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

[Sergei Barannikov via llvm-commits]

================
@@ -2630,57 +2789,121 @@ 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;
-  unsigned OpcodeMask = 0;
-  for (const auto &[NSAndByteSize, EncodingIDs] : OpcMap) {
-    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.FixupStack.clear();
-    TableInfo.FixupStack.emplace_back();
-    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.FixupStack.size() == 1 && "fixup stack phasing error!");
-    // Resolve any NumToSkip fixups in the current scope.
-    resolveTableFixups(TableInfo.Table, TableInfo.FixupStack.back(),
-                       TableInfo.Table.size());
-    TableInfo.FixupStack.clear();
+  bool HasCheckPredicate = false;
+
+  // 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) {
+    // Reset the Decoders for each non-templated type.
+    TableInfo.Decoders.clear();
+    unsigned OpcodeMask = 0;
+
+    const bool IsTemplate = Type.Kind == CPPType::TemplateTy;
+    if (!IsTemplate) {
+      OS << "// ------------------------------------------------------------\n";
+      OS << "// Decoder tables for " << Type.Bitwidth << "-bit instructions.\n";
+      OS << "// Using C++ type `" << Type.getName() << "` for payload.\n\n";
+    }
 
-    TableInfo.Table.push_back(MCD::OPC_Fail);
+    for (const auto &[NSAndByteSize, EncodingIDs] : OpcMap) {
+      const std::string &DecoderNamespace = NSAndByteSize.first;
+      const unsigned InstrBitwidth =
+          IsVarLenInst ? MaxInstLen : 8 * NSAndByteSize.second;
 
-    // Print the table to the output stream.
-    OpcodeMask |= emitTable(OS, TableInfo.Table, FC.getBitWidth(),
-                            DecoderNamespace, EncodingIDs);
-  }
+      if (!IsTemplate && InstrBitwidth != Type.Bitwidth)
+        continue;
 
-  // 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)
-    emitInstrLenTable(OS, InstrLen);
+      // Emit the decoder table for this namespace+ bitwidth combination.
+      FilterChooser FC(NumberedEncodings, EncodingIDs, Operands,
+                       IsVarLenInst ? MaxInstLen : InstrBitwidth, this);
+
+      // FIXME: Update this comment.
----------------
s-barannikov wrote:

Can it be updated now?

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