte_swap<value_type, endian>(val[0]);
+    val[1] = byte_swap<value_type, endian>(val[1]);
+
+    // Shift bits from the lower value into place.
+    unsigned lowerVal = val[0] >> startBit;
+    // Mask off upper bits after right shift in case of signed type.
+    unsigned numBitsFirstVal = (sizeof(value_type) * 8) - startBit;
+    lowerVal &= (1 << numBitsFirstVal) - 1;
+
+    // Get the bits from the upper value.
+    unsigned upperVal = val[1] & ((1 << startBit) - 1);
+    // Shift them in to place.
+    upperVal <<= numBitsFirstVal;
+
+    return lowerVal | upperVal;
+  }
+}
+
+/// Write a value to memory with a particular endianness, for a location
+/// that starts at the given bit offset within the first byte.
+template <typename value_type, endianness endian, std::size_t alignment>
+inline void writeAtBitAlignment(void *memory, value_type value,
+                                uint64_t startBit) {
+  assert(startBit < 8);
+  if (startBit == 0)
+    write<value_type, endian, alignment>(memory, value);
+  else {
+    // Read two values and shift the result into them.
+    value_type val[2];
+    memcpy(&val[0],
+           LLVM_ASSUME_ALIGNED(
+               memory, (detail::PickAlignment<value_type, alignment>::value)),
+           sizeof(value_type) * 2);
+    val[0] = byte_swap<value_type, endian>(val[0]);
+    val[1] = byte_swap<value_type, endian>(val[1]);
+
+    // Mask off any existing bits in the upper part of the lower value that
+    // we want to replace.
+    val[0] &= (1 << startBit) - 1;
+    // Now shift in the new bits
+    val[0] |= value << startBit;
+
+    // Mask off any existing bits in the lower part of the upper value that
+    // we want to replace.
+    val[1] &= ~((1 << startBit) - 1);
+    // Next shift the bits that go into the upper value into position.
+    unsigned numBitsFirstVal = (sizeof(value_type) * 8) - startBit;
+    unsigned upperVal = value >> numBitsFirstVal;
+    // Mask off upper bits after right shift in case of signed type.
+    upperVal &= (1 << startBit) - 1;
+    val[1] |= upperVal;
+
+    // Finally, rewrite values.
+    val[0] = byte_swap<value_type, endian>(val[0]);
+    val[1] = byte_swap<value_type, endian>(val[1]);
+    memcpy(LLVM_ASSUME_ALIGNED(
+               memory, (detail::PickAlignment<value_type, alignment>::value)),
+           &val[0], sizeof(value_type) * 2);
+  }
+}
 } // end namespace endian
 
 namespace detail {