[TableGen][DecoderEmitter] Returns insn_t / std::vector<Islands> by value (NFC) (#153354)

The containers passed by reference are always empty on entry to the
functions that fill them. Return them by value instead and let the
compiler do the return value optimization.

Author: s-barannikov

llvm/utils/TableGen/DecoderEmitter.cpp

@@ -533,10 +533,10 @@ class FilterChooser {
 
 protected:
   // Populates the insn given the uid.
-  void insnWithID(insn_t &Insn, unsigned Opcode) const {
+  insn_t insnWithID(unsigned Opcode) const {
     const Record *EncodingDef = AllInstructions[Opcode].EncodingDef;
     const BitsInit &Bits = getBitsField(*EncodingDef, "Inst");
-    Insn.resize(std::max(BitWidth, Bits.getNumBits()), BitValue::BIT_UNSET);
+    insn_t Insn(std::max(BitWidth, Bits.getNumBits()), BitValue::BIT_UNSET);
     // We may have a SoftFail bitmask, which specifies a mask where an encoding
     // may differ from the value in "Inst" and yet still be valid, but the
     // disassembler should return SoftFail instead of Success.
@@ -550,6 +550,7 @@ class FilterChooser {
       else
         Insn[i] = BitValue(Bits, i);
     }
+    return Insn;
   }
 
   // Populates the field of the insn given the start position and the number of
@@ -582,7 +583,7 @@ class FilterChooser {
   // This returns a list of undecoded bits of an instructions, for example,
   // Inst{20} = 1 && Inst{3-0} == 0b1111 represents two islands of yet-to-be
   // decoded bits in order to verify that the instruction matches the Opcode.
-  unsigned getIslands(std::vector<Island> &Islands, const insn_t &Insn) const;
+  std::vector<Island> getIslands(const insn_t &Insn) const;
 
   // Emits code to check the Predicates member of an instruction are true.
   // Returns true if predicate matches were emitted, false otherwise.
@@ -658,10 +659,8 @@ Filter::Filter(const FilterChooser &owner, unsigned startBit, unsigned numBits)
   LastOpcFiltered = {0, 0};
 
   for (const auto &OpcPair : Owner.Opcodes) {
-    insn_t Insn;
-
     // Populates the insn given the uid.
-    Owner.insnWithID(Insn, OpcPair.EncodingID);
+    insn_t Insn = Owner.insnWithID(OpcPair.EncodingID);
 
     // Scans the segment for possibly well-specified encoding bits.
     auto [Ok, Field] = Owner.fieldFromInsn(Insn, StartBit, NumBits);
@@ -1162,8 +1161,9 @@ void FilterChooser::dumpStack(raw_ostream &OS, const char *prefix) const {
 // This returns a list of undecoded bits of an instructions, for example,
 // Inst{20} = 1 && Inst{3-0} == 0b1111 represents two islands of yet-to-be
 // decoded bits in order to verify that the instruction matches the Opcode.
-unsigned FilterChooser::getIslands(std::vector<Island> &Islands,
-                                   const insn_t &Insn) const {
+std::vector<FilterChooser::Island>
+FilterChooser::getIslands(const insn_t &Insn) const {
+  std::vector<Island> Islands;
   uint64_t FieldVal;
   unsigned StartBit;
 
@@ -1203,7 +1203,7 @@ unsigned FilterChooser::getIslands(std::vector<Island> &Islands,
   if (State == 2)
     Islands.push_back({StartBit, BitWidth - StartBit, FieldVal});
 
-  return Islands.size();
+  return Islands;
 }
 
 bool FilterChooser::emitBinaryParser(raw_ostream &OS, indent Indent,
@@ -1452,12 +1452,10 @@ void FilterChooser::emitSoftFailTableEntry(DecoderTableInfo &TableInfo,
 // Emits table entries to decode the singleton.
 void FilterChooser::emitSingletonTableEntry(DecoderTableInfo &TableInfo,
                                             EncodingIDAndOpcode Opc) const {
-  std::vector<Island> Islands;
-  insn_t Insn;
-  insnWithID(Insn, Opc.EncodingID);
+  insn_t Insn = insnWithID(Opc.EncodingID);
 
   // Look for islands of undecoded bits of the singleton.
-  getIslands(Islands, Insn);
+  std::vector<Island> Islands = getIslands(Insn);
 
   // Emit the predicate table entry if one is needed.
   emitPredicateTableEntry(TableInfo, Opc.EncodingID);
@@ -1569,13 +1567,11 @@ bool FilterChooser::filterProcessor(bool AllowMixed, bool Greedy) {
     assert(numInstructions == 3);
 
     for (const auto &Opcode : Opcodes) {
-      std::vector<Island> Islands;
-      insn_t Insn;
-
-      insnWithID(Insn, Opcode.EncodingID);
+      insn_t Insn = insnWithID(Opcode.EncodingID);
 
       // Look for islands of undecoded bits of any instruction.
-      if (getIslands(Islands, Insn) > 0) {
+      std::vector<Island> Islands = getIslands(Insn);
+      if (!Islands.empty()) {
         // Found an instruction with island(s).  Now just assign a filter.
         runSingleFilter(Islands[0].StartBit, Islands[0].NumBits);
         return true;
@@ -1611,9 +1607,7 @@ bool FilterChooser::filterProcessor(bool AllowMixed, bool Greedy) {
       bitAttrs[BitIndex] = ATTR_FILTERED;
 
   for (const auto &OpcPair : Opcodes) {
-    insn_t insn;
-
-    insnWithID(insn, OpcPair.EncodingID);
+    insn_t insn = insnWithID(OpcPair.EncodingID);
 
     for (BitIndex = 0; BitIndex < BitWidth; ++BitIndex) {
       switch (bitAttrs[BitIndex]) {