use ref to avoid copy in range for-loop

[XinWang10]

Summary

Use big obj copy in range for-loop will call copy constructor every time,
which can be avoided by use ref instead.

[skan]

Could you help illustrate why "they're big obj copys"?

[XinWang10]

llvm/include/llvm/MC/MCParser/MCAsmParser.h

239

Every element here is type MCPendingError defined in MCAsmPaser.h, It contains 1 SMLoc(has 1 internal pointer as member), 1 SmallString, 1 SMRange(composed of 2 SMLoc).

struct MCPendingError {
    SMLoc Loc;
    SmallString<64> Msg;
    SMRange Range;
  };

SmallVector<MCPendingError, 0> PendingErrors;

This enumerated element has 4 pointers which is bigger than 64bit, I think it's better here to use ref.

llvm/lib/CodeGen/LiveDebugValues/VarLocBasedImpl.cpp

1315

The enumerated type is InstToEntryLocMap, every element is a pair of MachineInstr * and LocIndex(composed of 2 uint32_t.

using InstToEntryLocMap = std::multimap<const MachineInstr *, LocIndex>;

llvm/lib/CodeGen/MIRParser/MIRParser.cpp

405

YamlMF.CallSitesInfo is type CallSiteInfoMap, defined in MachineFunction.h line 452.

  struct ArgRegPair {
    Register Reg;
    uint16_t ArgNo;
    ArgRegPair(Register R, unsigned Arg) : Reg(R), ArgNo(Arg) {
      assert(Arg < (1 << 16) && "Arg out of range");
    }
  };
using CallSiteInfo = SmallVector<ArgRegPair, 1>;
...
using CallSiteInfoMap = DenseMap<const MachineInstr *, CallSiteInfo>;

Every element here is of type CallSiteInfo which contains a vector.

llvm/lib/ObjectYAML/MachOEmitter.cpp

429

The Opcode here is type MachOYAML::BindOpcode, defined in MachOYAML.h line 98.

struct BindOpcode {
  MachO::BindOpcode Opcode;
  uint8_t Imm;
  std::vector<yaml::Hex64> ULEBExtraData;
  std::vector<int64_t> SLEBExtraData;
  StringRef Symbol;
};

llvm/utils/TableGen/GlobalISel/GIMatchTree.cpp

736

It enumerated NewLeaves which is type GIMatchTreeBuilder::LeafVec and TraversedEdgesByNewLeaves.
In GIMatchTree.h

using LeafVec = std::vector<GIMatchTreeBuilderLeafInfo>;

and contains to many elements.

class GIMatchTreeBuilderLeafInfo {
protected:
  GIMatchTreeBuilder &Builder;
  GIMatchTreeLeafInfo Info;
  const GIMatchDag &MatchDag;
  /// The association between GIMatchDagInstr* and GIMatchTreeInstrInfo.
  /// The primary reason for this members existence is to allow the use of
  /// InstrIDToInfo.lookup() since that requires that the value is
  /// default-constructible.
  DenseMap<const GIMatchDagInstr *, GIMatchTreeInstrInfo> InstrNodeToInfo;
  /// The instruction information for a given ID in the context of this
  /// particular leaf.
  DenseMap<unsigned, GIMatchTreeInstrInfo *> InstrIDToInfo;
......

here I is a tuple composed of GIMatchTreeBuilderLeafInfo and BitVector, must be a big obj.

[skan]

LGTM

llvm/include/llvm/MC/MCParser/MCAsmParser.h

public:
   /// clients.
   virtual bool printError(SMLoc L, const Twine &Msg,
                           SMRange Range = std::nullopt) = 0;
 
   bool hasPendingError() { return !PendingErrors.empty(); }
 
   bool printPendingErrors() {
     bool rv = !PendingErrors.empty();
-    for (auto Err : PendingErrors) {
+    for (auto &Err : PendingErrors) {
       printError(Err.Loc, Twine(Err.Msg), Err.Range);
     }
     PendingErrors.clear();
     return rv;
   }
 
   void clearPendingErrors() { PendingErrors.clear(); }
 

llvm/lib/CodeGen/LiveDebugValues/VarLocBasedImpl.cpp

 /// Transfer, which uses the to-be-deleted \p EntryVL.
 void VarLocBasedLDV::cleanupEntryValueTransfers(
     const MachineInstr *TRInst, OpenRangesSet &OpenRanges, VarLocMap &VarLocIDs,
     const VarLoc &EntryVL, InstToEntryLocMap &EntryValTransfers) {
   if (EntryValTransfers.empty() || TRInst == nullptr)
     return;
 
   auto TransRange = EntryValTransfers.equal_range(TRInst);
-  for (auto TDPair : llvm::make_range(TransRange.first, TransRange.second)) {
+  for (auto &TDPair : llvm::make_range(TransRange.first, TransRange.second)) {
     const VarLoc &EmittedEV = VarLocIDs[TDPair.second];
     if (std::tie(EntryVL.Var, EntryVL.Locs[0].Value.RegNo, EntryVL.Expr) ==
         std::tie(EmittedEV.Var, EmittedEV.Locs[0].Value.RegNo,
                  EmittedEV.Expr)) {
       OpenRanges.erase(EmittedEV);
       EntryValTransfers.erase(TRInst);
       break;
     }

llvm/lib/CodeGen/MIRParser/MIRParser.cpp

if (MRI.getNumVirtRegs() == 0)
     Properties.set(MachineFunctionProperties::Property::NoVRegs);
 }
 
 bool MIRParserImpl::initializeCallSiteInfo(
     PerFunctionMIParsingState &PFS, const yaml::MachineFunction &YamlMF) {
   MachineFunction &MF = PFS.MF;
   SMDiagnostic Error;
   const LLVMTargetMachine &TM = MF.getTarget();
-  for (auto YamlCSInfo : YamlMF.CallSitesInfo) {
+  for (auto &YamlCSInfo : YamlMF.CallSitesInfo) {
     yaml::CallSiteInfo::MachineInstrLoc MILoc = YamlCSInfo.CallLocation;
     if (MILoc.BlockNum >= MF.size())
       return error(Twine(MF.getName()) +
                    Twine(" call instruction block out of range.") +
                    " Unable to reference bb:" + Twine(MILoc.BlockNum));
     auto CallB = std::next(MF.begin(), MILoc.BlockNum);
     if (MILoc.Offset >= CallB->size())
       return error(Twine(MF.getName()) +

llvm/lib/ObjectYAML/MachOEmitter.cpp

case MachO::LC_SEGMENT_64:
       }
     }
   }
 }
 
 void MachOWriter::writeBindOpcodes(
     raw_ostream &OS, std::vector<MachOYAML::BindOpcode> &BindOpcodes) {
 
-  for (auto Opcode : BindOpcodes) {
+  for (auto &Opcode : BindOpcodes) {
     uint8_t OpByte = Opcode.Opcode | Opcode.Imm;
     OS.write(reinterpret_cast<char *>(&OpByte), 1);
     for (auto Data : Opcode.ULEBExtraData) {
       encodeULEB128(Data, OS);
     }
     for (auto Data : Opcode.SLEBExtraData) {
       encodeSLEB128(Data, OS);
     }

llvm/utils/TableGen/GlobalISel/GIMatchTree.cpp

void GIMatchTreeVRegDefPartitioner::applyForPartition(
 
   // Nothing to do. The only thing we know is that it isn't a vreg-def.
   if (PartitionToResult[PartitionIdx] == false)
     return;
 
   NewInstrID = SubBuilder.allocInstrID();
 
   GIMatchTreeBuilder::LeafVec &NewLeaves = SubBuilder.getPossibleLeaves();
-  for (const auto I : zip(NewLeaves, TraversedEdgesByNewLeaves)) {
+  for (const auto &I : zip(NewLeaves, TraversedEdgesByNewLeaves)) {
     auto &Leaf = std::get<0>(I);
     auto &TraversedEdgesForLeaf = std::get<1>(I);
     GIMatchTreeInstrInfo *InstrInfo = Leaf.getInstrInfo(InstrID);
     // Skip any leaves that don't care about this instruction.
     if (!InstrInfo)
       continue;
     for (unsigned EIdx : TraversedEdgesForLeaf.set_bits()) {
       const GIMatchDagEdge *E = Leaf.getEdge(EIdx);