[llvm] [LLVM][DecoderEmitter] Add option to use lambdas in decodeToMCInst (PR #144814)

[Rahul Joshi via llvm-commits]

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@@ -1082,15 +1089,47 @@ void DecoderEmitter::emitDecoderFunction(formatted_raw_ostream &OS,
      << "using TmpType = "
         "std::conditional_t<std::is_integral<InsnType>::"
         "value, InsnType, uint64_t>;\n";
-  OS << Indent << "TmpType tmp;\n";
-  OS << Indent << "switch (Idx) {\n";
-  OS << Indent << "default: llvm_unreachable(\"Invalid index!\");\n";
-  for (const auto &[Index, Decoder] : enumerate(Decoders)) {
-    OS << Indent << "case " << Index << ":\n";
-    OS << Decoder;
-    OS << Indent + 2 << "return S;\n";
+
+  if (UseLambdaInDecodetoMCInst) {
+    // Emit one lambda for each case first.
+    for (const auto &[Index, Decoder] : enumerate(Decoders)) {
+      OS << Indent << "auto decodeLambda" << Index << " = [](DecodeStatus S,\n"
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jurahul wrote:

Switch to static functions and table of function pointers (instead of function_ref) so that it can be constexpr as well.

https://github.com/llvm/llvm-project/pull/144814