[llvm] [TableGen][DecoderEmitter] Add option to emit type-specialized code (PR #146593)

[Sergei Barannikov via llvm-commits]

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@@ -2562,53 +2764,143 @@ namespace {
     }
   }
 
+  bool GenerateTemplated =
+      Target.getInstructionSet()->getValueAsBit("GenerateTemplatedDecoder");
+
+  // For variable instruction, we emit a instruction length table to let the
+  // decoder know how long the instructions are. You can see example usage in
+  // M68k's disassembler.
+  if (IsVarLenInst) {
+    if (GenerateTemplated)
+      PrintFatalError(
+          "Templated decoder not needed for variable length instruction");
+    emitInstrLenTable(OS, InstrLen);
+  }
+
   DecoderTableInfo TableInfo;
+  bool HasCheckPredicate = false;
   unsigned OpcodeMask = 0;
-  for (const auto &[NSAndByteSize, EncodingIDs] : EncMap) {
-    const std::string &DecoderNamespace = NSAndByteSize.first;
-    const unsigned BitWidth = 8 * NSAndByteSize.second;
-    // Emit the decoder for this namespace+width combination.
-    FilterChooser FC(NumberedEncodings, EncodingIDs, Operands,
-                     IsVarLenInst ? MaxInstLen : BitWidth, this);
-
-    // The decode table is cleared for each top level decoder function. The
-    // predicates and decoders themselves, however, are shared across all
-    // decoders to give more opportunities for uniqueing.
-    TableInfo.Table.clear();
-    TableInfo.pushScope();
-    FC.emitTableEntries(TableInfo);
-    // Any NumToSkip fixups in the top level scope can resolve to the
-    // OPC_Fail at the end of the table.
-    assert(TableInfo.isOutermostScope() && "fixup stack phasing error!");
-    TableInfo.popScope();
 
-    TableInfo.Table.push_back(MCD::OPC_Fail);
+  // Helper lambda to emit the decoder code for a given instruction Bitwidth
+  // and associated C++ type. If `Bitwidth` is 0 (and CPPType is empty) it will
+  // generate templated decoder code.
+  auto emitDecoder = [&](const CPPType &Type, StringRef Suffix) {
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s-barannikov wrote:

This should be a function/method.


https://github.com/llvm/llvm-project/pull/146593