Index: utils/TableGen/FixedLenDecoderEmitter.cpp
===================================================================
--- utils/TableGen/FixedLenDecoderEmitter.cpp
+++ utils/TableGen/FixedLenDecoderEmitter.cpp
@@ -2067,21 +2067,59 @@
 // using the VS compiler. It has a bug which causes the function
 // to be optimized out in some circustances. See llvm.org/pr38292
 static void emitFieldFromInstruction(formatted_raw_ostream &OS) {
-  OS << "// Helper function for extracting fields from encoded instructions.\n"
-     << "template<typename InsnType>\n"
+  OS << "// Helper functions for extracting fields from encoded instructions.\n"
+     << "// InsnType must either be integral or an APInt-like object that "
+        "must:\n"
+     << "// * Have a static const max_size_in_bits equal to the number of bits "
+        "in the\n"
+     << "//   encoding.\n"
+     << "// * be default-constructible and copy-constructible\n"
+     << "// * be constructible from a uint64_t\n"
+     << "// * be constructible from an APInt (this can be private)\n"
+     << "// * Support getBitsSet(loBit, hiBit)\n"
+     << "// * be convertible to uint64_t\n"
+     << "// * Support the ~, &, ==, !=, and |= operators with other objects of "
+        "the same type\n"
+     << "// * Support shift (<<, >>) with signed and unsigned integers on the "
+        "RHS\n"
+     << "// * Support put (<<) to raw_ostream&\n"
      << "#if defined(_MSC_VER) && !defined(__clang__)\n"
      << "__declspec(noinline)\n"
      << "#endif\n"
-     << "static InsnType fieldFromInstruction(InsnType insn, unsigned startBit,\n"
+     << "template<typename InsnType>\n"
+     << "static InsnType fieldFromInstruction(InsnType insn, unsigned "
+        "startBit,\n"
+     << "                                     unsigned numBits, "
+        "std::true_type) {\n"
+     << "  assert(startBit + numBits <= (sizeof(InsnType) * 8) &&\n"
+     << "         \"Instruction field out of bounds!\");\n"
+     << "  InsnType fieldMask;\n"
+     << "  if (numBits == sizeof(InsnType) * 8)\n"
+     << "    fieldMask = (InsnType)(-1LL);\n"
+     << "  else\n"
+     << "    fieldMask = (((InsnType)1 << numBits) - 1) << startBit;\n"
+     << "  return (insn & fieldMask) >> startBit;\n"
+     << "}\n"
+     << "\n"
+     << "template<typename InsnType>\n"
+     << "static InsnType fieldFromInstruction(InsnType insn, unsigned "
+        "startBit,\n"
+     << "                                     unsigned numBits, "
+        "std::false_type) {\n"
+     << "  assert(startBit + numBits <= InsnType::max_size_in_bits && "
+        "\"Instruction field out of bounds!\");\n"
+     << "  InsnType fieldMask = InsnType::getBitsSet(0, numBits);\n"
+     << "  return (insn >> startBit) & fieldMask;\n"
+     << "}\n"
+     << "\n"
+     << "template<typename InsnType>\n"
+     << "static InsnType fieldFromInstruction(InsnType insn, unsigned "
+        "startBit,\n"
      << "                                     unsigned numBits) {\n"
-     << "    assert(startBit + numBits <= (sizeof(InsnType)*8) &&\n"
-     << "           \"Instruction field out of bounds!\");\n"
-     << "    InsnType fieldMask;\n"
-     << "    if (numBits == sizeof(InsnType)*8)\n"
-     << "      fieldMask = (InsnType)(-1LL);\n"
-     << "    else\n"
-     << "      fieldMask = (((InsnType)1 << numBits) - 1) << startBit;\n"
-     << "    return (insn & fieldMask) >> startBit;\n"
+     << "  assert(startBit + numBits <= 64 && \"Cannot support >64-bit "
+        "extractions!\");\n"
+     << "  return fieldFromInstruction(insn, startBit, numBits, "
+        "std::is_integral<InsnType>());\n"
      << "}\n\n";
 }