[NFC][TableGen] Change DecoderEmitter insertBits to only use integer types

The `insertBits` templated function generated by DecoderEmitter is
called with variable `tmp` of type `TmpType` which is:

```
using TmpType = std::conditional_t<std::is_integral<InsnType>::value, InsnType, uint64_t>;
```

That is, TmpType is always an integral type. Change the generated
`insertBits` to be valid only for integer types, and eliminate the
unused `insertBits` function from `DecoderUInt128` in AMDGPUDisassembler.h

Author: jurahul

llvm/lib/Target/AMDGPU/Disassembler/AMDGPUDisassembler.h

@@ -43,17 +43,6 @@ class DecoderUInt128 {
   DecoderUInt128() = default;
   DecoderUInt128(uint64_t Lo, uint64_t Hi = 0) : Lo(Lo), Hi(Hi) {}
   operator bool() const { return Lo || Hi; }
-  void insertBits(uint64_t SubBits, unsigned BitPosition, unsigned NumBits) {
-    assert(NumBits && NumBits <= 64);
-    assert(SubBits >> 1 >> (NumBits - 1) == 0);
-    assert(BitPosition < 128);
-    if (BitPosition < 64) {
-      Lo |= SubBits << BitPosition;
-      Hi |= SubBits >> 1 >> (63 - BitPosition);
-    } else {
-      Hi |= SubBits << (BitPosition - 64);
-    }
-  }
   uint64_t extractBitsAsZExtValue(unsigned NumBits,
                                   unsigned BitPosition) const {
     assert(NumBits && NumBits <= 64);

llvm/utils/TableGen/DecoderEmitter.cpp

@@ -2184,8 +2184,6 @@ static void emitFieldFromInstruction(formatted_raw_ostream &OS) {
 // Helper functions for extracting fields from encoded instructions.
 // InsnType must either be integral or an APInt-like object that must:
 // * be default-constructible and copy-constructible
-// * be constructible from an APInt (this can be private)
-// * Support insertBits(bits, startBit, numBits)
 // * Support extractBitsAsZExtValue(numBits, startBit)
 // * Support the ~, &, ==, and != operators with other objects of the same type
 // * Support the != and bitwise & with uint64_t
@@ -2221,17 +2219,17 @@ fieldFromInstruction(const InsnType &insn, unsigned startBit,
 static void emitInsertBits(formatted_raw_ostream &OS) {
   OS << R"(
 // Helper function for inserting bits extracted from an encoded instruction into
-// a field.
-template <typename InsnType>
-static void insertBits(InsnType &field, InsnType bits, unsigned startBit,
-                       unsigned numBits) {
-  if constexpr (std::is_integral<InsnType>::value) {
-    assert(startBit + numBits <= sizeof field * 8);
-    (void)numBits;
-    field |= (InsnType)bits << startBit;
-  } else {
-    field.insertBits(bits, startBit, numBits);
-  }
+// an integer-typed field.
+template <typename IntType>
+static std::enable_if_t<std::is_integral_v<IntType>, void>
+insertBits(IntType &field, IntType bits, unsigned startBit, unsigned numBits) {
+  // Check that no bit beyond numBits is set, so that a simple bitwise |
+  // is sufficient.
+  assert((~(((IntType)1 << numBits) - 1) & bits) == 0 &&
+           "bits has more than numBits bits set");
+  assert(startBit + numBits <= sizeof(IntType) * 8);
+  (void)numBits;
+  field |= bits << startBit;
 }
 )";
 }