[llvm] [LLVM][MC][DecoderEmitter] Fail fatally if `Insn` and decoder table bitwidths mismatch (PR #156734)

[Sergei Barannikov via llvm-commits]

================
@@ -2108,9 +2121,19 @@ static DecodeStatus decodeInstruction(const uint8_t DecodeTable[], MCInst &MI,
   if (HasCheckPredicate)
     OS << "  const FeatureBitset &Bits = STI.getFeatureBits();\n";
   OS << "  using namespace llvm::MCD;\n";
+  OS << "  const uint8_t *Ptr = DecodeTable;\n";
+
+  if (SpecializeDecodersPerBitwidth) {
+    // Fail with a fatal error if decoder table's bitwidth does not match
+    // `InsnType` bitwidth.
+    OS << R"(
+  uint32_t BitWidth = decodeULEB128AndIncUnsafe(Ptr);
+  if (InsnBitWidth<InsnType> != BitWidth)
+    llvm_unreachable("Table and instruction bitwidth mismatch");
----------------
s-barannikov wrote:

I guess it is more or less the same as:

```suggestion
  assert(InsnBitWidth<InsnType> == BitWidth &&
         "Table and instruction bitwidth mismatch")`
```

(This will require `BitWidth` to be annotated with `[[maybe_unused]]`.)


https://github.com/llvm/llvm-project/pull/156734