[llvm] [TableGen][Decoder] Decode operands with zero width or all bits known (PR #156358)

[Sam Elliott via llvm-commits]

================
@@ -1058,11 +1070,23 @@ FilterChooser::getIslands(const KnownBits &EncodingBits) const {
 
 void DecoderTableBuilder::emitBinaryParser(raw_ostream &OS, indent Indent,
                                            const OperandInfo &OpInfo) const {
-  bool UseInsertBits = OpInfo.numFields() != 1 || OpInfo.InitValue != 0;
+  // Special case for 'bits<0>'.
+  if (OpInfo.Fields.empty() && !OpInfo.InitValue) {
+    if (IgnoreNonDecodableOperands)
+      return;
+    assert(!OpInfo.Decoder.empty());
+    OS << Indent << OpInfo.Decoder << "(MI, Decoder);\n";
+    return;
+  }
----------------
lenary wrote:

Can we always pass `0` to the decoder function, for the encoded bits?

I'm not sure it would be entirely obvious that 0-width operands have a different decoder signature, and we already don't really document the decoder signature.

https://github.com/llvm/llvm-project/pull/156358