swc_ecma_transforms_optimization/simplify/expr/

mod.rs

use std::{borrow::Cow, iter, iter::once};

use swc_atoms::{
    atom,
    wtf8::{CodePoint, Wtf8, Wtf8Buf},
    Atom,
};
use swc_common::{
    pass::{CompilerPass, Repeated},
    util::take::Take,
    Mark, Span, Spanned, SyntaxContext, DUMMY_SP,
};
use swc_ecma_ast::*;
use swc_ecma_transforms_base::{
    ext::ExprRefExt,
    perf::{cpu_count, Parallel, ParallelExt},
};
use swc_ecma_utils::{
    is_literal, number::JsNumber, prop_name_eq, to_int32, BoolType, ExprCtx, ExprExt, NullType,
    NumberType, ObjectType, StringType, SymbolType, UndefinedType, Value,
};
use swc_ecma_visit::{noop_visit_mut_type, visit_mut_pass, VisitMut, VisitMutWith};
use Value::{Known, Unknown};

use crate::debug::debug_assert_valid;

#[cfg(test)]
mod tests;

macro_rules! try_val {
    ($v:expr) => {{
        match $v {
            Value::Known(v) => v,
            Value::Unknown => return Value::Unknown,
        }
    }};
}

/// All [bool] fields defaults to [false].
#[derive(Debug, Clone, Copy, Default, Hash)]
pub struct Config {}

/// Not intended for general use. Use [simplifier] instead.
///
/// Ported from `PeepholeFoldConstants` of google closure compiler.
pub fn expr_simplifier(
    unresolved_mark: Mark,
    config: Config,
) -> impl Repeated + Pass + CompilerPass + VisitMut + 'static {
    visit_mut_pass(SimplifyExpr {
        expr_ctx: ExprCtx {
            unresolved_ctxt: SyntaxContext::empty().apply_mark(unresolved_mark),
            is_unresolved_ref_safe: false,
            in_strict: false,
            remaining_depth: 4,
        },
        config,
        changed: false,
        is_arg_of_update: false,
        is_modifying: false,
        in_callee: false,
    })
}

impl Parallel for SimplifyExpr {
    fn create(&self) -> Self {
        Self { ..*self }
    }

    fn merge(&mut self, other: Self) {
        self.changed |= other.changed;
    }
}

#[derive(Debug)]
struct SimplifyExpr {
    expr_ctx: ExprCtx,
    config: Config,

    changed: bool,
    is_arg_of_update: bool,
    is_modifying: bool,
    in_callee: bool,
}

impl CompilerPass for SimplifyExpr {
    fn name(&self) -> Cow<'static, str> {
        Cow::Borrowed("simplify-expr")
    }
}

impl Repeated for SimplifyExpr {
    fn changed(&self) -> bool {
        self.changed
    }

    fn reset(&mut self) {
        self.changed = false;
    }
}

impl VisitMut for SimplifyExpr {
    noop_visit_mut_type!();

    fn visit_mut_assign_expr(&mut self, n: &mut AssignExpr) {
        let old = self.is_modifying;
        self.is_modifying = true;
        n.left.visit_mut_with(self);
        self.is_modifying = old;

        self.is_modifying = false;
        n.right.visit_mut_with(self);
        self.is_modifying = old;
    }

    /// This is overriden to preserve `this`.
    fn visit_mut_call_expr(&mut self, n: &mut CallExpr) {
        let old_in_callee = self.in_callee;

        self.in_callee = true;
        match &mut n.callee {
            Callee::Super(..) | Callee::Import(..) => {}
            Callee::Expr(e) => {
                let may_inject_zero = !need_zero_for_this(e);

                match &mut **e {
                    Expr::Seq(seq) => {
                        if seq.exprs.len() == 1 {
                            let mut expr = seq.exprs.take().into_iter().next().unwrap();
                            expr.visit_mut_with(self);
                            *e = expr;
                        } else if seq
                            .exprs
                            .last()
                            .map(|v| &**v)
                            .is_some_and(Expr::directness_matters)
                        {
                            match seq.exprs.first().map(|v| &**v) {
                                Some(Expr::Lit(..) | Expr::Ident(..)) => {}
                                _ => {
                                    tracing::debug!("Injecting `0` to preserve `this = undefined`");
                                    seq.exprs.insert(0, 0.0.into());
                                }
                            }

                            seq.visit_mut_with(self);
                        }
                    }

                    _ => {
                        e.visit_mut_with(self);
                    }
                }

                if may_inject_zero && need_zero_for_this(e) {
                    match &mut **e {
                        Expr::Seq(seq) => {
                            seq.exprs.insert(0, 0.into());
                        }
                        _ => {
                            let seq = SeqExpr {
                                span: DUMMY_SP,
                                exprs: vec![0.0.into(), e.take()],
                            };
                            **e = seq.into();
                        }
                    }
                }
            }
            #[cfg(swc_ast_unknown)]
            _ => panic!("unable to access unknown nodes"),
        }

        self.in_callee = false;
        n.args.visit_mut_with(self);

        self.in_callee = old_in_callee;
    }

    fn visit_mut_class_members(&mut self, members: &mut Vec<ClassMember>) {
        self.maybe_par(cpu_count(), members, |v, member| {
            member.visit_mut_with(v);
        });
    }

    fn visit_mut_export_default_expr(&mut self, expr: &mut ExportDefaultExpr) {
        fn is_paren_wrap_fn_or_class(expr: &mut Expr, visitor: &mut SimplifyExpr) -> bool {
            match &mut *expr {
                Expr::Fn(..) | Expr::Class(..) => {
                    expr.visit_mut_children_with(visitor);
                    true
                }
                Expr::Paren(p) => is_paren_wrap_fn_or_class(&mut p.expr, visitor),
                _ => false,
            }
        }

        if !is_paren_wrap_fn_or_class(&mut expr.expr, self) {
            expr.visit_mut_children_with(self);
        }
    }

    fn visit_mut_expr(&mut self, expr: &mut Expr) {
        if let Expr::Unary(UnaryExpr {
            op: op!("delete"), ..
        }) = expr
        {
            return;
        }
        // fold children before doing something more.
        expr.visit_mut_children_with(self);

        match expr {
            // Do nothing.
            // Note: Paren should be handled in fixer
            Expr::Lit(_) | Expr::This(..) | Expr::Paren(..) => return,

            Expr::Seq(seq) if seq.exprs.is_empty() => return,

            Expr::Unary(..)
            | Expr::Bin(..)
            | Expr::Member(..)
            | Expr::Cond(..)
            | Expr::Seq(..)
            | Expr::Array(..)
            | Expr::Object(..)
            | Expr::New(..) => {}

            _ => return,
        }

        match expr {
            Expr::Unary(_) => {
                optimize_unary_expr(self.expr_ctx, expr, &mut self.changed);
                debug_assert_valid(expr);
            }
            Expr::Bin(_) => {
                optimize_bin_expr(self.expr_ctx, expr, &mut self.changed);
                if expr.is_seq() {
                    expr.visit_mut_with(self);
                }

                debug_assert_valid(expr);
            }
            Expr::Member(_) => {
                if !self.is_modifying {
                    optimize_member_expr(self.expr_ctx, expr, self.in_callee, &mut self.changed);

                    debug_assert_valid(expr);
                }
            }

            Expr::Cond(CondExpr {
                span,
                test,
                cons,
                alt,
            }) => {
                if let (p, Known(val)) = test.cast_to_bool(self.expr_ctx) {
                    self.changed = true;

                    let expr_value = if val { cons } else { alt };
                    *expr = if p.is_pure() {
                        if expr_value.directness_matters() {
                            SeqExpr {
                                span: *span,
                                exprs: vec![0.into(), expr_value.take()],
                            }
                            .into()
                        } else {
                            *expr_value.take()
                        }
                    } else {
                        SeqExpr {
                            span: *span,
                            exprs: vec![test.take(), expr_value.take()],
                        }
                        .into()
                    }
                }
            }

            // Simplify sequence expression.
            Expr::Seq(SeqExpr { exprs, .. }) => {
                if exprs.len() == 1 {
                    //TODO: Respan
                    *expr = *exprs.take().into_iter().next().unwrap()
                } else {
                    assert!(!exprs.is_empty(), "sequence expression should not be empty");
                    //TODO: remove unused
                }
            }

            Expr::Array(ArrayLit { elems, .. }) => {
                let mut e = Vec::with_capacity(elems.len());

                for elem in elems.take() {
                    match elem {
                        Some(ExprOrSpread {
                            spread: Some(..),
                            expr,
                        }) if expr.is_array() => {
                            self.changed = true;

                            e.extend(expr.array().unwrap().elems.into_iter().map(|elem| {
                                Some(elem.unwrap_or_else(|| ExprOrSpread {
                                    spread: None,
                                    expr: Expr::undefined(DUMMY_SP),
                                }))
                            }));
                        }

                        _ => e.push(elem),
                    }
                }
                *elems = e;
            }

            Expr::Object(ObjectLit { props, .. }) => {
                let should_work = props.iter().any(|p| matches!(p, PropOrSpread::Spread(..)));
                if !should_work {
                    return;
                }

                let mut ps = Vec::with_capacity(props.len());

                for p in props.take() {
                    match p {
                        PropOrSpread::Spread(SpreadElement {
                            dot3_token, expr, ..
                        }) if expr.is_object() => {
                            if let Expr::Object(obj) = &*expr {
                                if obj.props.iter().any(|p| match p {
                                    PropOrSpread::Spread(..) => true,
                                    PropOrSpread::Prop(p) => !matches!(
                                        &**p,
                                        Prop::Shorthand(_) | Prop::KeyValue(_) | Prop::Method(_)
                                    ),
                                    #[cfg(swc_ast_unknown)]
                                    _ => panic!("unable to access unknown nodes"),
                                }) {
                                    ps.push(PropOrSpread::Spread(SpreadElement {
                                        dot3_token,
                                        expr,
                                    }));
                                    continue;
                                }
                            }
                            let props = expr.object().unwrap().props;
                            ps.extend(props);
                            self.changed = true;
                        }

                        _ => ps.push(p),
                    }
                }
                *props = ps;
            }

            // be conservative.
            _ => {}
        };
    }

    fn visit_mut_expr_or_spreads(&mut self, n: &mut Vec<ExprOrSpread>) {
        self.maybe_par(cpu_count(), n, |v, n| {
            n.visit_mut_with(v);
        });
    }

    fn visit_mut_exprs(&mut self, n: &mut Vec<Box<Expr>>) {
        self.maybe_par(cpu_count(), n, |v, n| {
            n.visit_mut_with(v);
        });
    }

    fn visit_mut_for_head(&mut self, n: &mut ForHead) {
        let old = self.is_modifying;
        self.is_modifying = true;
        n.visit_mut_children_with(self);
        self.is_modifying = old;
    }

    fn visit_mut_module_items(&mut self, n: &mut Vec<ModuleItem>) {
        let mut child = SimplifyExpr {
            expr_ctx: self.expr_ctx,
            config: self.config,
            changed: Default::default(),
            is_arg_of_update: Default::default(),
            is_modifying: Default::default(),
            in_callee: Default::default(),
        };

        child.maybe_par(cpu_count(), n, |v, n| {
            n.visit_mut_with(v);
        });
        self.changed |= child.changed;
    }

    /// Currently noop
    #[inline]
    fn visit_mut_opt_chain_expr(&mut self, _: &mut OptChainExpr) {}

    fn visit_mut_opt_var_decl_or_expr(&mut self, n: &mut Option<VarDeclOrExpr>) {
        if let Some(VarDeclOrExpr::Expr(e)) = n {
            match &mut **e {
                Expr::Seq(SeqExpr { exprs, .. }) if exprs.is_empty() => {
                    *n = None;
                    return;
                }
                _ => {}
            }
        }

        n.visit_mut_children_with(self);
    }

    fn visit_mut_opt_vec_expr_or_spreads(&mut self, n: &mut Vec<Option<ExprOrSpread>>) {
        self.maybe_par(cpu_count(), n, |v, n| {
            n.visit_mut_with(v);
        });
    }

    fn visit_mut_pat(&mut self, p: &mut Pat) {
        let old_in_callee = self.in_callee;
        self.in_callee = false;
        p.visit_mut_children_with(self);
        self.in_callee = old_in_callee;

        if let Pat::Assign(a) = p {
            if a.right.is_undefined(self.expr_ctx)
                || match *a.right {
                    Expr::Unary(UnaryExpr {
                        op: op!("void"),
                        ref arg,
                        ..
                    }) => !arg.may_have_side_effects(self.expr_ctx),
                    _ => false,
                }
            {
                self.changed = true;
                *p = *a.left.take();
            }
        }
    }

    fn visit_mut_prop_or_spreads(&mut self, n: &mut Vec<PropOrSpread>) {
        self.maybe_par(cpu_count(), n, |v, n| {
            n.visit_mut_with(v);
        });
    }

    /// Drops unused values
    fn visit_mut_seq_expr(&mut self, e: &mut SeqExpr) {
        if e.exprs.is_empty() {
            return;
        }

        let old_in_callee = self.in_callee;
        let len = e.exprs.len();
        for (idx, e) in e.exprs.iter_mut().enumerate() {
            if idx == len - 1 {
                self.in_callee = old_in_callee;
            } else {
                self.in_callee = false;
            }

            e.visit_mut_with(self);
        }
        self.in_callee = old_in_callee;

        let len = e.exprs.len();

        let last_expr = e.exprs.pop().expect("SeqExpr.exprs must not be empty");

        // Expressions except last one
        let mut exprs = Vec::with_capacity(e.exprs.len() + 1);

        for expr in e.exprs.take() {
            match *expr {
                Expr::Lit(Lit::Num(n)) if self.in_callee && n.value == 0.0 => {
                    if exprs.is_empty() {
                        exprs.push(0.0.into());

                        tracing::trace!("expr_simplifier: Preserving first zero");
                    }
                }

                Expr::Lit(..) | Expr::Ident(..)
                    if self.in_callee && !expr.may_have_side_effects(self.expr_ctx) =>
                {
                    if exprs.is_empty() {
                        self.changed = true;

                        exprs.push(0.0.into());

                        tracing::debug!("expr_simplifier: Injected first zero");
                    }
                }

                // Drop side-effect free nodes.
                Expr::Lit(_) => {}

                // Flatten array
                Expr::Array(ArrayLit { span, elems }) => {
                    let is_simple = elems
                        .iter()
                        .all(|elem| matches!(elem, None | Some(ExprOrSpread { spread: None, .. })));

                    if is_simple {
                        exprs.extend(elems.into_iter().flatten().map(|e| e.expr));
                    } else {
                        exprs.push(Box::new(ArrayLit { span, elems }.into()));
                    }
                }

                // Default case: preserve it
                _ => exprs.push(expr),
            }
        }

        exprs.push(last_expr);

        self.changed |= len != exprs.len();

        e.exprs = exprs;
    }

    fn visit_mut_stmt(&mut self, s: &mut Stmt) {
        let old_is_modifying = self.is_modifying;
        self.is_modifying = false;
        let old_is_arg_of_update = self.is_arg_of_update;
        self.is_arg_of_update = false;
        s.visit_mut_children_with(self);
        self.is_arg_of_update = old_is_arg_of_update;
        self.is_modifying = old_is_modifying;

        debug_assert_valid(s);
    }

    fn visit_mut_stmts(&mut self, n: &mut Vec<Stmt>) {
        let mut child = SimplifyExpr {
            expr_ctx: self.expr_ctx,
            config: self.config,
            changed: Default::default(),
            is_arg_of_update: Default::default(),
            is_modifying: Default::default(),
            in_callee: Default::default(),
        };

        child.maybe_par(cpu_count(), n, |v, n| {
            n.visit_mut_with(v);
        });
        self.changed |= child.changed;
    }

    fn visit_mut_tagged_tpl(&mut self, n: &mut TaggedTpl) {
        let old = self.in_callee;
        self.in_callee = true;

        n.tag.visit_mut_with(self);

        self.in_callee = false;
        n.tpl.visit_mut_with(self);

        self.in_callee = old;
    }

    fn visit_mut_update_expr(&mut self, n: &mut UpdateExpr) {
        let old = self.is_modifying;
        self.is_modifying = true;
        n.arg.visit_mut_with(self);
        self.is_modifying = old;
    }

    fn visit_mut_with_stmt(&mut self, n: &mut WithStmt) {
        n.obj.visit_mut_with(self);
    }
}

/// make a new boolean expression preserving side effects, if any.
fn make_bool_expr<I>(ctx: ExprCtx, span: Span, value: bool, orig: I) -> Box<Expr>
where
    I: IntoIterator<Item = Box<Expr>>,
{
    ctx.preserve_effects(span, Lit::Bool(Bool { value, span }).into(), orig)
}

/// Gets the `idx`-th UTF-16 code unit from the given [Wtf8].
/// Surrogate pairs are splitted into high and low surrogates and counted
/// separately.
fn nth_char(s: &Wtf8, idx: usize) -> CodePoint {
    match s.to_ill_formed_utf16().nth(idx) {
        Some(c) =>
        // SAFETY: `IllFormedUtf16CodeUnits` always returns code units in the range of UTF-16.
        unsafe { CodePoint::from_u32_unchecked(c as u32) },
        None => unreachable!("string is too short"),
    }
}

fn need_zero_for_this(e: &Expr) -> bool {
    e.directness_matters() || e.is_seq()
}

/// Gets the value of the given key from the given object properties, if the key
/// exists. If the key does exist, `Some` is returned and the property is
/// removed from the given properties.
fn get_key_value(key: &str, props: &mut Vec<PropOrSpread>) -> Option<Box<Expr>> {
    // It's impossible to know the value for certain if a spread property exists.
    let has_spread = props.iter().any(|prop| prop.is_spread());

    if has_spread {
        return None;
    }

    for (i, prop) in props.iter_mut().enumerate().rev() {
        let prop = match prop {
            PropOrSpread::Prop(x) => &mut **x,
            PropOrSpread::Spread(_) => unreachable!(),
            #[cfg(swc_ast_unknown)]
            _ => panic!("unable to access unknown nodes"),
        };

        match prop {
            Prop::Shorthand(ident) if ident.sym == key => {
                let prop = match props.remove(i) {
                    PropOrSpread::Prop(x) => *x,
                    _ => unreachable!(),
                };
                let ident = match prop {
                    Prop::Shorthand(x) => x,
                    _ => unreachable!(),
                };
                return Some(ident.into());
            }

            Prop::KeyValue(prop) => {
                if key != "__proto__" && prop_name_eq(&prop.key, "__proto__") {
                    // If __proto__ is defined, we need to check the contents of it,
                    // as well as any nested __proto__ objects
                    let Expr::Object(ObjectLit { props, .. }) = &mut *prop.value else {
                        // __proto__ is not an ObjectLiteral. It's unsafe to keep trying to find
                        // a value for this key, since __proto__ might also contain the key.
                        return None;
                    };

                    // Get key value from __props__ object. Only return if
                    // the result is Some. If None, we keep searching in the
                    // parent object.
                    let v = get_key_value(key, props);
                    if v.is_some() {
                        return v;
                    }
                } else if prop_name_eq(&prop.key, key) {
                    let prop = match props.remove(i) {
                        PropOrSpread::Prop(x) => *x,
                        _ => unreachable!(),
                    };
                    let prop = match prop {
                        Prop::KeyValue(x) => x,
                        _ => unreachable!(),
                    };
                    return Some(prop.value);
                }
            }

            _ => {}
        }
    }

    None
}

/// **NOTE**: This is **NOT** a public API. DO NOT USE.
pub fn optimize_member_expr(
    expr_ctx: ExprCtx,
    expr: &mut Expr,
    is_callee: bool,
    changed: &mut bool,
) {
    let MemberExpr { obj, prop, .. } = match expr {
        Expr::Member(member) => member,
        _ => return,
    };

    #[derive(Clone, PartialEq, Debug)]
    enum KnownOp {
        /// [a, b].length
        Len,

        /// [a, b][0]
        ///
        /// {0.5: "bar"}[0.5]
        /// Note: callers need to check `v.fract() == 0.0` in some cases.
        /// ie non-integer indexes for arrays result in `undefined`
        /// but not for objects (because indexing an object
        /// returns the value of the key, ie `0.5` will not
        /// return `undefined` if a key `0.5` exists
        /// and its value is not `undefined`).
        Index(f64),

        /// ({}).foo
        IndexStr(Atom),
    }
    let op = match prop {
        MemberProp::Ident(IdentName { sym, .. }) if &**sym == "length" && !obj.is_object() => {
            KnownOp::Len
        }
        MemberProp::Ident(IdentName { sym, .. }) => {
            if is_callee {
                return;
            }

            KnownOp::IndexStr(sym.clone())
        }
        MemberProp::Computed(ComputedPropName { expr, .. }) => {
            if is_callee {
                return;
            }

            if let Expr::Lit(Lit::Num(Number { value, .. })) = &**expr {
                // x[5]
                KnownOp::Index(*value)
            } else if let Known(s) = expr.as_pure_string(expr_ctx) {
                if s == "length" && !obj.is_object() {
                    // Length of non-object type
                    KnownOp::Len
                } else if let Ok(n) = s.parse::<f64>() {
                    // x['0'] is treated as x[0]
                    KnownOp::Index(n)
                } else {
                    // x[''] or x[...] where ... is an expression like [], ie x[[]]
                    KnownOp::IndexStr(s.into())
                }
            } else {
                return;
            }
        }
        _ => return,
    };

    // Note: pristine_globals refers to the compress config option pristine_globals.
    // Any potential cases where globals are not pristine are handled in compress,
    // e.g. x[-1] is not changed as the object's prototype may be modified.
    // For example, Array.prototype[-1] = "foo" will result in [][-1] returning
    // "foo".

    match &mut **obj {
        Expr::Lit(Lit::Str(Str { value, span, .. })) => match op {
            // 'foo'.length
            //
            // Prototype changes do not affect .length, so we don't need to worry
            // about pristine_globals here.
            KnownOp::Len => {
                *changed = true;

                let Some(value) = value.as_str() else {
                    return;
                };

                *expr = Lit::Num(Number {
                    value: value.chars().map(|c| c.len_utf16()).sum::<usize>() as _,
                    span: *span,
                    raw: None,
                })
                .into();
            }

            // 'foo'[1]
            KnownOp::Index(idx) => {
                if idx.fract() != 0.0 || idx < 0.0 || idx as usize >= value.len() {
                    // Prototype changes affect indexing if the index is out of bounds, so we
                    // don't replace out-of-bound indexes.
                    return;
                }

                let c = nth_char(value, idx as _);
                *changed = true;

                // `nth_char` always returns a code point within the UTF-16 range.
                let mut value = Wtf8Buf::with_capacity(2);
                value.push(c);

                *expr = Lit::Str(Str {
                    raw: None,
                    value: value.into(),
                    span: *span,
                })
                .into()
            }

            // 'foo'['']
            //
            // Handled in compress
            KnownOp::IndexStr(..) => {}
        },

        // [1, 2, 3].length
        //
        // [1, 2, 3][0]
        Expr::Array(ArrayLit { elems, span }) => {
            // do nothing if spread exists
            let has_spread = elems.iter().any(|elem| {
                elem.as_ref()
                    .map(|elem| elem.spread.is_some())
                    .unwrap_or(false)
            });

            if has_spread {
                return;
            }

            match op {
                KnownOp::Len => {
                    // do nothing if replacement will have side effects
                    let may_have_side_effects = elems
                        .iter()
                        .filter_map(|e| e.as_ref())
                        .any(|e| e.expr.may_have_side_effects(expr_ctx));

                    if may_have_side_effects {
                        return;
                    }

                    // Prototype changes do not affect .length
                    *changed = true;

                    *expr = Lit::Num(Number {
                        value: elems.len() as _,
                        span: *span,
                        raw: None,
                    })
                    .into();
                }

                KnownOp::Index(idx) => {
                    // If the fraction part is non-zero, or if the index is out of bounds,
                    // then we handle this in compress as Array's prototype may be modified.
                    if idx.fract() != 0.0 || idx < 0.0 || idx as usize >= elems.len() {
                        return;
                    }

                    // Don't change if after has side effects.
                    let after_has_side_effect =
                        elems
                            .iter()
                            .skip((idx as usize + 1) as _)
                            .any(|elem| match elem {
                                Some(elem) => elem.expr.may_have_side_effects(expr_ctx),
                                None => false,
                            });

                    if after_has_side_effect {
                        return;
                    }

                    *changed = true;

                    // elements before target element
                    let before: Vec<Option<ExprOrSpread>> = elems.drain(..(idx as usize)).collect();
                    let mut iter = elems.take().into_iter();
                    // element at idx
                    let e = iter.next().flatten();
                    // elements after target element
                    let after: Vec<Option<ExprOrSpread>> = iter.collect();

                    // element value
                    let v = match e {
                        None => Expr::undefined(*span),
                        Some(e) => e.expr,
                    };

                    // Replacement expressions.
                    let mut exprs = Vec::new();

                    // Add before side effects.
                    for elem in before.into_iter().flatten() {
                        expr_ctx.extract_side_effects_to(&mut exprs, *elem.expr);
                    }

                    // Element value.
                    let val = v;

                    // Add after side effects.
                    for elem in after.into_iter().flatten() {
                        expr_ctx.extract_side_effects_to(&mut exprs, *elem.expr);
                    }

                    // Note: we always replace with a SeqExpr so that
                    // `this` remains undefined in strict mode.

                    // No side effects exist, replace with:
                    // (0, val)
                    if exprs.is_empty() && val.directness_matters() {
                        exprs.push(0.into());
                    }

                    // Add value and replace with SeqExpr
                    exprs.push(val);
                    *expr = *Expr::from_exprs(exprs);
                }

                // Handled in compress
                KnownOp::IndexStr(..) => {}
            }
        }

        // { foo: true }['foo']
        //
        // { 0.5: true }[0.5]
        Expr::Object(ObjectLit { props, span }) => {
            // get key
            let key = match op {
                KnownOp::Index(i) => Atom::from(i.to_string()),
                KnownOp::IndexStr(key) if key != *"yield" && is_literal(props) => key,
                _ => return,
            };

            // Get `key`s value. Non-existent keys are handled in compress.
            // This also checks if spread exists.
            let Some(v) = get_key_value(&key, props) else {
                return;
            };

            *changed = true;

            *expr = *expr_ctx.preserve_effects(
                *span,
                v,
                once(
                    ObjectLit {
                        props: props.take(),
                        span: *span,
                    }
                    .into(),
                ),
            );
        }

        _ => {}
    }
}

/// **NOTE**: This is **NOT** a public API. DO NOT USE.
pub fn optimize_bin_expr(expr_ctx: ExprCtx, expr: &mut Expr, changed: &mut bool) {
    let BinExpr {
        left,
        op,
        right,
        span,
    } = match expr {
        Expr::Bin(bin) => bin,
        _ => return,
    };
    let op = *op;

    macro_rules! try_replace {
        ($v:expr) => {{
            match $v {
                Known(v) => {
                    // TODO: Optimize
                    *changed = true;

                    *expr = *make_bool_expr(expr_ctx, *span, v, {
                        iter::once(left.take()).chain(iter::once(right.take()))
                    });
                    return;
                }
                _ => {}
            }
        }};
        (number, $v:expr) => {{
            match $v {
                Known(v) => {
                    *changed = true;

                    let value_expr = if !v.is_nan() {
                        Expr::Lit(Lit::Num(Number {
                            value: v,
                            span: *span,
                            raw: None,
                        }))
                    } else {
                        Expr::Ident(Ident::new(atom!("NaN"), *span, expr_ctx.unresolved_ctxt))
                    };

                    *expr = *expr_ctx.preserve_effects(*span, value_expr.into(), {
                        iter::once(left.take()).chain(iter::once(right.take()))
                    });
                    return;
                }
                _ => {}
            }
        }};
    }

    match op {
        op!(bin, "+") => {
            // It's string concatenation if either left or right is string.
            if let (Known(l), Known(r)) =
                (left.as_pure_wtf8(expr_ctx), right.as_pure_wtf8(expr_ctx))
            {
                if left.is_str() || left.is_array_lit() || right.is_str() || right.is_array_lit() {
                    let mut l = l.into_owned();

                    l.push_wtf8(&r);

                    *changed = true;

                    *expr = Lit::Str(Str {
                        raw: None,
                        value: l.into(),
                        span: *span,
                    })
                    .into();
                    return;
                }
            }

            match expr.get_type(expr_ctx) {
                // String concatenation
                Known(StringType) => match expr {
                    Expr::Bin(BinExpr {
                        left, right, span, ..
                    }) => {
                        if !left.may_have_side_effects(expr_ctx)
                            && !right.may_have_side_effects(expr_ctx)
                        {
                            if let (Known(l), Known(r)) =
                                (left.as_pure_wtf8(expr_ctx), right.as_pure_wtf8(expr_ctx))
                            {
                                *changed = true;

                                let mut value = l.into_owned();
                                value.push_wtf8(&r);

                                *expr = Lit::Str(Str {
                                    raw: None,
                                    value: value.into(),
                                    span: *span,
                                })
                                .into();
                            }
                        }
                    }
                    _ => unreachable!(),
                },
                // Numerical calculation
                Known(BoolType) | Known(NullType) | Known(NumberType) | Known(UndefinedType) => {
                    match expr {
                        Expr::Bin(BinExpr {
                            left, right, span, ..
                        }) => {
                            if let Known(v) = perform_arithmetic_op(expr_ctx, op, left, right) {
                                *changed = true;
                                let span = *span;

                                let value_expr = if !v.is_nan() {
                                    Lit::Num(Number {
                                        value: v,
                                        span,
                                        raw: None,
                                    })
                                    .into()
                                } else {
                                    Ident::new(atom!("NaN"), span, expr_ctx.unresolved_ctxt).into()
                                };

                                *expr = *expr_ctx.preserve_effects(
                                    span,
                                    value_expr,
                                    iter::once(left.take()).chain(iter::once(right.take())),
                                );
                            }
                        }
                        _ => unreachable!(),
                    };
                }
                _ => {}
            }

            //TODO: try string concat
        }

        op!("&&") | op!("||") => {
            if let (_, Known(val)) = left.cast_to_bool(expr_ctx) {
                let node = if op == op!("&&") {
                    if val {
                        // 1 && $right
                        right
                    } else {
                        *changed = true;

                        // 0 && $right
                        *expr = *left.take();
                        return;
                    }
                } else if val {
                    *changed = true;

                    // 1 || $right
                    *expr = *(left.take());
                    return;
                } else {
                    // 0 || $right
                    right
                };

                if !left.may_have_side_effects(expr_ctx) {
                    *changed = true;

                    if node.directness_matters() {
                        *expr = SeqExpr {
                            span: node.span(),
                            exprs: vec![0.into(), node.take()],
                        }
                        .into();
                    } else {
                        *expr = *node.take();
                    }
                } else {
                    *changed = true;

                    let seq = SeqExpr {
                        span: *span,
                        exprs: vec![left.take(), node.take()],
                    };

                    *expr = seq.into()
                };
            }
        }
        op!("instanceof") => {
            fn is_non_obj(e: &Expr) -> bool {
                match e {
                    // Non-object types are never instances.
                    Expr::Lit(Lit::Str { .. })
                    | Expr::Lit(Lit::Num(..))
                    | Expr::Lit(Lit::Null(..))
                    | Expr::Lit(Lit::Bool(..)) => true,
                    Expr::Ident(Ident { sym, .. }) if &**sym == "undefined" => true,
                    Expr::Ident(Ident { sym, .. }) if &**sym == "Infinity" => true,
                    Expr::Ident(Ident { sym, .. }) if &**sym == "NaN" => true,

                    Expr::Unary(UnaryExpr {
                        op: op!("!"),
                        ref arg,
                        ..
                    })
                    | Expr::Unary(UnaryExpr {
                        op: op!(unary, "-"),
                        ref arg,
                        ..
                    })
                    | Expr::Unary(UnaryExpr {
                        op: op!("void"),
                        ref arg,
                        ..
                    }) => is_non_obj(arg),
                    _ => false,
                }
            }

            fn is_obj(e: &Expr) -> bool {
                matches!(
                    *e,
                    Expr::Array { .. } | Expr::Object { .. } | Expr::Fn { .. } | Expr::New { .. }
                )
            }

            // Non-object types are never instances.
            if is_non_obj(left) {
                *changed = true;

                *expr = *make_bool_expr(expr_ctx, *span, false, iter::once(right.take()));
                return;
            }

            if is_obj(left) && right.is_global_ref_to(expr_ctx, "Object") {
                *changed = true;

                *expr = *make_bool_expr(expr_ctx, *span, true, iter::once(left.take()));
            }
        }

        // Arithmetic operations
        op!(bin, "-") | op!("/") | op!("%") | op!("**") => {
            try_replace!(number, perform_arithmetic_op(expr_ctx, op, left, right))
        }

        // Bit shift operations
        op!("<<") | op!(">>") | op!(">>>") => {
            fn try_fold_shift(ctx: ExprCtx, op: BinaryOp, left: &Expr, right: &Expr) -> Value<f64> {
                if !left.is_number() || !right.is_number() {
                    return Unknown;
                }

                let (lv, rv) = match (left.as_pure_number(ctx), right.as_pure_number(ctx)) {
                    (Known(lv), Known(rv)) => (lv, rv),
                    _ => unreachable!(),
                };
                let (lv, rv) = (JsNumber::from(lv), JsNumber::from(rv));

                Known(match op {
                    op!("<<") => *(lv << rv),
                    op!(">>") => *(lv >> rv),
                    op!(">>>") => *(lv.unsigned_shr(rv)),

                    _ => unreachable!("Unknown bit operator {:?}", op),
                })
            }
            try_replace!(number, try_fold_shift(expr_ctx, op, left, right))
        }

        // These needs one more check.
        //
        // (a * 1) * 2 --> a * (1 * 2) --> a * 2
        op!("*") | op!("&") | op!("|") | op!("^") => {
            try_replace!(number, perform_arithmetic_op(expr_ctx, op, left, right));

            // Try left.rhs * right
            if let Expr::Bin(BinExpr {
                span: _,
                left: left_lhs,
                op: left_op,
                right: left_rhs,
            }) = &mut **left
            {
                if *left_op == op {
                    if let Known(value) = perform_arithmetic_op(expr_ctx, op, left_rhs, right) {
                        let value_expr = if !value.is_nan() {
                            Lit::Num(Number {
                                value,
                                span: *span,
                                raw: None,
                            })
                            .into()
                        } else {
                            Ident::new(atom!("NaN"), *span, expr_ctx.unresolved_ctxt).into()
                        };

                        *changed = true;
                        *left = left_lhs.take();
                        *right = Box::new(value_expr);
                    }
                }
            }
        }

        // Comparisons
        op!("<") => {
            try_replace!(perform_abstract_rel_cmp(expr_ctx, left, right, false))
        }
        op!(">") => {
            try_replace!(perform_abstract_rel_cmp(expr_ctx, right, left, false))
        }
        op!("<=") => {
            try_replace!(!perform_abstract_rel_cmp(expr_ctx, right, left, true))
        }
        op!(">=") => {
            try_replace!(!perform_abstract_rel_cmp(expr_ctx, left, right, true))
        }

        op!("==") => try_replace!(perform_abstract_eq_cmp(expr_ctx, *span, left, right)),
        op!("!=") => try_replace!(!perform_abstract_eq_cmp(expr_ctx, *span, left, right)),
        op!("===") => try_replace!(perform_strict_eq_cmp(expr_ctx, left, right)),
        op!("!==") => try_replace!(!perform_strict_eq_cmp(expr_ctx, left, right)),
        _ => {}
    };
}

/// **NOTE**: This is **NOT** a public API. DO NOT USE.
pub fn optimize_unary_expr(expr_ctx: ExprCtx, expr: &mut Expr, changed: &mut bool) {
    let UnaryExpr { op, arg, span } = match expr {
        Expr::Unary(unary) => unary,
        _ => return,
    };
    let may_have_side_effects = arg.may_have_side_effects(expr_ctx);

    match op {
        op!("typeof") if !may_have_side_effects => {
            try_fold_typeof(expr_ctx, expr, changed);
        }
        op!("!") => {
            match &**arg {
                // Don't expand booleans.
                Expr::Lit(Lit::Num(..)) => return,

                // Don't remove ! from negated iifes.
                Expr::Call(call) => {
                    if let Callee::Expr(callee) = &call.callee {
                        if let Expr::Fn(..) = &**callee {
                            return;
                        }
                    }
                }
                _ => {}
            }

            if let (_, Known(val)) = arg.cast_to_bool(expr_ctx) {
                *changed = true;

                *expr = *make_bool_expr(expr_ctx, *span, !val, iter::once(arg.take()));
            }
        }
        op!(unary, "+") => {
            if let Known(v) = arg.as_pure_number(expr_ctx) {
                *changed = true;

                if v.is_nan() {
                    *expr = *expr_ctx.preserve_effects(
                        *span,
                        Ident::new(atom!("NaN"), *span, expr_ctx.unresolved_ctxt).into(),
                        iter::once(arg.take()),
                    );
                    return;
                }

                *expr = *expr_ctx.preserve_effects(
                    *span,
                    Lit::Num(Number {
                        value: v,
                        span: *span,
                        raw: None,
                    })
                    .into(),
                    iter::once(arg.take()),
                );
            }
        }
        op!(unary, "-") => match &**arg {
            Expr::Ident(Ident { sym, .. }) if &**sym == "Infinity" => {}
            // "-NaN" is "NaN"
            Expr::Ident(Ident { sym, .. }) if &**sym == "NaN" => {
                *changed = true;
                *expr = *(arg.take());
            }
            Expr::Lit(Lit::Num(Number { value: f, .. })) => {
                *changed = true;
                *expr = Lit::Num(Number {
                    value: -f,
                    span: *span,
                    raw: None,
                })
                .into();
            }
            _ => {

                // TODO: Report that user is something bad (negating
                // non-number value)
            }
        },
        op!("void") if !may_have_side_effects => {
            match &**arg {
                Expr::Lit(Lit::Num(Number { value, .. })) if *value == 0.0 => return,
                _ => {}
            }
            *changed = true;

            *arg = Lit::Num(Number {
                value: 0.0,
                span: arg.span(),
                raw: None,
            })
            .into();
        }

        op!("~") => {
            if let Known(value) = arg.as_pure_number(expr_ctx) {
                if value.fract() == 0.0 {
                    *changed = true;
                    *expr = Lit::Num(Number {
                        span: *span,
                        value: if value < 0.0 {
                            !(value as i32 as u32) as i32 as f64
                        } else {
                            !(value as u32) as i32 as f64
                        },
                        raw: None,
                    })
                    .into();
                }
                // TODO: Report error
            }
        }
        _ => {}
    }
}

/// Folds 'typeof(foo)' if foo is a literal, e.g.
///
/// typeof("bar") --> "string"
///
/// typeof(6) --> "number"
fn try_fold_typeof(_expr_ctx: ExprCtx, expr: &mut Expr, changed: &mut bool) {
    let UnaryExpr { op, arg, span } = match expr {
        Expr::Unary(unary) => unary,
        _ => return,
    };
    assert_eq!(*op, op!("typeof"));

    let val = match &**arg {
        Expr::Fn(..) => "function",
        Expr::Lit(Lit::Str { .. }) => "string",
        Expr::Lit(Lit::Num(..)) => "number",
        Expr::Lit(Lit::Bool(..)) => "boolean",
        Expr::Lit(Lit::Null(..)) | Expr::Object { .. } | Expr::Array { .. } => "object",
        Expr::Unary(UnaryExpr {
            op: op!("void"), ..
        }) => "undefined",

        Expr::Ident(Ident { sym, .. }) if &**sym == "undefined" => {
            // We can assume `undefined` is `undefined`,
            // because overriding `undefined` is always hard error in swc.
            "undefined"
        }

        _ => {
            return;
        }
    };

    *changed = true;

    *expr = Lit::Str(Str {
        span: *span,
        raw: None,
        value: val.into(),
    })
    .into();
}

/// Try to fold arithmetic binary operators
fn perform_arithmetic_op(expr_ctx: ExprCtx, op: BinaryOp, left: &Expr, right: &Expr) -> Value<f64> {
    /// Replace only if it becomes shorter
    macro_rules! try_replace {
        ($value:expr) => {{
            let (ls, rs) = (left.span(), right.span());
            if ls.is_dummy() || rs.is_dummy() {
                Known($value)
            } else {
                let new_len = format!("{}", $value).len();
                if right.span().hi() > left.span().lo() {
                    let orig_len =
                        right.span().hi() - right.span().lo() + left.span().hi() - left.span().lo();
                    if new_len <= orig_len.0 as usize + 1 {
                        Known($value)
                    } else {
                        Unknown
                    }
                } else {
                    Known($value)
                }
            }
        }};
        (i32, $value:expr) => {
            try_replace!($value as f64)
        };
    }

    let (lv, rv) = (
        left.as_pure_number(expr_ctx),
        right.as_pure_number(expr_ctx),
    );

    if (lv.is_unknown() && rv.is_unknown())
        || op == op!(bin, "+")
            && (!left.get_type(expr_ctx).casted_to_number_on_add()
                || !right.get_type(expr_ctx).casted_to_number_on_add())
    {
        return Unknown;
    }

    match op {
        op!(bin, "+") => {
            if let (Known(lv), Known(rv)) = (lv, rv) {
                return try_replace!(lv + rv);
            }

            if lv == Known(0.0) {
                return rv;
            } else if rv == Known(0.0) {
                return lv;
            }

            return Unknown;
        }
        op!(bin, "-") => {
            if let (Known(lv), Known(rv)) = (lv, rv) {
                return try_replace!(lv - rv);
            }

            // 0 - x => -x
            if lv == Known(0.0) {
                return rv;
            }

            // x - 0 => x
            if rv == Known(0.0) {
                return lv;
            }

            return Unknown;
        }
        op!("*") => {
            if let (Known(lv), Known(rv)) = (lv, rv) {
                return try_replace!(lv * rv);
            }
            // NOTE: 0*x != 0 for all x, if x==0, then it is NaN.  So we can't take
            // advantage of that without some kind of non-NaN proof.  So the special cases
            // here only deal with 1*x
            if Known(1.0) == lv {
                return rv;
            }
            if Known(1.0) == rv {
                return lv;
            }

            return Unknown;
        }

        op!("/") => {
            if let (Known(lv), Known(rv)) = (lv, rv) {
                if rv == 0.0 {
                    return Unknown;
                }
                return try_replace!(lv / rv);
            }

            // NOTE: 0/x != 0 for all x, if x==0, then it is NaN

            if rv == Known(1.0) {
                // TODO: cloneTree
                // x/1->x
                return lv;
            }
            return Unknown;
        }

        op!("**") => {
            if Known(0.0) == rv {
                return Known(1.0);
            }

            if let (Known(lv), Known(rv)) = (lv, rv) {
                let lv: JsNumber = lv.into();
                let rv: JsNumber = rv.into();
                let result: f64 = lv.pow(rv).into();
                return try_replace!(result);
            }

            return Unknown;
        }
        _ => {}
    }
    let (lv, rv) = match (lv, rv) {
        (Known(lv), Known(rv)) => (lv, rv),
        _ => return Unknown,
    };

    match op {
        op!("&") => try_replace!(i32, to_int32(lv) & to_int32(rv)),
        op!("|") => try_replace!(i32, to_int32(lv) | to_int32(rv)),
        op!("^") => try_replace!(i32, to_int32(lv) ^ to_int32(rv)),
        op!("%") => {
            if rv == 0.0 {
                return Unknown;
            }
            try_replace!(lv % rv)
        }
        _ => unreachable!("unknown binary operator: {:?}", op),
    }
}

/// This actually performs `<`.
///
/// https://tc39.github.io/ecma262/#sec-abstract-relational-comparison
fn perform_abstract_rel_cmp(
    expr_ctx: ExprCtx,
    left: &Expr,
    right: &Expr,
    will_negate: bool,
) -> Value<bool> {
    match (left, right) {
        // Special case: `x < x` is always false.
        (
            &Expr::Ident(
                Ident {
                    sym: ref li,
                    ctxt: l_ctxt,
                    ..
                },
                ..,
            ),
            &Expr::Ident(Ident {
                sym: ref ri,
                ctxt: r_ctxt,
                ..
            }),
        ) if !will_negate && li == ri && l_ctxt == r_ctxt => {
            return Known(false);
        }
        // Special case: `typeof a < typeof a` is always false.
        (
            &Expr::Unary(UnaryExpr {
                op: op!("typeof"),
                arg: ref la,
                ..
            }),
            &Expr::Unary(UnaryExpr {
                op: op!("typeof"),
                arg: ref ra,
                ..
            }),
        ) if la.as_ident().is_some()
            && la.as_ident().map(|i| i.to_id()) == ra.as_ident().map(|i| i.to_id()) =>
        {
            return Known(false)
        }
        _ => {}
    }

    // Try to evaluate based on the general type.
    let (lt, rt) = (left.get_type(expr_ctx), right.get_type(expr_ctx));

    if let (Known(StringType), Known(StringType)) = (lt, rt) {
        if let (Known(lv), Known(rv)) = (
            left.as_pure_string(expr_ctx),
            right.as_pure_string(expr_ctx),
        ) {
            // In JS, browsers parse \v differently. So do not compare strings if one
            // contains \v.
            if lv.contains('\u{000B}') || rv.contains('\u{000B}') {
                return Unknown;
            } else {
                return Known(lv < rv);
            }
        }
    }

    // Then, try to evaluate based on the value of the node. Try comparing as
    // numbers.
    let (lv, rv) = (
        try_val!(left.as_pure_number(expr_ctx)),
        try_val!(right.as_pure_number(expr_ctx)),
    );
    if lv.is_nan() || rv.is_nan() {
        return Known(will_negate);
    }

    Known(lv < rv)
}

/// https://tc39.github.io/ecma262/#sec-abstract-equality-comparison
fn perform_abstract_eq_cmp(
    expr_ctx: ExprCtx,
    span: Span,
    left: &Expr,
    right: &Expr,
) -> Value<bool> {
    let (lt, rt) = (
        try_val!(left.get_type(expr_ctx)),
        try_val!(right.get_type(expr_ctx)),
    );

    if lt == rt {
        return perform_strict_eq_cmp(expr_ctx, left, right);
    }

    match (lt, rt) {
        (NullType, UndefinedType) | (UndefinedType, NullType) => Known(true),
        (NumberType, StringType) | (_, BoolType) => {
            let rv = try_val!(right.as_pure_number(expr_ctx));
            perform_abstract_eq_cmp(
                expr_ctx,
                span,
                left,
                &Lit::Num(Number {
                    value: rv,
                    span,
                    raw: None,
                })
                .into(),
            )
        }

        (StringType, NumberType) | (BoolType, _) => {
            let lv = try_val!(left.as_pure_number(expr_ctx));
            perform_abstract_eq_cmp(
                expr_ctx,
                span,
                &Lit::Num(Number {
                    value: lv,
                    span,
                    raw: None,
                })
                .into(),
                right,
            )
        }

        (StringType, ObjectType)
        | (NumberType, ObjectType)
        | (ObjectType, StringType)
        | (ObjectType, NumberType) => Unknown,

        _ => Known(false),
    }
}

/// https://tc39.github.io/ecma262/#sec-strict-equality-comparison
fn perform_strict_eq_cmp(expr_ctx: ExprCtx, left: &Expr, right: &Expr) -> Value<bool> {
    // Any strict equality comparison against NaN returns false.
    if left.is_nan() || right.is_nan() {
        return Known(false);
    }
    match (left, right) {
        // Special case, typeof a == typeof a is always true.
        (
            &Expr::Unary(UnaryExpr {
                op: op!("typeof"),
                arg: ref la,
                ..
            }),
            &Expr::Unary(UnaryExpr {
                op: op!("typeof"),
                arg: ref ra,
                ..
            }),
        ) if la.as_ident().is_some()
            && la.as_ident().map(|i| i.to_id()) == ra.as_ident().map(|i| i.to_id()) =>
        {
            return Known(true)
        }
        _ => {}
    }

    let (lt, rt) = (
        try_val!(left.get_type(expr_ctx)),
        try_val!(right.get_type(expr_ctx)),
    );
    // Strict equality can only be true for values of the same type.
    if lt != rt {
        return Known(false);
    }

    match lt {
        UndefinedType | NullType => Known(true),
        NumberType => Known(
            try_val!(left.as_pure_number(expr_ctx)) == try_val!(right.as_pure_number(expr_ctx)),
        ),
        StringType => {
            let (lv, rv) = (
                try_val!(left.as_pure_string(expr_ctx)),
                try_val!(right.as_pure_string(expr_ctx)),
            );
            // In JS, browsers parse \v differently. So do not consider strings
            // equal if one contains \v.
            if lv.contains('\u{000B}') || rv.contains('\u{000B}') {
                return Unknown;
            }
            Known(lv == rv)
        }
        BoolType => {
            let (lv, rv) = (left.as_pure_bool(expr_ctx), right.as_pure_bool(expr_ctx));

            // lv && rv || !lv && !rv

            lv.and(rv).or((!lv).and(!rv))
        }
        ObjectType | SymbolType => Unknown,
    }
}