zeek/src/script_opt/Stmt.cc

// See the file "COPYING" in the main distribution directory for copyright.

// Optimization-related methods for Stmt classes.

#include "zeek/Stmt.h"

#include "zeek/Desc.h"
#include "zeek/Expr.h"
#include "zeek/Frame.h"
#include "zeek/Reporter.h"
#include "zeek/Traverse.h"
#include "zeek/script_opt/Expr.h"
#include "zeek/script_opt/IDOptInfo.h"
#include "zeek/script_opt/Reduce.h"

namespace zeek::detail {

bool Stmt::IsReduced(Reducer* c) const { return true; }

StmtPtr Stmt::Reduce(Reducer* c) {
    auto this_ptr = ThisPtr();

    auto repl = c->ReplacementStmt(this_ptr);
    if ( repl )
        return repl;

    if ( c->ShouldOmitStmt(this) )
        return with_location_of(make_intrusive<NullStmt>(), this);

    c->SetCurrStmt(this);

    return DoReduce(c);
}

StmtPtr Stmt::TransformMe(StmtPtr new_me, Reducer* c) {
    ASSERT(new_me != this);

    // Set the original prior to reduction, to support "original chains"
    // to ultimately resolve back to the source statement.
    new_me->SetLocationInfo(GetLocationInfo());
    return new_me->Reduce(c);
}

void ExprListStmt::Inline(Inliner* inl) {
    auto& e = l->Exprs();
    for ( auto i = 0; i < e.length(); ++i )
        e.replace(i, e[i]->Inline(inl).release());
}

bool ExprListStmt::IsReduced(Reducer* c) const {
    const ExprPList& e = l->Exprs();
    for ( const auto& expr : e )
        if ( ! expr->IsSingleton(c) )
            return NonReduced(expr);

    return true;
}

StmtPtr ExprListStmt::DoReduce(Reducer* c) {
    if ( ! c->Optimizing() && IsReduced(c) )
        return ThisPtr();

    auto new_l = with_location_of(make_intrusive<ListExpr>(), this);
    auto s = with_location_of(make_intrusive<StmtList>(), this);

    ExprPList& e = l->Exprs();
    for ( auto& expr : e ) {
        if ( c->Optimizing() )
            new_l->Append(c->OptExpr(expr));

        else if ( expr->IsSingleton(c) )
            new_l->Append({NewRef{}, expr});

        else {
            StmtPtr red_e_stmt;
            auto red_e = expr->ReduceToSingleton(c, red_e_stmt);
            new_l->Append(red_e);

            if ( red_e_stmt )
                s->Stmts().push_back(red_e_stmt);
        }
    }

    if ( c->Optimizing() ) {
        l = new_l;
        return ThisPtr();
    }

    else {
        s->Stmts().push_back(DoSubclassReduce(new_l, c));
        return s->Reduce(c);
    }
}

StmtPtr PrintStmt::Duplicate() { return SetSucc(new PrintStmt(l->Duplicate()->AsListExprPtr())); }

StmtPtr PrintStmt::DoSubclassReduce(ListExprPtr singletons, Reducer* c) {
    return with_location_of(make_intrusive<PrintStmt>(singletons), this);
}

StmtPtr ExprStmt::Duplicate() { return SetSucc(new ExprStmt(e ? e->Duplicate() : nullptr)); }

void ExprStmt::Inline(Inliner* inl) {
    if ( e )
        e = e->Inline(inl);
}

bool ExprStmt::IsReduced(Reducer* c) const {
    if ( ! e || e->IsReduced(c) )
        return true;

    return NonReduced(e.get());
}

StmtPtr ExprStmt::DoReduce(Reducer* c) {
    if ( ! e )
        // e can be nil for our derived classes (like ReturnStmt).
        return TransformMe(make_intrusive<NullStmt>(), c);

    auto t = e->Tag();

    if ( t == EXPR_NOP )
        return TransformMe(make_intrusive<NullStmt>(), c);

    if ( c->Optimizing() ) {
        e = c->OptExpr(e);
        return ThisPtr();
    }

    if ( e->IsSingleton(c) )
        // No point evaluating.
        return TransformMe(make_intrusive<NullStmt>(), c);

    if ( (t == EXPR_ASSIGN || t == EXPR_CALL || t == EXPR_INDEX_ASSIGN || t == EXPR_FIELD_LHS_ASSIGN ||
          t == EXPR_APPEND_TO || t == EXPR_ADD_TO || t == EXPR_REMOVE_FROM) &&
         e->IsReduced(c) )
        return ThisPtr();

    StmtPtr red_e_stmt;

    if ( t == EXPR_CALL )
        // A bare call.  If we reduce it regularly, if
        // it has a non-void type it'll generate an
        // assignment to a temporary.
        red_e_stmt = e->ReduceToSingletons(c);
    else {
        e = e->Reduce(c, red_e_stmt);
        // It's possible that 'e' has gone away because it was a call
        // to an inlined function that doesn't have a return value.
        if ( ! e )
            return red_e_stmt;
    }

    if ( red_e_stmt ) {
        auto s = make_intrusive<StmtList>(red_e_stmt, ThisPtr());
        return TransformMe(s, c);
    }

    else
        return ThisPtr();
}

StmtPtr IfStmt::Duplicate() { return SetSucc(new IfStmt(e->Duplicate(), s1->Duplicate(), s2->Duplicate())); }

void IfStmt::Inline(Inliner* inl) {
    ExprStmt::Inline(inl);

    if ( s1 )
        s1->Inline(inl);
    if ( s2 )
        s2->Inline(inl);
}

bool IfStmt::IsReduced(Reducer* c) const {
    if ( e->IsConst() || ! e->IsReducedConditional(c) || IsMinMaxConstruct() )
        return NonReduced(e.get());

    return s1->IsReduced(c) && s2->IsReduced(c);
}

StmtPtr IfStmt::DoReduce(Reducer* c) {
    StmtPtr red_e_stmt;

    if ( e->WillTransformInConditional(c) )
        e = e->ReduceToConditional(c, red_e_stmt);

    // First, assess some fundamental transformations.
    if ( IsMinMaxConstruct() )
        return ConvertToMinMaxConstruct()->Reduce(c);

    if ( e->Tag() == EXPR_NOT ) { // Change "if ( ! x ) s1 else s2" to "if ( x ) s2 else s1".
        std::swap(s1, s2);
        e = e->GetOp1();
    }

    if ( e->Tag() == EXPR_OR_OR && c->BifurcationOkay() ) {
        c->PushBifurcation();

        // Expand "if ( a || b ) s1 else s2" to
        // "if ( a ) s1 else { if ( b ) s1 else s2 }"
        auto a = e->GetOp1();
        auto b = e->GetOp2();

        auto s1_dup = s1 ? s1->Duplicate() : nullptr;
        s2 = with_location_of(make_intrusive<IfStmt>(b, s1_dup, s2), s2);
        e = a;

        auto res = DoReduce(c);
        c->PopBifurcation();
        return res;
    }

    if ( e->Tag() == EXPR_AND_AND && c->BifurcationOkay() ) {
        c->PushBifurcation();

        // Expand "if ( a && b ) s1 else s2" to
        // "if ( a ) { if ( b ) s1 else s2 } else s2"
        auto a = e->GetOp1();
        auto b = e->GetOp2();

        auto s2_dup = s2 ? s2->Duplicate() : nullptr;
        s1 = with_location_of(make_intrusive<IfStmt>(b, s1, s2_dup), s1);
        e = a;

        auto res = DoReduce(c);
        c->PopBifurcation();
        return res;
    }

    s1 = s1->Reduce(c);
    s2 = s2->Reduce(c);

    if ( s1->Tag() == STMT_NULL && s2->Tag() == STMT_NULL )
        return TransformMe(make_intrusive<NullStmt>(), c);

    if ( c->Optimizing() )
        e = c->OptExpr(e);
    else {
        StmtPtr cond_red_stmt;
        e = e->ReduceToConditional(c, cond_red_stmt);

        if ( red_e_stmt && cond_red_stmt )
            red_e_stmt = with_location_of(make_intrusive<StmtList>(red_e_stmt, cond_red_stmt), this);
        else if ( cond_red_stmt )
            red_e_stmt = cond_red_stmt;
    }

    StmtPtr sl;

    if ( e->IsConst() ) {
        auto c_e = e->AsConstExprPtr();
        auto t = c_e->Value()->AsBool();

        if ( c->Optimizing() )
            return t ? s1 : s2;

        sl = make_intrusive<StmtList>(red_e_stmt, t ? s1 : s2);
    }

    else if ( red_e_stmt )
        sl = make_intrusive<StmtList>(red_e_stmt, ThisPtr());

    if ( sl )
        return TransformMe(std::move(sl), c);

    return ThisPtr();
}

bool IfStmt::NoFlowAfter(bool ignore_break) const {
    if ( s1 && s2 )
        return s1->NoFlowAfter(ignore_break) && s2->NoFlowAfter(ignore_break);

    // Assuming the test isn't constant, the nonexistent branch
    // could be picked, so flow definitely continues afterwards.
    // (Constant branches will be pruned during reduction.)
    return false;
}

bool IfStmt::CouldReturn(bool ignore_break) const {
    return (s1 && s1->CouldReturn(ignore_break)) || (s2 && s2->CouldReturn(ignore_break));
}

bool IfStmt::IsMinMaxConstruct() const {
    if ( ! s1 || ! s2 )
        // not an if-else construct
        return false;

    if ( s1->Tag() != STMT_EXPR || s2->Tag() != STMT_EXPR )
        // definitely not if-else assignments
        return false;

    auto es1 = s1->AsExprStmt()->StmtExpr();
    auto es2 = s2->AsExprStmt()->StmtExpr();

    if ( es1->Tag() != EXPR_ASSIGN || es2->Tag() != EXPR_ASSIGN )
        return false;

    switch ( e->Tag() ) {
        case EXPR_LT:
        case EXPR_LE:
        case EXPR_GE:
        case EXPR_GT: break;

        default:
            // Not an apt conditional.
            return false;
    }

    auto a1 = es1->AsAssignExpr();
    auto a2 = es2->AsAssignExpr();
    auto a1_lhs = a1->GetOp1();
    auto a2_lhs = a2->GetOp1();

    if ( ! same_expr(a1_lhs, a2_lhs) )
        // if-else assignments are not to the same variable
        return false;

    auto a1_rhs = a1->GetOp2();
    auto a2_rhs = a2->GetOp2();
    auto op1 = e->GetOp1();
    auto op2 = e->GetOp2();

    if ( ! same_expr(op1, a1_rhs) && ! same_expr(op1, a2_rhs) )
        // Operand does not appear in the assignment RHS.
        return false;

    if ( ! same_expr(op2, a1_rhs) && ! same_expr(op2, a2_rhs) )
        // Operand does not appear in the assignment RHS.
        return false;

    if ( same_expr(op1, op2) )
        // This is degenerate and should be found by other reductions.
        return false;

    return true;
}

StmtPtr IfStmt::ConvertToMinMaxConstruct() {
    auto relop1 = e->GetOp1();
    auto relop2 = e->GetOp2();

    auto is_min = (e->Tag() == EXPR_LT || e->Tag() == EXPR_LE);
    auto assign2 = s2->AsExprStmt()->StmtExpr();
    auto lhs2 = assign2->GetOp1();
    auto rhs2 = assign2->GetOp2();

    if ( same_expr(relop1, rhs2) )
        is_min = ! is_min;

    auto built_in = is_min ? ScriptOptBuiltinExpr::MINIMUM : ScriptOptBuiltinExpr::MAXIMUM;

    auto bi = with_location_of(make_intrusive<ScriptOptBuiltinExpr>(built_in, relop1, relop2), this);
    auto new_assign = with_location_of(make_intrusive<AssignExpr>(lhs2, bi, false), this);

    return with_location_of(make_intrusive<ExprStmt>(new_assign), this);
}

IntrusivePtr<Case> Case::Duplicate() {
    if ( expr_cases ) {
        auto new_exprs = expr_cases->Duplicate()->AsListExprPtr();
        return make_intrusive<Case>(new_exprs, nullptr, s->Duplicate());
    }

    IDPList* new_type_cases = nullptr;

    if ( type_cases ) {
        new_type_cases = new IDPList();

        for ( auto tc : *type_cases ) {
            zeek::Ref(tc);
            new_type_cases->append(tc);
        }
    }

    return make_intrusive<Case>(nullptr, new_type_cases, s->Duplicate());
}

StmtPtr SwitchStmt::Duplicate() {
    auto new_cases = new case_list;

    loop_over_list(*cases, i) new_cases->append((*cases)[i]->Duplicate().release());

    return SetSucc(new SwitchStmt(e->Duplicate(), new_cases));
}

void SwitchStmt::Inline(Inliner* inl) {
    ExprStmt::Inline(inl);

    for ( auto c : *cases )
        // In principle this can do the operation multiple times
        // for a given body, but that's no big deal as repeated
        // calls won't do anything.
        c->Body()->Inline(inl);
}

bool SwitchStmt::IsReduced(Reducer* r) const {
    if ( ! e->IsReduced(r) )
        return NonReduced(e.get());

    if ( cases->length() == 0 )
        return false;

    for ( const auto& c : *cases ) {
        if ( c->ExprCases() && ! c->ExprCases()->IsReduced(r) )
            return false;

        if ( c->TypeCases() && ! r->IDsAreReduced(c->TypeCases()) )
            return false;

        if ( ! c->Body()->IsReduced(r) )
            return false;
    }

    return true;
}

StmtPtr SwitchStmt::DoReduce(Reducer* rc) {
    if ( cases->length() == 0 )
        // Degenerate.
        return TransformMe(make_intrusive<NullStmt>(), rc);

    auto s = with_location_of(make_intrusive<StmtList>(), this);
    StmtPtr red_e_stmt;

    if ( rc->Optimizing() )
        e = rc->OptExpr(e);
    else
        e = e->Reduce(rc, red_e_stmt);

    // Note, the compiler checks for constant switch expressions.

    if ( red_e_stmt )
        s->Stmts().push_back(red_e_stmt);

    // Update type cases.
    for ( auto& i : case_label_type_list ) {
        IDPtr idp = {NewRef{}, i.first};
        if ( idp->Name() )
            i.first = rc->UpdateID(idp).release();
    }

    for ( const auto& c : *cases ) {
        auto c_e = c->ExprCases();
        if ( c_e ) {
            StmtPtr c_e_stmt;
            auto red_cases = c_e->Reduce(rc, c_e_stmt);

            if ( c_e_stmt )
                s->Stmts().push_back(c_e_stmt);
        }

        auto c_t = c->TypeCases();
        if ( c_t ) {
            for ( auto& c_t_i : *c_t )
                if ( c_t_i->Name() )
                    c_t_i = rc->UpdateID({NewRef{}, c_t_i}).release();
        }

        c->UpdateBody(c->Body()->Reduce(rc));
    }

    if ( ! s->Stmts().empty() )
        return TransformMe(make_intrusive<StmtList>(s, ThisPtr()), rc);

    return ThisPtr();
}

bool SwitchStmt::NoFlowAfter(bool ignore_break) const {
    bool control_reaches_end = false;
    bool default_seen_with_no_flow_after = false;

    for ( const auto& c : *Cases() ) {
        if ( ! c->Body()->NoFlowAfter(true) )
            return false;

        if ( (! c->ExprCases() || c->ExprCases()->Exprs().length() == 0) &&
             (! c->TypeCases() || c->TypeCases()->length() == 0) )
            // We saw the default, and the test before this
            // one established that it has no flow after it.
            default_seen_with_no_flow_after = true;
    }

    return default_seen_with_no_flow_after;
}

bool SwitchStmt::CouldReturn(bool ignore_break) const {
    for ( const auto& c : *Cases() )
        if ( c->Body()->CouldReturn(true) )
            return true;

    return false;
}

bool AddDelStmt::IsReduced(Reducer* c) const { return e->HasReducedOps(c); }

StmtPtr AddDelStmt::DoReduce(Reducer* c) {
    if ( c->Optimizing() ) {
        e = c->OptExpr(e);
        return ThisPtr();
    }

    auto red_e_stmt = e->ReduceToSingletons(c);

    if ( red_e_stmt )
        return TransformMe(make_intrusive<StmtList>(red_e_stmt, ThisPtr()), c);
    else
        return ThisPtr();
}

StmtPtr AddStmt::Duplicate() { return SetSucc(new AddStmt(e->Duplicate())); }

StmtPtr DelStmt::Duplicate() { return SetSucc(new DelStmt(e->Duplicate())); }

StmtPtr EventStmt::Duplicate() { return SetSucc(new EventStmt(e->Duplicate()->AsEventExprPtr())); }

StmtPtr EventStmt::DoReduce(Reducer* c) {
    if ( c->Optimizing() ) {
        e = c->OptExpr(e);
        event_expr = e->AsEventExprPtr();
    }

    else if ( ! event_expr->IsSingleton(c) ) {
        StmtPtr red_e_stmt;
        auto ee_red = event_expr->Reduce(c, red_e_stmt);

        event_expr = ee_red->AsEventExprPtr();
        e = event_expr;

        if ( red_e_stmt )
            return TransformMe(make_intrusive<StmtList>(red_e_stmt, ThisPtr()), c);
    }

    return ThisPtr();
}

StmtPtr WhileStmt::Duplicate() { return SetSucc(new WhileStmt(loop_condition->Duplicate(), body->Duplicate())); }

void WhileStmt::Inline(Inliner* inl) {
    loop_condition = loop_condition->Inline(inl);

    if ( loop_cond_pred_stmt )
        loop_cond_pred_stmt->Inline(inl);
    if ( body )
        body->Inline(inl);
}

bool WhileStmt::IsReduced(Reducer* c) const {
    // No need to check loop_cond_pred_stmt, as we create it reduced.
    return loop_condition->IsReducedConditional(c) && body->IsReduced(c);
}

StmtPtr WhileStmt::DoReduce(Reducer* c) {
    if ( loop_cond_pred_stmt )
        // Important to do this before updating the loop_condition, since
        // changes to the predecessor statement can alter the condition.
        loop_cond_pred_stmt = loop_cond_pred_stmt->Reduce(c);

    if ( c->Optimizing() )
        loop_condition = c->OptExpr(loop_condition);
    else {
        if ( IsReduced(c) ) {
            if ( ! c->IsPruning() ) {
                // See comment below for the particulars
                // of this constructor.
                stmt_loop_condition = with_location_of(make_intrusive<ExprStmt>(STMT_EXPR, loop_condition), this);
                return ThisPtr();
            }
        }
        else
            loop_condition = loop_condition->ReduceToConditional(c, loop_cond_pred_stmt);
    }

    body = body->Reduce(c);

    // We use the more involved ExprStmt constructor here to bypass
    // its check for whether the expression is being ignored, since
    // we're not actually creating an ExprStmt for execution.
    stmt_loop_condition = with_location_of(make_intrusive<ExprStmt>(STMT_EXPR, loop_condition), this);

    return ThisPtr();
}

bool WhileStmt::CouldReturn(bool ignore_break) const { return body->CouldReturn(false); }

StmtPtr ForStmt::Duplicate() {
    auto expr_copy = e->Duplicate();

    auto new_loop_vars = new zeek::IDPList;
    loop_over_list(*loop_vars, i) {
        auto id = (*loop_vars)[i];
        zeek::Ref(id);
        new_loop_vars->append(id);
    }

    ForStmt* f;
    if ( value_var )
        f = new ForStmt(new_loop_vars, expr_copy, value_var);
    else
        f = new ForStmt(new_loop_vars, expr_copy);

    f->AddBody(body->Duplicate());

    return SetSucc(f);
}

void ForStmt::Inline(Inliner* inl) {
    ExprStmt::Inline(inl);
    body->Inline(inl);
}

bool ForStmt::IsReduced(Reducer* c) const {
    if ( ! e->IsReduced(c) )
        return NonReduced(e.get());

    if ( ! c->IDsAreReduced(loop_vars) )
        return false;

    if ( value_var && (value_var->IsBlank() || ! c->ID_IsReduced(value_var)) )
        return false;

    return body->IsReduced(c);
}

StmtPtr ForStmt::DoReduce(Reducer* c) {
    if ( value_var && value_var->IsBlank() ) {
        auto no_vv = make_intrusive<ForStmt>(loop_vars, e);
        no_vv->AddBody(body);
        return TransformMe(no_vv, c);
    }

    StmtPtr red_e_stmt;

    if ( c->Optimizing() )
        e = c->OptExpr(e);
    else {
        e = e->Reduce(c, red_e_stmt);
        c->UpdateIDs(loop_vars);

        if ( value_var )
            value_var = c->UpdateID(value_var);
    }

    body = body->Reduce(c);

    if ( body->Tag() == STMT_NULL )
        Warn("empty \"for\" body leaves loop variables in indeterminate state");

    if ( red_e_stmt )
        return TransformMe(make_intrusive<StmtList>(red_e_stmt, ThisPtr()), c);

    return ThisPtr();
}

bool ForStmt::CouldReturn(bool ignore_break) const { return body->CouldReturn(false); }

StmtPtr ReturnStmt::Duplicate() { return SetSucc(new ReturnStmt(e ? e->Duplicate() : nullptr, true)); }

ReturnStmt::ReturnStmt(ExprPtr arg_e, bool ignored) : ExprStmt(STMT_RETURN, std::move(arg_e)) {}

bool ReturnStmt::IsReduced(Reducer* c) const {
    if ( ! e || e->IsSingleton(c) )
        return true;

    return NonReduced(e.get());
}

StmtPtr ReturnStmt::DoReduce(Reducer* c) {
    if ( ! e )
        return ThisPtr();

    if ( c->Optimizing() )
        e = c->OptExpr(e);

    else if ( ! e->IsSingleton(c) ) {
        StmtPtr red_e_stmt;
        e = e->ReduceToSingleton(c, red_e_stmt);

        if ( red_e_stmt )
            return TransformMe(make_intrusive<StmtList>(red_e_stmt, ThisPtr()), c);
    }

    return ThisPtr();
}

StmtList::StmtList(StmtPtr s1, StmtPtr s2) : Stmt(STMT_LIST) {
    if ( s1 )
        stmts.push_back(std::move(s1));
    if ( s2 )
        stmts.push_back(std::move(s2));
}

StmtList::StmtList(StmtPtr s1, StmtPtr s2, StmtPtr s3) : Stmt(STMT_LIST) {
    if ( s1 )
        stmts.push_back(std::move(s1));
    if ( s2 )
        stmts.push_back(std::move(s2));
    if ( s3 )
        stmts.push_back(std::move(s3));
}

StmtPtr StmtList::Duplicate() {
    auto new_sl = new StmtList();

    for ( auto& stmt : stmts )
        new_sl->stmts.push_back(stmt->Duplicate());

    return SetSucc(new_sl);
}

void StmtList::Inline(Inliner* inl) {
    for ( const auto& stmt : stmts )
        stmt->Inline(inl);
}

bool StmtList::IsReduced(Reducer* c) const {
    auto n = stmts.size();

    for ( auto i = 0U; i < n; ++i ) {
        auto& s_i = stmts[i];
        if ( ! s_i->IsReduced(c) )
            return false;

        if ( s_i->NoFlowAfter(false) && i < n - 1 )
            return false;
    }

    return true;
}

StmtPtr StmtList::DoReduce(Reducer* c) {
    std::vector<StmtPtr> f_stmts;
    bool did_change = false;

    auto n = stmts.size();

    for ( auto i = 0U; i < n; ++i ) {
        if ( ReduceStmt(i, f_stmts, c) )
            did_change = true;

        if ( i < n - 1 && stmts[i]->NoFlowAfter(false) ) {
            did_change = true;
            break;
        }

        if ( reporter->Errors() > 0 )
            return ThisPtr();
    }

    if ( f_stmts.empty() )
        return TransformMe(make_intrusive<NullStmt>(), c);

    if ( f_stmts.size() == 1 )
        return f_stmts[0]->Reduce(c);

    if ( did_change ) {
        ResetStmts(std::move(f_stmts));
        return Reduce(c);
    }

    return ThisPtr();
}

unsigned int StmtList::FindRecAssignmentChain(unsigned int i) const {
    const NameExpr* targ_rec = nullptr;
    std::set<int> fields_seen;

    for ( ; i < stmts.size(); ++i ) {
        const auto& s = stmts[i];

        // We're looking for either "x$a = y$b" or "x$a = x$a + y$b".
        if ( s->Tag() != STMT_EXPR )
            // No way it's an assignment.
            return i;

        auto se = s->AsExprStmt()->StmtExpr();
        if ( se->Tag() != EXPR_ASSIGN )
            return i;

        // The LHS of an assignment starts with a RefExpr.
        auto lhs_ref = se->GetOp1();
        ASSERT(lhs_ref->Tag() == EXPR_REF);

        auto lhs = lhs_ref->GetOp1();
        if ( lhs->Tag() != EXPR_FIELD )
            // Not of the form "x$a = ...".
            return i;

        auto lhs_field = lhs->AsFieldExpr()->Field();
        if ( fields_seen.count(lhs_field) > 0 )
            // Earlier in this chain we've already seen "x$a", so end the
            // chain at this repeated use because it's no longer a simple
            // block of field assignments.
            return i;

        fields_seen.insert(lhs_field);

        auto lhs_rec = lhs->GetOp1();
        if ( lhs_rec->Tag() != EXPR_NAME )
            // Not a simple field reference, e.g. "x$y$a".
            return i;

        auto lhs_rec_n = lhs_rec->AsNameExpr();

        if ( targ_rec ) {
            if ( lhs_rec_n->Id() != targ_rec->Id() )
                // It's no longer "x$..." but some new variable "z$...".
                return i;
        }
        else
            targ_rec = lhs_rec_n;
    }

    return i;
}

void StmtList::UpdateAssignmentChains(const StmtPtr& s, OpChain& assign_chains, OpChain& add_chains) const {
    auto se = s->AsExprStmt()->StmtExpr();
    ASSERT(se->Tag() == EXPR_ASSIGN);

    // We dig three times into the LHS. The first gets the EXPR_ASSIGN's
    // first operand, which is a RefExpr; the second gets its operand,
    // which we've guaranteed in FindRecAssignmentChain is a FieldExpr;
    // and the third is the FieldExpr's operand, which we've guaranteed
    // is a NameExpr.
    auto lhs_id = se->GetOp1()->GetOp1()->GetOp1()->AsNameExpr()->Id();
    auto rhs = se->GetOp2();
    const FieldExpr* f;
    OpChain* c;

    // Check whether RHS is either "y$b" or "x$a + y$b".

    if ( rhs->Tag() == EXPR_ADD ) {
        auto rhs_op1 = rhs->GetOp1(); // need to see that it's "x$a"

        if ( rhs_op1->Tag() != EXPR_FIELD )
            return;

        auto rhs_op1_rec = rhs_op1->GetOp1();
        if ( rhs_op1_rec->Tag() != EXPR_NAME || rhs_op1_rec->AsNameExpr()->Id() != lhs_id )
            return;

        auto rhs_op2 = rhs->GetOp2(); // need to see that it's "y$b"
        if ( rhs_op2->Tag() != EXPR_FIELD )
            return;

        if ( ! IsArithmetic(rhs_op2->GetType()->Tag()) )
            // Avoid esoteric forms of adding.
            return;

        f = rhs_op2->AsFieldExpr();
        c = &add_chains;
    }

    else if ( rhs->Tag() == EXPR_FIELD ) {
        f = rhs->AsFieldExpr();
        c = &assign_chains;
    }

    else
        // Not a RHS we know how to leverage.
        return;

    auto f_rec = f->GetOp1();
    if ( f_rec->Tag() != EXPR_NAME )
        // Not a simple RHS, instead something like "y$z$b".
        return;

    // If we get here, it's a keeper, record the associated statement.
    auto id = f_rec->AsNameExpr()->Id();
    auto cf = c->find(id);
    if ( cf == c->end() )
        (*c)[id] = std::vector<const Stmt*>{s.get()};
    else
        cf->second.push_back(s.get());
}

StmtPtr StmtList::TransformChain(const OpChain& c, ExprTag t, std::set<const Stmt*>& chain_stmts) const {
    IntrusivePtr<StmtList> sl;

    for ( auto& id_stmts : c ) {
        auto orig_s = id_stmts.second;

        if ( ! sl )
            // Now that we have a statement, create our list and associate
            // its location with the statement.
            sl = with_location_of(make_intrusive<StmtList>(), orig_s[0]);

        ExprPtr e;
        if ( t == EXPR_ASSIGN )
            e = make_intrusive<AssignRecordFields>(orig_s, chain_stmts);
        else
            e = make_intrusive<AddRecordFields>(orig_s, chain_stmts);

        e->SetLocationInfo(sl->GetLocationInfo());
        auto es = with_location_of(make_intrusive<ExprStmt>(std::move(e)), sl);
        sl->Stmts().emplace_back(std::move(es));
    }

    return sl;
}

bool StmtList::SimplifyChain(unsigned int start, unsigned int end, std::vector<StmtPtr>& f_stmts) const {
    OpChain assign_chains;
    OpChain add_chains;
    std::set<const Stmt*> chain_stmts;

    static bool skip_chains = getenv("ZAM_SKIP_CHAINS");
    if ( skip_chains )
        return false;

    for ( auto i = start; i <= end; ++i ) {
        auto& s = stmts[i];
        chain_stmts.insert(s.get());
        UpdateAssignmentChains(s, assign_chains, add_chains);
    }

    // An add-chain of any size is a win. For an assign-chain to be a win,
    // it needs to have at least two elements, because a single "x$a = y$b"
    // can be expressed using one ZAM instructino (but "x$a += y$b" cannot).
    if ( add_chains.empty() ) {
        bool have_useful_assign_chain = false;
        for ( auto& ac : assign_chains )
            if ( ac.second.size() > 1 ) {
                have_useful_assign_chain = true;
                break;
            }

        if ( ! have_useful_assign_chain )
            // No gains available.
            return false;
    }

    auto as_c = TransformChain(assign_chains, EXPR_ASSIGN, chain_stmts);
    auto ad_c = TransformChain(add_chains, EXPR_ADD, chain_stmts);

    ASSERT(as_c || ad_c);

    if ( as_c )
        f_stmts.push_back(as_c);
    if ( ad_c )
        f_stmts.push_back(ad_c);

    // At this point, chain_stmts has only the remainders that weren't removed.
    for ( auto s : stmts )
        if ( chain_stmts.count(s.get()) > 0 )
            f_stmts.push_back(s);

    return true;
}

bool StmtList::ReduceStmt(unsigned int& s_i, std::vector<StmtPtr>& f_stmts, Reducer* c) {
    bool did_change = false;
    auto& stmt_i = stmts[s_i];
    auto old_stmt = stmt_i;

    auto chain_end = FindRecAssignmentChain(s_i);
    if ( chain_end > s_i && SimplifyChain(s_i, chain_end - 1, f_stmts) ) {
        s_i = chain_end - 1;
        return true;
    }

    auto stmt = stmt_i->Reduce(c);

    if ( stmt != old_stmt )
        did_change = true;

    if ( c->Optimizing() && stmt->Tag() == STMT_EXPR ) {
        // There are two potential optimizations that affect
        // whether we keep assignment statements.  The first is
        // for potential assignment chains like
        //
        //	tmp1 = x;
        //	tmp2 = tmp1;
        //
        // where we can change this pair to simply "tmp2 = x", assuming
        // no later use of tmp1.
        //
        // In addition, if we have "tmp1 = e" and "e" is an expression
        // already computed into another temporary (say tmp0) that's
        // safely usable at this point, then we can elide the tmp1
        // assignment entirely.
        auto s_e = stmt->AsExprStmt();
        auto e = s_e->StmtExpr();

        if ( e->Tag() != EXPR_ASSIGN ) {
            f_stmts.push_back(std::move(stmt));
            return did_change;
        }

        auto a = e->AsAssignExpr();
        auto lhs = a->Op1()->AsRefExprPtr()->Op();

        if ( lhs->Tag() != EXPR_NAME ) {
            f_stmts.push_back(std::move(stmt));
            return did_change;
        }

        auto var = lhs->AsNameExpr();
        auto rhs = a->GetOp2();

        if ( s_i < stmts.size() - 1 ) {
            // See if we can compress an assignment chain.
            auto& s_i_succ = stmts[s_i + 1];

            // Don't reduce s_i_succ.  If it's what we're
            // looking for, it's already reduced.  Plus
            // that's what Reducer::MergeStmts (not that
            // it really matters, per the comment there).
            auto merge = c->MergeStmts(var, rhs, s_i_succ);
            if ( merge ) {
                f_stmts.push_back(std::move(merge));

                // Skip both this statement and the next,
                // now that we've substituted the merge.
                ++s_i;
                return true;
            }
        }

        if ( c->IsTemporary(var->Id()) && ! c->IsParamTemp(var->Id()) && c->IsCSE(a, var, rhs.get()) ) {
            // printf("discarding %s as unnecessary\n", var->Id()->Name());
            // Skip this now unnecessary statement.
            return true;
        }
    }

    if ( stmt->Tag() == STMT_LIST ) { // inline the list
        auto sl = stmt->AsStmtList();

        for ( auto& sub_stmt : sl->Stmts() )
            f_stmts.push_back(sub_stmt);

        did_change = true;
    }

    else if ( stmt->Tag() == STMT_NULL )
        // skip it
        did_change = true;

    else
        f_stmts.push_back(std::move(stmt));

    return did_change;
}

bool StmtList::NoFlowAfter(bool ignore_break) const {
    for ( auto& s : stmts ) {
        // For "break" statements, if ignore_break is set then
        // by construction flow *does* go to after this statement
        // list.  If we just used the second test below, then
        // while the "break" would indicate there's flow after it,
        // if there's dead code following that includes a "return",
        // this would in fact be incorrect.
        if ( ignore_break && s->Tag() == STMT_BREAK )
            return false;

        if ( s->NoFlowAfter(ignore_break) )
            return true;
    }

    return false;
}

bool StmtList::CouldReturn(bool ignore_break) const {
    for ( auto& s : stmts )
        if ( s->CouldReturn(ignore_break) )
            return true;

    return false;
}

StmtPtr InitStmt::Duplicate() {
    // Need to duplicate the initializer list since later reductions
    // can modify it in place.
    std::vector<IDPtr> new_inits;
    for ( const auto& id : inits )
        new_inits.push_back(id);

    return SetSucc(new InitStmt(new_inits));
}

bool InitStmt::IsReduced(Reducer* c) const { return c->IDsAreReduced(inits); }

StmtPtr InitStmt::DoReduce(Reducer* c) {
    c->UpdateIDs(inits);
    return ThisPtr();
}

StmtPtr AssertStmt::Duplicate() { return SetSucc(new AssertStmt(cond->Duplicate(), msg ? msg->Duplicate() : nullptr)); }

bool AssertStmt::IsReduced(Reducer* c) const { return false; }

StmtPtr AssertStmt::DoReduce(Reducer* c) { return TransformMe(make_intrusive<NullStmt>(), c); }

bool WhenInfo::HasUnreducedIDs(Reducer* c) const {
    for ( auto& cp : *cl ) {
        auto cid = cp.Id();

        if ( when_new_locals.count(cid.get()) == 0 && ! c->ID_IsReduced(cp.Id()) )
            return true;
    }

    for ( auto& l : when_expr_locals )
        if ( ! c->ID_IsReduced(l) )
            return true;

    return false;
}

void WhenInfo::UpdateIDs(Reducer* c) {
    for ( auto& cp : *cl ) {
        auto& cid = cp.Id();
        if ( when_new_locals.count(cid.get()) == 0 )
            cp.SetID(c->UpdateID(cid));
    }

    for ( auto& l : when_expr_locals )
        l = c->UpdateID(l);
}

StmtPtr WhenStmt::Duplicate() { return SetSucc(new WhenStmt(std::make_shared<WhenInfo>(wi.get()))); }

bool WhenStmt::IsReduced(Reducer* c) const {
    if ( wi->HasUnreducedIDs(c) )
        return false;

    if ( ! wi->Lambda()->IsReduced(c) )
        return false;

    if ( ! wi->TimeoutExpr() )
        return true;

    return wi->TimeoutExpr()->IsReduced(c);
}

StmtPtr WhenStmt::DoReduce(Reducer* c) {
    if ( ! c->Optimizing() ) {
        wi->UpdateIDs(c);
        (void)wi->Lambda()->ReduceToSingletons(c);
    }

    auto e = wi->TimeoutExpr();

    if ( ! e )
        return ThisPtr();

    if ( c->Optimizing() )
        wi->SetTimeoutExpr(c->OptExpr(e));

    else if ( ! e->IsSingleton(c) ) {
        StmtPtr red_e_stmt;
        auto new_e = e->ReduceToSingleton(c, red_e_stmt);
        wi->SetTimeoutExpr(new_e);

        if ( red_e_stmt )
            return TransformMe(make_intrusive<StmtList>(red_e_stmt, ThisPtr()), c);
    }

    return ThisPtr();
}

CatchReturnStmt::CatchReturnStmt(ScriptFuncPtr _sf, StmtPtr _block, NameExprPtr _ret_var) : Stmt(STMT_CATCH_RETURN) {
    sf = std::move(_sf);
    block = std::move(_block);
    ret_var = std::move(_ret_var);
}

ValPtr CatchReturnStmt::Exec(Frame* f, StmtFlowType& flow) {
    RegisterAccess();

    auto val = block->Exec(f, flow);

    if ( flow == FLOW_RETURN )
        flow = FLOW_NEXT;

    if ( ret_var )
        f->SetElement(ret_var->Id()->Offset(), val);

    // Note, do *not* return the value!  That's taken as a signal
    // that a full return executed.
    return nullptr;
}

bool CatchReturnStmt::IsPure() const {
    // The ret_var is pure by construction.
    return block->IsPure();
}

StmtPtr CatchReturnStmt::Duplicate() {
    auto rv_dup = ret_var->Duplicate();
    auto rv_dup_ptr = rv_dup->AsNameExprPtr();
    return SetSucc(new CatchReturnStmt(sf, block->Duplicate(), rv_dup_ptr));
}

StmtPtr CatchReturnStmt::DoReduce(Reducer* c) {
    block = block->Reduce(c);

    if ( block->Tag() == STMT_RETURN ) {
        // The whole thing reduced to a bare return.  This can
        // happen due to constant propagation.
        auto ret = block->AsReturnStmt();
        auto ret_e = ret->StmtExprPtr();

        if ( ! ret_e ) {
            if ( ret_var )
                reporter->InternalError("inlining inconsistency: no return value");

            return TransformMe(make_intrusive<NullStmt>(), c);
        }

        auto rv_dup = ret_var->Duplicate();
        auto ret_e_dup = ret_e->Duplicate();

        auto assign = with_location_of(make_intrusive<AssignExpr>(rv_dup, ret_e_dup, false), this);
        assign_stmt = with_location_of(make_intrusive<ExprStmt>(assign), this);

        if ( ret_e_dup->Tag() == EXPR_CONST ) {
            auto ce = ret_e_dup->AsConstExpr();
            rv_dup->AsNameExpr()->Id()->GetOptInfo()->SetConst(ce);
        }

        return assign_stmt;
    }

    return ThisPtr();
}

void CatchReturnStmt::StmtDescribe(ODesc* d) const {
    Stmt::StmtDescribe(d);
    block->Describe(d);
    DescribeDone(d);
}

TraversalCode CatchReturnStmt::Traverse(TraversalCallback* cb) const {
    TraversalCode tc = cb->PreStmt(this);
    HANDLE_TC_STMT_PRE(tc);

    tc = block->Traverse(cb);
    HANDLE_TC_STMT_PRE(tc);

    if ( ret_var ) {
        tc = ret_var->Traverse(cb);
        HANDLE_TC_STMT_PRE(tc);
    }

    tc = cb->PostStmt(this);
    HANDLE_TC_STMT_POST(tc);
}

CheckAnyLenStmt::CheckAnyLenStmt(ExprPtr arg_e, int _expected_len) : ExprStmt(STMT_CHECK_ANY_LEN, std::move(arg_e)) {
    expected_len = _expected_len;
}

ValPtr CheckAnyLenStmt::Exec(Frame* f, StmtFlowType& flow) {
    RegisterAccess();
    flow = FLOW_NEXT;

    auto& v = e->Eval(f)->AsListVal()->Vals();

    if ( v.size() != static_cast<size_t>(expected_len) )
        reporter->ExprRuntimeError(e.get(), "mismatch in list lengths");

    return nullptr;
}

StmtPtr CheckAnyLenStmt::Duplicate() { return SetSucc(new CheckAnyLenStmt(e->Duplicate(), expected_len)); }

bool CheckAnyLenStmt::IsReduced(Reducer* c) const { return true; }

StmtPtr CheckAnyLenStmt::DoReduce(Reducer* c) {
    // These are created in reduced form.
    return ThisPtr();
}

void CheckAnyLenStmt::StmtDescribe(ODesc* d) const {
    Stmt::StmtDescribe(d);

    e->Describe(d);
    if ( ! d->IsBinary() )
        d->Add(".length == ");

    d->Add(expected_len);

    DescribeDone(d);
}

} // namespace zeek::detail