diff --git a/llvm/tools/bugpoint/BugDriver.h b/llvm/tools/bugpoint/BugDriver.h
index e3117ec0ed536..cc127bdf23d6e 100644
--- a/llvm/tools/bugpoint/BugDriver.h
+++ b/llvm/tools/bugpoint/BugDriver.h
@@ -183,7 +183,7 @@ class BugDriver {
   /// MayModifySemantics argument is true, then the cleanups is allowed to
   /// modify how the code behaves.
   ///
-  std::unique_ptr<Module> performFinalCleanups(std::unique_ptr<Module> M,
+  std::unique_ptr<Module> performFinalCleanups(const std::unique_ptr<Module> &M,
                                                bool MayModifySemantics = false);
 
   /// Given a module, extract up to one loop from it into a new function. This
diff --git a/llvm/tools/bugpoint/ExtractFunction.cpp b/llvm/tools/bugpoint/ExtractFunction.cpp
index dd9a82c320359..d9ec6ddd83c02 100644
--- a/llvm/tools/bugpoint/ExtractFunction.cpp
+++ b/llvm/tools/bugpoint/ExtractFunction.cpp
@@ -126,7 +126,7 @@ BugDriver::deleteInstructionFromProgram(const Instruction *I,
 }
 
 std::unique_ptr<Module>
-BugDriver::performFinalCleanups(std::unique_ptr<Module> M,
+BugDriver::performFinalCleanups(const std::unique_ptr<Module> &M,
                                 bool MayModifySemantics) {
   // Make all functions external, so GlobalDCE doesn't delete them...
   for (Module::iterator I = M->begin(), E = M->end(); I != E; ++I)
diff --git a/llvm/tools/dsymutil/DwarfLinkerForBinary.cpp b/llvm/tools/dsymutil/DwarfLinkerForBinary.cpp
index f6a35708dc076..9b10393c2cabe 100644
--- a/llvm/tools/dsymutil/DwarfLinkerForBinary.cpp
+++ b/llvm/tools/dsymutil/DwarfLinkerForBinary.cpp
@@ -119,7 +119,7 @@ static void dumpDIE(const DWARFDie *DIE, bool Verbose) {
 
 /// Report a warning to the user, optionally including information about a
 /// specific \p DIE related to the warning.
-void DwarfLinkerForBinary::reportWarning(Twine Warning, Twine Context,
+void DwarfLinkerForBinary::reportWarning(const Twine &Warning, const Twine &Context,
                                          const DWARFDie *DIE) const {
   // FIXME: implement warning logging which does not block other threads.
   if (ErrorHandlerMutex.try_lock()) {
@@ -129,7 +129,7 @@ void DwarfLinkerForBinary::reportWarning(Twine Warning, Twine Context,
   }
 }
 
-void DwarfLinkerForBinary::reportError(Twine Error, Twine Context,
+void DwarfLinkerForBinary::reportError(const Twine &Error, const Twine &Context,
                                        const DWARFDie *DIE) const {
   // FIXME: implement error logging which does not block other threads.
   if (ErrorHandlerMutex.try_lock()) {
diff --git a/llvm/tools/dsymutil/DwarfLinkerForBinary.h b/llvm/tools/dsymutil/DwarfLinkerForBinary.h
index 53f9e183ebe88..6f2ea2db70742 100644
--- a/llvm/tools/dsymutil/DwarfLinkerForBinary.h
+++ b/llvm/tools/dsymutil/DwarfLinkerForBinary.h
@@ -80,9 +80,9 @@ class DwarfLinkerForBinary {
   /// Link the contents of the DebugMap.
   bool link(const DebugMap &);
 
-  void reportWarning(Twine Warning, Twine Context = {},
+  void reportWarning(const Twine &Warning, const Twine &Context = {},
                      const DWARFDie *DIE = nullptr) const;
-  void reportError(Twine Error, Twine Context = {},
+  void reportError(const Twine &Error, const Twine &Context = {},
                    const DWARFDie *DIE = nullptr) const;
 
   /// Returns true if input verification is enabled and verification errors were
diff --git a/llvm/tools/llvm-cov/CoverageExporterLcov.cpp b/llvm/tools/llvm-cov/CoverageExporterLcov.cpp
index d6b9367ae4c51..df3ff9c009b7c 100644
--- a/llvm/tools/llvm-cov/CoverageExporterLcov.cpp
+++ b/llvm/tools/llvm-cov/CoverageExporterLcov.cpp
@@ -105,8 +105,8 @@ collectNestedBranches(const coverage::CoverageMapping &Coverage,
   return Branches;
 }
 
-bool sortLine(llvm::coverage::CountedRegion I,
-              llvm::coverage::CountedRegion J) {
+bool sortLine(const llvm::coverage::CountedRegion &I,
+              const llvm::coverage::CountedRegion &J) {
   return (I.LineStart < J.LineStart) ||
          ((I.LineStart == J.LineStart) && (I.ColumnStart < J.ColumnStart));
 }
diff --git a/llvm/tools/llvm-exegesis/lib/Assembler.cpp b/llvm/tools/llvm-exegesis/lib/Assembler.cpp
index 1e97cd24fe099..52673926103c9 100644
--- a/llvm/tools/llvm-exegesis/lib/Assembler.cpp
+++ b/llvm/tools/llvm-exegesis/lib/Assembler.cpp
@@ -240,7 +240,7 @@ BitVector getFunctionReservedRegs(const TargetMachine &TM) {
 }
 
 Error assembleToStream(const ExegesisTarget &ET,
-                       std::unique_ptr<TargetMachine> TM,
+                       const std::unique_ptr<TargetMachine> &TM,
                        ArrayRef<MCRegister> LiveIns, const FillFunction &Fill,
                        raw_pwrite_stream &AsmStream, const BenchmarkKey &Key,
                        bool GenerateMemoryInstructions) {
diff --git a/llvm/tools/llvm-exegesis/lib/Assembler.h b/llvm/tools/llvm-exegesis/lib/Assembler.h
index 1c8854c21b9a7..186f4e5e14172 100644
--- a/llvm/tools/llvm-exegesis/lib/Assembler.h
+++ b/llvm/tools/llvm-exegesis/lib/Assembler.h
@@ -89,7 +89,7 @@ using FillFunction = std::function<void(FunctionFiller &)>;
 // epilogue. Once the MachineFunction is ready, it is assembled for TM to
 // AsmStream, the temporary function is eventually discarded.
 Error assembleToStream(const ExegesisTarget &ET,
-                       std::unique_ptr<TargetMachine> TM,
+                       const std::unique_ptr<TargetMachine> &TM,
                        ArrayRef<MCRegister> LiveIns, const FillFunction &Fill,
                        raw_pwrite_stream &AsmStreamm, const BenchmarkKey &Key,
                        bool GenerateMemoryInstructions);
diff --git a/llvm/tools/llvm-exegesis/lib/BenchmarkResult.cpp b/llvm/tools/llvm-exegesis/lib/BenchmarkResult.cpp
index 1823a534a301a..f9add64c692e3 100644
--- a/llvm/tools/llvm-exegesis/lib/BenchmarkResult.cpp
+++ b/llvm/tools/llvm-exegesis/lib/BenchmarkResult.cpp
@@ -346,7 +346,7 @@ template <> struct MappingTraits<exegesis::Benchmark::TripleAndCpu> {
 namespace exegesis {
 
 Expected<std::set<Benchmark::TripleAndCpu>>
-Benchmark::readTriplesAndCpusFromYamls(MemoryBufferRef Buffer) {
+Benchmark::readTriplesAndCpusFromYamls(const MemoryBufferRef &Buffer) {
   // We're only mapping a field, drop other fields and silence the corresponding
   // warnings.
   yaml::Input Yin(Buffer, nullptr, +[](const SMDiagnostic &, void *Context) {});
@@ -365,7 +365,7 @@ Benchmark::readTriplesAndCpusFromYamls(MemoryBufferRef Buffer) {
 }
 
 Expected<Benchmark> Benchmark::readYaml(const LLVMState &State,
-                                        MemoryBufferRef Buffer) {
+                                        const MemoryBufferRef &Buffer) {
   yaml::Input Yin(Buffer);
   YamlContext Context(State);
   Benchmark Benchmark;
@@ -377,7 +377,7 @@ Expected<Benchmark> Benchmark::readYaml(const LLVMState &State,
 }
 
 Expected<std::vector<Benchmark>> Benchmark::readYamls(const LLVMState &State,
-                                                      MemoryBufferRef Buffer) {
+                                                      const MemoryBufferRef &Buffer) {
   yaml::Input Yin(Buffer);
   YamlContext Context(State);
   std::vector<Benchmark> Benchmarks;
diff --git a/llvm/tools/llvm-exegesis/lib/BenchmarkResult.h b/llvm/tools/llvm-exegesis/lib/BenchmarkResult.h
index 7984c8805cadc..94c91d3aedde4 100644
--- a/llvm/tools/llvm-exegesis/lib/BenchmarkResult.h
+++ b/llvm/tools/llvm-exegesis/lib/BenchmarkResult.h
@@ -81,8 +81,8 @@ struct BenchmarkKey {
 struct BenchmarkMeasure {
   // A helper to create an unscaled BenchmarkMeasure.
   static BenchmarkMeasure
-  Create(std::string Key, double Value,
-         std::map<ValidationEvent, int64_t> ValCounters) {
+  Create(const std::string &Key, double Value,
+         const std::map<ValidationEvent, int64_t> &ValCounters) {
     return {Key, Value, Value, Value, ValCounters};
   }
   std::string Key;
@@ -134,10 +134,10 @@ struct Benchmark {
 
   // Read functions.
   static Expected<Benchmark> readYaml(const LLVMState &State,
-                                                 MemoryBufferRef Buffer);
+                                                 const MemoryBufferRef &Buffer);
 
   static Expected<std::vector<Benchmark>>
-  readYamls(const LLVMState &State, MemoryBufferRef Buffer);
+  readYamls(const LLVMState &State, const MemoryBufferRef &Buffer);
 
   // Given a set of serialized instruction benchmarks, returns the set of
   // triples and CPUs that appear in the list of benchmarks.
@@ -149,7 +149,7 @@ struct Benchmark {
     }
   };
   static Expected<std::set<TripleAndCpu>>
-  readTriplesAndCpusFromYamls(MemoryBufferRef Buffer);
+  readTriplesAndCpusFromYamls(const MemoryBufferRef &Buffer);
 
   class Error readYamlFrom(const LLVMState &State, StringRef InputContent);
 
diff --git a/llvm/tools/llvm-pdbutil/DumpOutputStyle.cpp b/llvm/tools/llvm-pdbutil/DumpOutputStyle.cpp
index b2362ecb75703..afe01d33904dc 100644
--- a/llvm/tools/llvm-pdbutil/DumpOutputStyle.cpp
+++ b/llvm/tools/llvm-pdbutil/DumpOutputStyle.cpp
@@ -1201,7 +1201,7 @@ static void
 dumpFullTypeStream(LinePrinter &Printer, LazyRandomTypeCollection &Types,
                    TypeReferenceTracker *RefTracker, uint32_t NumTypeRecords,
                    uint32_t NumHashBuckets,
-                   FixedStreamArray<support::ulittle32_t> HashValues,
+                   const FixedStreamArray<support::ulittle32_t> &HashValues,
                    TpiStream *Stream, bool Bytes, bool Extras) {
 
   Printer.formatLine("Showing {0:N} records", NumTypeRecords);
diff --git a/llvm/tools/llvm-readobj/COFFDumper.cpp b/llvm/tools/llvm-readobj/COFFDumper.cpp
index 1666f7692ad5f..b10c6a45e44b3 100644
--- a/llvm/tools/llvm-readobj/COFFDumper.cpp
+++ b/llvm/tools/llvm-readobj/COFFDumper.cpp
@@ -134,7 +134,7 @@ class COFFDumper : public ObjDumper {
   void printCodeViewTypeSection(StringRef SectionName, const SectionRef &Section);
   StringRef getFileNameForFileOffset(uint32_t FileOffset);
   void printFileNameForOffset(StringRef Label, uint32_t FileOffset);
-  void printTypeIndex(StringRef FieldName, TypeIndex TI) {
+  void printTypeIndex(StringRef FieldName, const TypeIndex &TI) {
     // Forward to CVTypeDumper for simplicity.
     codeview::printTypeIndex(Writer, FieldName, TI, Types);
   }
diff --git a/llvm/tools/llvm-readobj/ELFDumper.cpp b/llvm/tools/llvm-readobj/ELFDumper.cpp
index bfca65aad52b4..f42c66d1c01c6 100644
--- a/llvm/tools/llvm-readobj/ELFDumper.cpp
+++ b/llvm/tools/llvm-readobj/ELFDumper.cpp
@@ -5840,7 +5840,7 @@ struct CoreNote {
   std::vector<CoreFileMapping> Mappings;
 };
 
-static Expected<CoreNote> readCoreNote(DataExtractor Desc) {
+static Expected<CoreNote> readCoreNote(const DataExtractor &Desc) {
   // Expected format of the NT_FILE note description:
   // 1. # of file mappings (call it N)
   // 2. Page size
diff --git a/llvm/tools/llvm-readtapi/DiffEngine.cpp b/llvm/tools/llvm-readtapi/DiffEngine.cpp
index 6434c871fa64b..394cf90c679f3 100644
--- a/llvm/tools/llvm-readtapi/DiffEngine.cpp
+++ b/llvm/tools/llvm-readtapi/DiffEngine.cpp
@@ -98,7 +98,7 @@ std::string SymScalar::getFlagString(const MachO::Symbol *Sym) {
   return std::string(Flags);
 }
 
-void SymScalar::print(raw_ostream &OS, std::string Indent, MachO::Target Targ) {
+void SymScalar::print(raw_ostream &OS, const std::string &Indent, const MachO::Target &Targ) {
   if (Val->getKind() == MachO::EncodeKind::ObjectiveCClass) {
     if (Targ.Arch == MachO::AK_i386 && Targ.Platform == MachO::PLATFORM_MACOS) {
       OS << Indent << "\t\t" << ((Order == lhs) ? "< " : "> ")
@@ -144,7 +144,7 @@ void addDiffForTargSlice(V Val, Target Targ, DiffOutput &Diff,
 }
 
 DiffOutput getSingleAttrDiff(const std::vector<InterfaceFileRef> &IRefVec,
-                             std::string Name, InterfaceInputOrder Order) {
+                             const std::string &Name, InterfaceInputOrder Order) {
   DiffOutput Diff(Name);
   Diff.Kind = AD_Str_Vec;
   for (const auto &IRef : IRefVec)
@@ -157,7 +157,7 @@ DiffOutput getSingleAttrDiff(const std::vector<InterfaceFileRef> &IRefVec,
 
 DiffOutput
 getSingleAttrDiff(const std::vector<std::pair<Target, std::string>> &PairVec,
-                  std::string Name, InterfaceInputOrder Order) {
+                  const std::string &Name, InterfaceInputOrder Order) {
   DiffOutput Diff(Name);
   Diff.Kind = AD_Str_Vec;
   for (const auto &Pair : PairVec)
@@ -168,7 +168,7 @@ getSingleAttrDiff(const std::vector<std::pair<Target, std::string>> &PairVec,
 }
 
 DiffOutput getSingleAttrDiff(InterfaceFile::const_symbol_range SymRange,
-                             std::string Name, InterfaceInputOrder Order) {
+                             const std::string &Name, InterfaceInputOrder Order) {
   DiffOutput Diff(Name);
   Diff.Kind = AD_Sym_Vec;
   for (const auto *Sym : SymRange)
@@ -305,7 +305,7 @@ DiffOutput recordDifferences(const std::vector<T> &LHS,
 
 DiffOutput recordDifferences(llvm::MachO::InterfaceFile::const_symbol_range LHS,
                              llvm::MachO::InterfaceFile::const_symbol_range RHS,
-                             std::string Attr) {
+                             const std::string &Attr) {
   DiffOutput Diff(Attr);
   Diff.Kind = AD_Sym_Vec;
   findAndAddDiff(LHS, RHS, Diff, lhs);
diff --git a/llvm/tools/llvm-readtapi/DiffEngine.h b/llvm/tools/llvm-readtapi/DiffEngine.h
index 58b0c1b3ea9cb..fe5ae8b6bb36f 100644
--- a/llvm/tools/llvm-readtapi/DiffEngine.h
+++ b/llvm/tools/llvm-readtapi/DiffEngine.h
@@ -85,7 +85,7 @@ class SymScalar {
 
   std::string getFlagString(const MachO::Symbol *Sym);
 
-  void print(raw_ostream &OS, std::string Indent, MachO::Target Targ);
+  void print(raw_ostream &OS, const std::string &Indent, const MachO::Target &Targ);
 
   const MachO::Symbol *getVal() const { return Val; }
   InterfaceInputOrder getOrder() const { return Order; }
diff --git a/llvm/utils/FileCheck/FileCheck.cpp b/llvm/utils/FileCheck/FileCheck.cpp
index 9cf3a3164dfec..70428aa877c0d 100644
--- a/llvm/utils/FileCheck/FileCheck.cpp
+++ b/llvm/utils/FileCheck/FileCheck.cpp
@@ -343,7 +343,7 @@ struct InputAnnotation {
 };
 
 /// Get an abbreviation for the check type.
-static std::string GetCheckTypeAbbreviation(Check::FileCheckType Ty) {
+static std::string GetCheckTypeAbbreviation(const Check::FileCheckType &Ty) {
   switch (Ty) {
   case Check::CheckPlain:
     if (Ty.getCount() > 1)
diff --git a/llvm/utils/TableGen/DFAPacketizerEmitter.cpp b/llvm/utils/TableGen/DFAPacketizerEmitter.cpp
index a6c0d09f69ba3..cd95face03290 100644
--- a/llvm/utils/TableGen/DFAPacketizerEmitter.cpp
+++ b/llvm/utils/TableGen/DFAPacketizerEmitter.cpp
@@ -85,7 +85,7 @@ class DFAPacketizerEmitter {
   // Emit code for a subset of itineraries.
   void emitForItineraries(raw_ostream &OS,
                           std::vector<const CodeGenProcModel *> &ProcItinList,
-                          std::string DFAName);
+                          const std::string &DFAName);
 
   void run(raw_ostream &OS);
 };
@@ -231,7 +231,7 @@ void DFAPacketizerEmitter::run(raw_ostream &OS) {
 
 void DFAPacketizerEmitter::emitForItineraries(
     raw_ostream &OS, std::vector<const CodeGenProcModel *> &ProcModels,
-    std::string DFAName) {
+    const std::string &DFAName) {
   OS << "} // end namespace llvm\n\n";
   OS << "namespace {\n";
   collectAllFuncUnits(ProcModels);
diff --git a/llvm/utils/TableGen/DXILEmitter.cpp b/llvm/utils/TableGen/DXILEmitter.cpp
index 70f2aa6522640..34bc470b7a3d6 100644
--- a/llvm/utils/TableGen/DXILEmitter.cpp
+++ b/llvm/utils/TableGen/DXILEmitter.cpp
@@ -374,7 +374,7 @@ static void emitDXILAttributes(const RecordKeeper &Records, raw_ostream &OS) {
 
 // Helper function to determine if the given Attr is defined in the vector
 // Attrs, by comparing the names
-static bool attrIsDefined(std::vector<const Record *> Attrs,
+static bool attrIsDefined(const std::vector<const Record *> &Attrs,
                           const Record *Attr) {
   for (auto CurAttr : Attrs)
     if (CurAttr->getName() == Attr->getName())
diff --git a/llvm/utils/TableGen/GlobalISelCombinerEmitter.cpp b/llvm/utils/TableGen/GlobalISelCombinerEmitter.cpp
index 770494405810d..6ca17e9691b33 100644
--- a/llvm/utils/TableGen/GlobalISelCombinerEmitter.cpp
+++ b/llvm/utils/TableGen/GlobalISelCombinerEmitter.cpp
@@ -665,9 +665,9 @@ class CombineRuleBuilder {
   std::optional<LLTCodeGenOrTempType>
   getLLTCodeGenOrTempType(const PatternType &PT, RuleMatcher &RM);
 
-  void PrintError(Twine Msg) const { ::PrintError(&RuleDef, Msg); }
-  void PrintWarning(Twine Msg) const { ::PrintWarning(RuleDef.getLoc(), Msg); }
-  void PrintNote(Twine Msg) const { ::PrintNote(RuleDef.getLoc(), Msg); }
+  void PrintError(const Twine &Msg) const { ::PrintError(&RuleDef, Msg); }
+  void PrintWarning(const Twine &Msg) const { ::PrintWarning(RuleDef.getLoc(), Msg); }
+  void PrintNote(const Twine &Msg) const { ::PrintNote(RuleDef.getLoc(), Msg); }
 
   void print(raw_ostream &OS, const PatternAlternatives &Alts) const;
 
@@ -710,7 +710,7 @@ class CombineRuleBuilder {
   /// \param AdditionalComment Comment string to be added to the
   ///        `DebugCommentAction`.
   RuleMatcher &addRuleMatcher(const PatternAlternatives &Alts,
-                              Twine AdditionalComment = "");
+                              const Twine &AdditionalComment = "");
   bool addFeaturePredicates(RuleMatcher &M);
 
   bool findRoots();
@@ -1350,7 +1350,7 @@ bool CombineRuleBuilder::checkSemantics() {
 }
 
 RuleMatcher &CombineRuleBuilder::addRuleMatcher(const PatternAlternatives &Alts,
-                                                Twine AdditionalComment) {
+                                                const Twine &AdditionalComment) {
   auto &RM = OutRMs.emplace_back(RuleDef.getLoc());
   addFeaturePredicates(RM);
   RM.setPermanentGISelFlags(GISF_IgnoreCopies);