diff --git a/src/Expr.cc b/src/Expr.cc
index 5db22848c9..f65dfcb2ab 100644
--- a/src/Expr.cc
+++ b/src/Expr.cc
@@ -4270,7 +4270,7 @@ LambdaExpr::LambdaExpr(LambdaExpr* orig) : Expr(EXPR_LAMBDA) {
 
     // We need to have our own copies of the outer IDs and captures so
     // we can rename them when inlined.
-    for ( auto i : orig->outer_ids )
+    for ( auto& i : orig->outer_ids )
         outer_ids.emplace_back(i);
 
     if ( orig->captures ) {
@@ -4325,7 +4325,7 @@ bool LambdaExpr::CheckCaptures(StmtPtr when_parent) {
             }
     }
 
-    for ( auto id : outer_ids )
+    for ( const auto& id : outer_ids )
         if ( ! outer_is_matched.contains(id) ) {
             auto msg = util::fmt("%s is used inside %s but not captured", id->Name(), desc);
             if ( when_parent )
diff --git a/src/Trigger.cc b/src/Trigger.cc
index 62f085ef25..6bcbe7a343 100644
--- a/src/Trigger.cc
+++ b/src/Trigger.cc
@@ -183,7 +183,7 @@ void Trigger::ReInit(std::vector<ValPtr> index_expr_results) {
         have_trigger_elems = true;
     }
 
-    for ( auto g : globals ) {
+    for ( const auto& g : globals ) {
         Register(g.get());
 
         auto& v = g->GetVal();
@@ -191,7 +191,7 @@ void Trigger::ReInit(std::vector<ValPtr> index_expr_results) {
             Register(v.get());
     }
 
-    for ( auto l : locals ) {
+    for ( const auto& l : locals ) {
         ASSERT(! l->GetVal());
     }
 
diff --git a/src/Var.cc b/src/Var.cc
index 270f4cea48..8d7d8d68f8 100644
--- a/src/Var.cc
+++ b/src/Var.cc
@@ -761,7 +761,7 @@ IDPList gather_outer_ids(ScopePtr scope, StmtPtr body) {
     IDPList idl;
 
     for ( auto& id : cb.outer_id_references )
-        idl.emplace_back(std::move(id));
+        idl.emplace_back(id);
 
     return idl;
 }
diff --git a/src/script_opt/CPP/DeclFunc.cc b/src/script_opt/CPP/DeclFunc.cc
index 8e79110fbd..d2d8ec7516 100644
--- a/src/script_opt/CPP/DeclFunc.cc
+++ b/src/script_opt/CPP/DeclFunc.cc
@@ -30,7 +30,7 @@ void CPPCompile::DeclareLambda(const LambdaExpr* l, const ProfileFunc* pf) {
     auto l_id = l->Ingredients()->GetID();
     auto& ids = l->OuterIDs();
 
-    for ( auto lid : ids ) {
+    for ( const auto& lid : ids ) {
         if ( lambda_names.contains(lid) ) {
             ASSERT(lambda_names[lid] == CaptureName(lid));
         }
@@ -227,7 +227,7 @@ void CPPCompile::BuildLambda(const FuncTypePtr& ft, const ProfileFunc* pf, const
     Emit("void SetLambdaCaptures(Frame* f) override");
     StartBlock();
     for ( size_t i = 0; i < nl; ++i ) {
-        auto l_i = (*lambda_ids)[i];
+        const auto& l_i = (*lambda_ids)[i];
         const auto& t_i = l_i->GetType();
         auto cap_i = string("f->GetElement(") + Fmt(static_cast<int>(i)) + ")";
         Emit("%s = %s;", lambda_names[l_i], GenericValPtrToGT(cap_i, t_i, GEN_NATIVE));
@@ -239,7 +239,7 @@ void CPPCompile::BuildLambda(const FuncTypePtr& ft, const ProfileFunc* pf, const
     StartBlock();
     Emit("std::vector<ValPtr> vals;");
     for ( size_t i = 0; i < nl; ++i ) {
-        auto l_i = (*lambda_ids)[i];
+        const auto& l_i = (*lambda_ids)[i];
         const auto& t_i = l_i->GetType();
         Emit("vals.emplace_back(%s);", NativeToGT(lambda_names[l_i], t_i, GEN_VAL_PTR));
     }
diff --git a/src/script_opt/CPP/GenFunc.cc b/src/script_opt/CPP/GenFunc.cc
index e6307e76de..d171dc5fd4 100644
--- a/src/script_opt/CPP/GenFunc.cc
+++ b/src/script_opt/CPP/GenFunc.cc
@@ -145,7 +145,7 @@ void CPPCompile::DeclareLocals(const ProfileFunc* pf, const IDPList* lambda_ids)
     // latter can be inconsistent when inlining.
     set<string> capture_names;
     if ( lambda_ids )
-        for ( auto li : *lambda_ids )
+        for ( const auto& li : *lambda_ids )
             capture_names.insert(CaptureName(li));
 
     const auto& ls = pf->Locals();
diff --git a/src/script_opt/CPP/Stmts.cc b/src/script_opt/CPP/Stmts.cc
index 40d3b27eb6..0d684a5b3e 100644
--- a/src/script_opt/CPP/Stmts.cc
+++ b/src/script_opt/CPP/Stmts.cc
@@ -178,7 +178,7 @@ void CPPCompile::GenTypeSwitchStmt(const Expr* e, const case_list* cases) {
     for ( const auto& c : *cases ) {
         auto tc = c->TypeCases();
         if ( tc )
-            for ( auto id : *tc )
+            for ( const auto& id : *tc )
                 Emit(Fmt(TypeOffset(id->GetType())) + ",");
     }
     EndBlock(true);
@@ -206,7 +206,7 @@ void CPPCompile::GenTypeSwitchStmt(const Expr* e, const case_list* cases) {
         auto tc = c->TypeCases();
         if ( tc ) {
             bool is_multi = tc->size() > 1;
-            for ( auto id : *tc )
+            for ( const auto& id : *tc )
                 GenTypeSwitchCase(id, case_offset++, is_multi);
         }
         else
@@ -432,7 +432,7 @@ void CPPCompile::GenForOverTable(const ExprPtr& tbl, const IDPtr& value_var, con
 
     int n = static_cast<int>(loop_vars->size());
     for ( int i = 0; i < n; ++i ) {
-        auto var = (*loop_vars)[i];
+        const auto& var = (*loop_vars)[i];
         if ( var->IsBlank() )
             continue;
 
@@ -454,7 +454,7 @@ void CPPCompile::GenForOverVector(const ExprPtr& vec, const IDPtr& value_var, co
 
     Emit("if ( ! vv__CPP->Has(i__CPP) ) continue;");
 
-    auto lv0 = (*loop_vars)[0];
+    const auto& lv0 = (*loop_vars)[0];
 
     if ( ! lv0->IsBlank() )
         Emit("%s = i__CPP;", IDName(lv0));
@@ -475,7 +475,7 @@ void CPPCompile::GenForOverString(const ExprPtr& str, const IDPList* loop_vars)
 
     Emit("auto sv__CPP = make_intrusive<StringVal>(1, (const char*) sval__CPP->Bytes() + i__CPP);");
 
-    auto lv0 = (*loop_vars)[0];
+    const auto& lv0 = (*loop_vars)[0];
     if ( ! lv0->IsBlank() )
         Emit("%s = std::move(sv__CPP);", IDName(lv0));
 }
diff --git a/src/script_opt/CSE.cc b/src/script_opt/CSE.cc
index e846483126..8464d561fa 100644
--- a/src/script_opt/CSE.cc
+++ b/src/script_opt/CSE.cc
@@ -212,7 +212,7 @@ TraversalCode CSE_ValidityChecker::PostExpr(const Expr* e) {
 }
 
 bool CSE_ValidityChecker::CheckID(const IDPtr& id, bool ignore_orig) {
-    for ( auto i : ids ) {
+    for ( const auto& i : ids ) {
         if ( ignore_orig && i == ids.front() )
             continue;
 
@@ -227,7 +227,7 @@ bool CSE_ValidityChecker::CheckAggrMod(const TypePtr& t) {
     if ( ! IsAggr(t) )
         return false;
 
-    for ( auto i : ids )
+    for ( const auto& i : ids )
         if ( same_type(t, i->GetType()) )
             return Invalid();
 
@@ -287,8 +287,8 @@ bool CSE_ValidityChecker::CheckSideEffects(const IDSet& non_local_ids, const Typ
         // This is far and away the most common case.
         return false;
 
-    for ( auto i : ids ) {
-        for ( auto nli : non_local_ids )
+    for ( const auto& i : ids ) {
+        for ( const auto& nli : non_local_ids )
             if ( nli == i )
                 return Invalid();
 
diff --git a/src/script_opt/GenIDDefs.cc b/src/script_opt/GenIDDefs.cc
index f9a354710c..95b59100da 100644
--- a/src/script_opt/GenIDDefs.cc
+++ b/src/script_opt/GenIDDefs.cc
@@ -426,7 +426,7 @@ void GenIDDefs::StartConfluenceBlock(const Stmt* s) {
 }
 
 void GenIDDefs::EndConfluenceBlock(bool no_orig) {
-    for ( auto id : modified_IDs.back() )
+    for ( const auto& id : modified_IDs.back() )
         id->GetOptInfo()->ConfluenceBlockEndsAfter(last_stmt_traversed, no_orig);
 
     confluence_blocks.pop_back();
@@ -434,12 +434,12 @@ void GenIDDefs::EndConfluenceBlock(bool no_orig) {
 }
 
 void GenIDDefs::BranchBackTo(const Stmt* from, const Stmt* to, bool close_all) {
-    for ( auto id : modified_IDs.back() )
+    for ( const auto& id : modified_IDs.back() )
         id->GetOptInfo()->BranchBackTo(from, to, close_all);
 }
 
 void GenIDDefs::BranchBeyond(const Stmt* from, const Stmt* to, bool close_all) {
-    for ( auto id : modified_IDs.back() )
+    for ( const auto& id : modified_IDs.back() )
         id->GetOptInfo()->BranchBeyond(from, to, close_all);
 
     to->GetOptInfo()->contains_branch_beyond = true;
@@ -479,7 +479,7 @@ void GenIDDefs::ReturnAt(const Stmt* s) {
     // identifier as encountering a scope-ending "return" here.  By avoiding
     // that, we're able to do optimization across catch-return blocks.
     if ( cr_active.empty() || cr_active.back() != confluence_blocks.size() )
-        for ( auto id : modified_IDs.back() )
+        for ( const auto& id : modified_IDs.back() )
             id->GetOptInfo()->ReturnAt(s);
 }
 
diff --git a/src/script_opt/ProfileFunc.cc b/src/script_opt/ProfileFunc.cc
index 39d6733609..6c4660a4e9 100644
--- a/src/script_opt/ProfileFunc.cc
+++ b/src/script_opt/ProfileFunc.cc
@@ -35,7 +35,7 @@ ProfileFunc::ProfileFunc(const Func* func, const StmtPtr& body, bool _abs_rec_fi
         int offset = 0;
 
         for ( auto& c : *fcaps ) {
-            auto cid = c.Id();
+            const auto& cid = c.Id();
             captures.insert(cid);
             captures_offsets[cid] = offset++;
         }
@@ -51,7 +51,7 @@ ProfileFunc::ProfileFunc(const Func* func, const StmtPtr& body, bool _abs_rec_fi
     // declaration.
     num_params = profiled_func_t->Params()->NumFields();
 
-    for ( auto l : locals ) {
+    for ( const auto& l : locals ) {
         if ( ! captures.contains(l) && l->Offset() < num_params )
             params.insert(l);
     }
@@ -76,7 +76,7 @@ ProfileFunc::ProfileFunc(const Expr* e, bool _abs_rec_fields) {
 
         int offset = 0;
 
-        for ( auto oid : func->OuterIDs() ) {
+        for ( const auto& oid : func->OuterIDs() ) {
             captures.insert(oid);
             captures_offsets[oid] = offset++;
         }
@@ -130,7 +130,7 @@ TraversalCode ProfileFunc::PreStmt(const Stmt* s) {
             auto w = s->AsWhenStmt();
             auto wi = w->Info();
 
-            for ( auto wl : wi->WhenNewLocals() )
+            for ( const auto& wl : wi->WhenNewLocals() )
                 when_locals.insert(wl);
         } break;
 
@@ -139,7 +139,7 @@ TraversalCode ProfileFunc::PreStmt(const Stmt* s) {
             auto loop_vars = sf->LoopVars();
             auto value_var = sf->ValueVar();
 
-            for ( auto id : *loop_vars )
+            for ( const auto& id : *loop_vars )
                 locals.insert(id);
 
             if ( value_var )
@@ -161,7 +161,7 @@ TraversalCode ProfileFunc::PreStmt(const Stmt* s) {
             for ( auto& c : *sw->Cases() ) {
                 auto idl = c->TypeCases();
                 if ( idl ) {
-                    for ( auto id : *idl )
+                    for ( const auto& id : *idl )
                         // Make sure it's not a placeholder
                         // identifier, used when there's
                         // no explicit one.
@@ -541,7 +541,7 @@ void ProfileFunc::TrackID(const IDPtr id) {
 }
 
 void ProfileFunc::TrackAssignment(const IDPtr id) {
-    if ( assignees.count(id) > 0 )
+    if ( assignees.contains(id) )
         ++assignees[id];
     else
         assignees[id] = 1;
@@ -685,7 +685,7 @@ bool ProfileFuncs::GetCallSideEffects(const NameExpr* n, IDSet& non_local_ids, T
 
     for ( auto a : seo->ModAggrs() )
         aggrs.insert(a);
-    for ( auto nl : seo->ModNonLocals() )
+    for ( const auto& nl : seo->ModNonLocals() )
         non_local_ids.insert(nl);
 
     return true;
@@ -878,7 +878,7 @@ void ProfileFuncs::ComputeProfileHash(std::shared_ptr<ProfileFunc> pf) {
         h = merge_p_hashes(h, p_hash(i->Tag()));
 
     h = merge_p_hashes(h, p_hash("ids"));
-    for ( auto i : pf->OrderedIdentifiers() )
+    for ( const auto& i : pf->OrderedIdentifiers() )
         h = merge_p_hashes(h, p_hash(i->Name()));
 
     h = merge_p_hashes(h, p_hash("constants"));
@@ -1408,7 +1408,7 @@ bool ProfileFuncs::AssessSideEffects(const SideEffectsOp* se, SideEffectsOp::Acc
 
     for ( auto a : se->ModAggrs() )
         aggrs.insert(a);
-    for ( auto nl : se->ModNonLocals() )
+    for ( const auto& nl : se->ModNonLocals() )
         non_local_ids.insert(nl);
 
     return false;
diff --git a/src/script_opt/Reduce.cc b/src/script_opt/Reduce.cc
index 4f4cf0a05f..d3a81ef604 100644
--- a/src/script_opt/Reduce.cc
+++ b/src/script_opt/Reduce.cc
@@ -322,7 +322,7 @@ bool Reducer::ID_IsReducedOrTopLevel(const IDPtr& id) {
 }
 
 bool Reducer::ID_IsReduced(const IDPtr& id) const {
-    return inline_block_level == 0 || tracked_ids.count(id) > 0 || id->IsGlobal() || IsTemporary(id);
+    return inline_block_level == 0 || tracked_ids.contains(id) || id->IsGlobal() || IsTemporary(id);
 }
 
 StmtPtr Reducer::GenParam(const IDPtr& id, ExprPtr rhs, bool is_modified) {
@@ -518,8 +518,7 @@ bool Reducer::ExprValid(const IDPtr& id, const Expr* e1, const Expr* e2) const {
 
     // Tracks which ID's are germane for our analysis.
     std::vector<IDPtr> ids;
-
-    ids.push_back(std::move(id));
+    ids.push_back(id);
 
     // Identify variables involved in the expression.
     CheckIDs(e1->GetOp1(), ids);
@@ -550,7 +549,7 @@ void Reducer::CheckIDs(const ExprPtr& e, std::vector<IDPtr>& ids) const {
 }
 
 bool Reducer::IsCSE(const AssignExpr* a, const NameExpr* lhs, const Expr* rhs) {
-    auto lhs_id = lhs->IdPtr();
+    const auto& lhs_id = lhs->IdPtr();
     auto lhs_tmp = FindTemporary(lhs_id); // nil if LHS not a temporary
     auto rhs_tmp = FindExprTmp(rhs, a, lhs_tmp);
 
@@ -705,7 +704,7 @@ ExprPtr Reducer::UpdateExpr(ExprPtr e) {
 
 StmtPtr Reducer::MergeStmts(const NameExpr* lhs, ExprPtr rhs, const StmtPtr& succ_stmt) {
     // First check for tmp=rhs.
-    auto lhs_id = lhs->IdPtr();
+    const auto& lhs_id = lhs->IdPtr();
     auto lhs_tmp = FindTemporary(lhs_id);
 
     if ( ! lhs_tmp )
diff --git a/src/script_opt/Reduce.h b/src/script_opt/Reduce.h
index b03a706752..b6547b39aa 100644
--- a/src/script_opt/Reduce.h
+++ b/src/script_opt/Reduce.h
@@ -83,7 +83,7 @@ public:
     bool IsTemporary(const IDPtr& id) const { return FindTemporary(id) != nullptr; }
     bool IsParamTemp(const IDPtr& id) const { return param_temps.contains(id); }
 
-    bool IsConstantVar(const IDPtr& id) const { return constant_vars.find(id) != constant_vars.end(); }
+    bool IsConstantVar(const IDPtr& id) const { return constant_vars.contains(id); }
 
     // True if the Reducer is being used in the context of a second
     // pass over for AST optimization.
diff --git a/src/script_opt/UsageAnalyzer.cc b/src/script_opt/UsageAnalyzer.cc
index d3015f2456..d84a353b1e 100644
--- a/src/script_opt/UsageAnalyzer.cc
+++ b/src/script_opt/UsageAnalyzer.cc
@@ -69,7 +69,7 @@ UsageAnalyzer::UsageAnalyzer(std::vector<FuncInfo>& funcs) {
     for ( auto& gpair : globals ) {
         auto& id = gpair.second;
 
-        if ( reachables.count(id) > 0 )
+        if ( reachables.contains(id) )
             continue;
 
         auto f = GetFuncIfAny(id);
@@ -119,7 +119,7 @@ public:
     TraversalCode PreID(const ID* raw_id) override {
         IDPtr id{NewRef{}, const_cast<ID*>(raw_id)};
 
-        if ( ids.count(id) > 0 )
+        if ( ids.contains(id) )
             return TC_ABORTSTMT;
 
         if ( attr_depth > 0 )
@@ -151,7 +151,7 @@ void UsageAnalyzer::FindSeeds(IDSet& seeds) const {
         auto f = GetFuncIfAny(id);
 
         if ( f && id->GetType<FuncType>()->Flavor() == FUNC_FLAVOR_EVENT ) {
-            if ( script_events.count(f->GetName()) == 0 )
+            if ( ! script_events.contains(f->GetName()) )
                 seeds.insert(id);
 
             continue;
@@ -201,7 +201,7 @@ void UsageAnalyzer::FullyExpandReachables() {
 bool UsageAnalyzer::ExpandReachables(const IDSet& curr_r) {
     new_reachables.clear();
 
-    for ( auto r : curr_r )
+    for ( const auto& r : curr_r )
         Expand(r);
 
     return ! new_reachables.empty();
@@ -229,7 +229,7 @@ void UsageAnalyzer::Expand(const IDPtr& id) {
 TraversalCode UsageAnalyzer::PreID(const ID* raw_id) {
     IDPtr id{NewRef{}, const_cast<ID*>(raw_id)};
 
-    if ( analyzed_IDs.count(id) > 0 )
+    if ( analyzed_IDs.contains(id) )
         // No need to repeat the analysis.
         return TC_ABORTSTMT;
 
diff --git a/src/script_opt/UseDefs.cc b/src/script_opt/UseDefs.cc
index 8739ea9fe4..7b9d1839fd 100644
--- a/src/script_opt/UseDefs.cc
+++ b/src/script_opt/UseDefs.cc
@@ -609,11 +609,11 @@ void UseDefs::FoldInUDs(UDs& main_UDs, const UDs& u1, const UDs& u2) {
         main_UDs->Replicate(old_main);
 
     if ( u1 )
-        for ( auto ud : u1->IterateOver() )
+        for ( const auto& ud : u1->IterateOver() )
             main_UDs->Add(ud);
 
     if ( u2 )
-        for ( auto ud : u2->IterateOver() )
+        for ( const auto& ud : u2->IterateOver() )
             main_UDs->Add(ud);
 }
 
@@ -633,7 +633,7 @@ void UseDefs::UpdateUDs(const Stmt* s, const UDs& uds) {
     }
 
     if ( uds ) {
-        for ( auto u : uds->IterateOver() )
+        for ( const auto& u : uds->IterateOver() )
             curr_uds->Add(u);
     }
 }
diff --git a/src/script_opt/UseDefs.h b/src/script_opt/UseDefs.h
index 290e349300..83e678b7a1 100644
--- a/src/script_opt/UseDefs.h
+++ b/src/script_opt/UseDefs.h
@@ -27,7 +27,7 @@ public:
 
     void Replicate(const UDs& from) { use_defs = from->use_defs; }
 
-    bool HasID(const IDPtr& id) { return use_defs.find(id) != use_defs.end(); }
+    bool HasID(const IDPtr& id) const { return use_defs.contains(id); }
 
     void Add(IDPtr id) { use_defs.insert(std::move(id)); }
     void Remove(const IDPtr& id) { use_defs.erase(id); }
diff --git a/src/script_opt/ZAM/AM-Opt.cc b/src/script_opt/ZAM/AM-Opt.cc
index 83e7592c25..af45025490 100644
--- a/src/script_opt/ZAM/AM-Opt.cc
+++ b/src/script_opt/ZAM/AM-Opt.cc
@@ -510,7 +510,7 @@ void ZAMCompiler::ReMapFrame() {
             continue;
 
         auto vars = inst_beginnings[inst];
-        for ( auto v : vars ) {
+        for ( const auto& v : vars ) {
             // Don't remap variables whose values aren't actually used.
             int slot = frame_layout1[v];
             if ( denizen_ending.contains(slot) )
diff --git a/src/script_opt/ZAM/Driver.cc b/src/script_opt/ZAM/Driver.cc
index 34e3f926c2..6a50828ced 100644
--- a/src/script_opt/ZAM/Driver.cc
+++ b/src/script_opt/ZAM/Driver.cc
@@ -77,7 +77,7 @@ void ZAMCompiler::InitArgs() {
 }
 
 void ZAMCompiler::InitCaptures() {
-    for ( auto c : pf->Captures() )
+    for ( const auto& c : pf->Captures() )
         (void)AddToFrame(c);
 }
 
diff --git a/src/script_opt/ZAM/Stmt.cc b/src/script_opt/ZAM/Stmt.cc
index 1874d3a240..842719119c 100644
--- a/src/script_opt/ZAM/Stmt.cc
+++ b/src/script_opt/ZAM/Stmt.cc
@@ -528,7 +528,7 @@ const ZAMStmt ZAMCompiler::GenSwitch(const SwitchStmt* sw, int slot, InternalTyp
     // For type switches, we map them to consecutive numbers, and then use
     // a integer-valued switch on those.
     int tm_ctr = 0;
-    for ( auto [_, index] : *sw->TypeMap() ) {
+    for ( const auto& [_, index] : *sw->TypeMap() ) {
         auto case_body_start = case_start[index];
         new_int_cases[tm_ctr++] = case_body_start;
     }
diff --git a/src/script_opt/ZAM/Vars.cc b/src/script_opt/ZAM/Vars.cc
index c22b0784a1..0479caf9dd 100644
--- a/src/script_opt/ZAM/Vars.cc
+++ b/src/script_opt/ZAM/Vars.cc
@@ -68,7 +68,7 @@ const ZAMStmt ZAMCompiler::LoadGlobal(const IDPtr& id) {
 
     // We use the id_val for reporting used-but-not-set errors.
     z.aux = new ZInstAux(0);
-    z.aux->id_val = std::move(id);
+    z.aux->id_val = id;
 
     return AddInst(z, true);
 }