[TableGen][DecoderEmitter] Factor populateFixedLenEncoding (NFC) (#154511)

Also drop the debug code under `#if 0` and a seemingly outdated comment.

Author: s-barannikov

llvm/utils/TableGen/DecoderEmitter.cpp

@@ -156,7 +156,14 @@ class InstructionEncoding {
       Name = (EncodingDef->getName() + Twine(':')).str();
     Name.append(InstDef->getName());
 
-    BitWidth = populateEncoding();
+    StringRef DecoderMethod = EncodingDef->getValueAsString("DecoderMethod");
+    if (!DecoderMethod.empty()) {
+      bool HasCompleteDecoder =
+          EncodingDef->getValueAsBit("hasCompleteDecoder");
+      Operands.push_back(OperandInfo(DecoderMethod.str(), HasCompleteDecoder));
+    }
+
+    populateEncoding();
   }
 
   /// Returns the Record this encoding originates from.
@@ -175,9 +182,9 @@ class InstructionEncoding {
   ArrayRef<OperandInfo> getOperands() const { return Operands; }
 
 private:
-  void populateVarLenEncoding();
-
-  unsigned populateEncoding();
+  void populateVarLenEncoding(const VarLenInst &VLI);
+  void populateFixedLenEncoding(const BitsInit &Bits);
+  void populateEncoding();
 };
 
 typedef std::vector<uint32_t> FixupList;
@@ -1862,9 +1869,7 @@ OperandInfo getOpInfo(const Record *TypeRecord) {
   return OperandInfo(findOperandDecoderMethod(TypeRecord), HasCompleteDecoder);
 }
 
-void InstructionEncoding::populateVarLenEncoding() {
-  const RecordVal *RV = EncodingDef->getValue("Inst");
-  VarLenInst VLI(cast<DagInit>(RV->getValue()), RV);
+void InstructionEncoding::populateVarLenEncoding(const VarLenInst &VLI) {
   SmallVector<int> TiedTo;
 
   for (const auto &[Idx, Op] : enumerate(Inst->Operands)) {
@@ -1961,28 +1966,9 @@ static void addOneOperandFields(const Record *EncodingDef, const BitsInit &Bits,
   }
 }
 
-unsigned InstructionEncoding::populateEncoding() {
-  bool IsVarLenInst = isa<DagInit>(EncodingDef->getValueInit("Inst"));
+void InstructionEncoding::populateFixedLenEncoding(const BitsInit &Bits) {
   const Record &Def = *Inst->TheDef;
 
-  const BitsInit &Bits = getBitsField(*EncodingDef, "Inst");
-  assert(!Bits.allInComplete() &&
-         "Invalid encodings should have been filtered out");
-
-  // If the instruction has specified a custom decoding hook, use that instead
-  // of trying to auto-generate the decoder.
-  StringRef InstDecoder = EncodingDef->getValueAsString("DecoderMethod");
-  if (!InstDecoder.empty()) {
-    bool HasCompleteInstDecoder =
-        EncodingDef->getValueAsBit("hasCompleteDecoder");
-    Operands.push_back(OperandInfo(InstDecoder.str(), HasCompleteInstDecoder));
-    return Bits.getNumBits();
-  }
-
-  // Generate a description of the operand of the instruction that we know
-  // how to decode automatically.
-  // FIXME: We'll need to have a way to manually override this as needed.
-
   // Gather the outputs/inputs of the instruction, so we can find their
   // positions in the encoding.  This assumes for now that they appear in the
   // MCInst in the order that they're listed.
@@ -2015,108 +2001,100 @@ unsigned InstructionEncoding::populateEncoding() {
     }
   }
 
-  if (IsVarLenInst) {
-    populateVarLenEncoding();
-  } else {
-    // For each operand, see if we can figure out where it is encoded.
-    for (const auto &Op : InOutOperands) {
-      const Init *OpInit = Op.first;
-      StringRef OpName = Op.second;
-
-      // We're ready to find the instruction encoding locations for this
-      // operand.
-
-      // First, find the operand type ("OpInit"), and sub-op names
-      // ("SubArgDag") if present.
-      const DagInit *SubArgDag = dyn_cast<DagInit>(OpInit);
-      if (SubArgDag)
-        OpInit = SubArgDag->getOperator();
-      const Record *OpTypeRec = cast<DefInit>(OpInit)->getDef();
-      // Lookup the sub-operands from the operand type record (note that only
-      // Operand subclasses have MIOperandInfo, see CodeGenInstruction.cpp).
-      const DagInit *SubOps = OpTypeRec->isSubClassOf("Operand")
-                                  ? OpTypeRec->getValueAsDag("MIOperandInfo")
-                                  : nullptr;
-
-      // Lookup the decoder method and construct a new OperandInfo to hold our
-      // result.
-      OperandInfo OpInfo = getOpInfo(OpTypeRec);
-
-      // If we have named sub-operands...
-      if (SubArgDag) {
-        // Then there should not be a custom decoder specified on the top-level
-        // type.
-        if (!OpInfo.Decoder.empty()) {
-          PrintError(EncodingDef,
-                     "DecoderEmitter: operand \"" + OpName + "\" has type \"" +
-                         OpInit->getAsString() +
-                         "\" with a custom DecoderMethod, but also named "
-                         "sub-operands.");
-          continue;
-        }
-
-        // Decode each of the sub-ops separately.
-        assert(SubOps && SubArgDag->getNumArgs() == SubOps->getNumArgs());
-        for (const auto &[I, Arg] : enumerate(SubOps->getArgs())) {
-          StringRef SubOpName = SubArgDag->getArgNameStr(I);
-          OperandInfo SubOpInfo = getOpInfo(cast<DefInit>(Arg)->getDef());
-
-          addOneOperandFields(EncodingDef, Bits, TiedNames, SubOpName,
-                              SubOpInfo);
-          Operands.push_back(std::move(SubOpInfo));
-        }
+  // For each operand, see if we can figure out where it is encoded.
+  for (const auto &Op : InOutOperands) {
+    const Init *OpInit = Op.first;
+    StringRef OpName = Op.second;
+
+    // We're ready to find the instruction encoding locations for this
+    // operand.
+
+    // First, find the operand type ("OpInit"), and sub-op names
+    // ("SubArgDag") if present.
+    const DagInit *SubArgDag = dyn_cast<DagInit>(OpInit);
+    if (SubArgDag)
+      OpInit = SubArgDag->getOperator();
+    const Record *OpTypeRec = cast<DefInit>(OpInit)->getDef();
+    // Lookup the sub-operands from the operand type record (note that only
+    // Operand subclasses have MIOperandInfo, see CodeGenInstruction.cpp).
+    const DagInit *SubOps = OpTypeRec->isSubClassOf("Operand")
+                                ? OpTypeRec->getValueAsDag("MIOperandInfo")
+                                : nullptr;
+
+    // Lookup the decoder method and construct a new OperandInfo to hold our
+    // result.
+    OperandInfo OpInfo = getOpInfo(OpTypeRec);
+
+    // If we have named sub-operands...
+    if (SubArgDag) {
+      // Then there should not be a custom decoder specified on the top-level
+      // type.
+      if (!OpInfo.Decoder.empty()) {
+        PrintError(EncodingDef,
+                   "DecoderEmitter: operand \"" + OpName + "\" has type \"" +
+                       OpInit->getAsString() +
+                       "\" with a custom DecoderMethod, but also named "
+                       "sub-operands.");
         continue;
       }
 
-      // Otherwise, if we have an operand with sub-operands, but they aren't
-      // named...
-      if (SubOps && OpInfo.Decoder.empty()) {
-        // If it's a single sub-operand, and no custom decoder, use the decoder
-        // from the one sub-operand.
-        if (SubOps->getNumArgs() == 1)
-          OpInfo = getOpInfo(cast<DefInit>(SubOps->getArg(0))->getDef());
-
-        // If we have multiple sub-ops, there'd better have a custom
-        // decoder. (Otherwise we don't know how to populate them properly...)
-        if (SubOps->getNumArgs() > 1) {
-          PrintError(EncodingDef,
-                     "DecoderEmitter: operand \"" + OpName +
-                         "\" uses MIOperandInfo with multiple ops, but doesn't "
-                         "have a custom decoder!");
-          debugDumpRecord(*EncodingDef);
-          continue;
-        }
-      }
+      // Decode each of the sub-ops separately.
+      assert(SubOps && SubArgDag->getNumArgs() == SubOps->getNumArgs());
+      for (const auto &[I, Arg] : enumerate(SubOps->getArgs())) {
+        StringRef SubOpName = SubArgDag->getArgNameStr(I);
+        OperandInfo SubOpInfo = getOpInfo(cast<DefInit>(Arg)->getDef());
 
-      addOneOperandFields(EncodingDef, Bits, TiedNames, OpName, OpInfo);
-      // FIXME: it should be an error not to find a definition for a given
-      // operand, rather than just failing to add it to the resulting
-      // instruction! (This is a longstanding bug, which will be addressed in an
-      // upcoming change.)
-      if (OpInfo.numFields() > 0)
-        Operands.push_back(std::move(OpInfo));
+        addOneOperandFields(EncodingDef, Bits, TiedNames, SubOpName, SubOpInfo);
+        Operands.push_back(std::move(SubOpInfo));
+      }
+      continue;
     }
-  }
-
-#if 0
-  LLVM_DEBUG({
-      // Dumps the instruction encoding bits.
-      dumpBits(errs(), Bits);
 
-      errs() << '\n';
-
-      // Dumps the list of operand info.
-      for (unsigned i = 0, e = Inst->Operands.size(); i != e; ++i) {
-        const CGIOperandList::OperandInfo &Info = Inst->Operands[i];
-        const std::string &OperandName = Info.Name;
-        const Record &OperandDef = *Info.Rec;
-
-        errs() << "\t" << OperandName << " (" << OperandDef.getName() << ")\n";
+    // Otherwise, if we have an operand with sub-operands, but they aren't
+    // named...
+    if (SubOps && OpInfo.Decoder.empty()) {
+      // If it's a single sub-operand, and no custom decoder, use the decoder
+      // from the one sub-operand.
+      if (SubOps->getNumArgs() == 1)
+        OpInfo = getOpInfo(cast<DefInit>(SubOps->getArg(0))->getDef());
+
+      // If we have multiple sub-ops, there'd better have a custom
+      // decoder. (Otherwise we don't know how to populate them properly...)
+      if (SubOps->getNumArgs() > 1) {
+        PrintError(EncodingDef,
+                   "DecoderEmitter: operand \"" + OpName +
+                       "\" uses MIOperandInfo with multiple ops, but doesn't "
+                       "have a custom decoder!");
+        debugDumpRecord(*EncodingDef);
+        continue;
       }
-    });
-#endif
+    }
+
+    addOneOperandFields(EncodingDef, Bits, TiedNames, OpName, OpInfo);
+    // FIXME: it should be an error not to find a definition for a given
+    // operand, rather than just failing to add it to the resulting
+    // instruction! (This is a longstanding bug, which will be addressed in an
+    // upcoming change.)
+    if (OpInfo.numFields() > 0)
+      Operands.push_back(std::move(OpInfo));
+  }
+}
 
-  return Bits.getNumBits();
+void InstructionEncoding::populateEncoding() {
+  const RecordVal *InstField = EncodingDef->getValue("Inst");
+  if (const auto *DI = dyn_cast<DagInit>(InstField->getValue())) {
+    VarLenInst VLI(DI, InstField);
+    BitWidth = VLI.size();
+    // If the encoding has a custom decoder, don't bother parsing the operands.
+    if (Operands.empty())
+      populateVarLenEncoding(VLI);
+  } else {
+    const auto *BI = cast<BitsInit>(InstField->getValue());
+    BitWidth = BI->getNumBits();
+    // If the encoding has a custom decoder, don't bother parsing the operands.
+    if (Operands.empty())
+      populateFixedLenEncoding(*BI);
+  }
 }
 
 // emitFieldFromInstruction - Emit the templated helper function