perf(semantic): reduce storage size for symbol redeclarations (#4463)

Most symbols don't have redeclarations.

So instead of storing `Vec<Span>` directly in `redeclare_variables` (24 bytes per symbol), store `Option<RedeclarationId>` (4 bytes).

`RedeclarationId` indexes into `redeclarations` where the actual `Vec<Span>` is stored. But for symbols with no redeclarations (the vast majority), it takes 4 bytes per symbol only.

crates/oxc_linter/src/rules/eslint/no_redeclare.rs

@@ -66,7 +66,7 @@ impl Rule for NoRedeclare {
                 AstKind::VariableDeclarator(var) => {
                     if let BindingPatternKind::BindingIdentifier(ident) = &var.id.kind {
                         if symbol_name == ident.name.as_str() {
-                            for span in ctx.symbols().get_redeclare_variables(symbol_id) {
+                            for span in ctx.symbols().get_redeclarations(symbol_id) {
                                 self.report_diagnostic(ctx, *span, ident);
                             }
                         }
@@ -75,7 +75,7 @@ impl Rule for NoRedeclare {
                 AstKind::FormalParameter(param) => {
                     if let BindingPatternKind::BindingIdentifier(ident) = &param.pattern.kind {
                         if symbol_name == ident.name.as_str() {
-                            for span in ctx.symbols().get_redeclare_variables(symbol_id) {
+                            for span in ctx.symbols().get_redeclarations(symbol_id) {
                                 self.report_diagnostic(ctx, *span, ident);
                             }
                         }

crates/oxc_linter/src/rules/eslint/no_shadow_restricted_names.rs

@@ -64,7 +64,7 @@ impl Rule for NoShadowRestrictedNames {
             }
 
             check_and_diagnostic(name, ctx.symbols().get_span(symbol_id), ctx);
-            for span in ctx.symbols().get_redeclare_variables(symbol_id) {
+            for span in ctx.symbols().get_redeclarations(symbol_id) {
                 check_and_diagnostic(name, *span, ctx);
             }
         });

crates/oxc_semantic/src/builder.rs

@@ -465,7 +465,7 @@ impl<'a> SemanticBuilder<'a> {
     }
 
     pub fn add_redeclare_variable(&mut self, symbol_id: SymbolId, span: Span) {
-        self.symbols.add_redeclare_variable(symbol_id, span);
+        self.symbols.add_redeclaration(symbol_id, span);
     }
 
     fn add_export_flag_to_export_identifiers(&mut self, program: &Program<'a>) {

crates/oxc_semantic/src/symbol.rs

@@ -5,7 +5,7 @@ use oxc_index::IndexVec;
 use oxc_span::{CompactStr, Span};
 pub use oxc_syntax::{
     scope::ScopeId,
-    symbol::{SymbolFlags, SymbolId},
+    symbol::{RedeclarationId, SymbolFlags, SymbolId},
 };
 #[cfg(feature = "serialize")]
 use serde::Serialize;
@@ -26,6 +26,10 @@ export type IndexVec<I, T> = Array<T>;
 /// Symbol Table
 ///
 /// `SoA` (Struct of Arrays) for memory efficiency.
+///
+/// Most symbols won't have redeclarations, so instead of storing `Vec<Span>` directly in
+/// `redeclare_variables` (32 bytes per symbol), store `Option<RedeclarationId>` (4 bytes).
+/// That ID indexes into `redeclarations` where the actual `Vec<Span>` is stored.
 #[derive(Debug, Default)]
 #[cfg_attr(feature = "serialize", derive(Serialize, Tsify), serde(rename_all = "camelCase"))]
 pub struct SymbolTable {
@@ -36,7 +40,9 @@ pub struct SymbolTable {
     /// Pointer to the AST Node where this symbol is declared
     pub declarations: IndexVec<SymbolId, AstNodeId>,
     pub resolved_references: IndexVec<SymbolId, Vec<ReferenceId>>,
-    pub redeclare_variables: IndexVec<SymbolId, Vec<Span>>,
+    redeclarations: IndexVec<SymbolId, Option<RedeclarationId>>,
+
+    redeclaration_spans: IndexVec<RedeclarationId, Vec<Span>>,
 
     pub references: IndexVec<ReferenceId, Reference>,
 }
@@ -90,8 +96,13 @@ impl SymbolTable {
         self.flags[symbol_id]
     }
 
-    pub fn get_redeclare_variables(&self, symbol_id: SymbolId) -> &Vec<Span> {
-        &self.redeclare_variables[symbol_id]
+    pub fn get_redeclarations(&self, symbol_id: SymbolId) -> &[Span] {
+        if let Some(redeclaration_id) = self.redeclarations[symbol_id] {
+            &self.redeclaration_spans[redeclaration_id]
+        } else {
+            static EMPTY: &[Span] = &[];
+            EMPTY
+        }
     }
 
     pub fn union_flag(&mut self, symbol_id: SymbolId, includes: SymbolFlags) {
@@ -128,11 +139,16 @@ impl SymbolTable {
         self.scope_ids.push(scope_id);
         self.declarations.push(node_id);
         self.resolved_references.push(vec![]);
-        self.redeclare_variables.push(vec![])
+        self.redeclarations.push(None)
     }
 
-    pub fn add_redeclare_variable(&mut self, symbol_id: SymbolId, span: Span) {
-        self.redeclare_variables[symbol_id].push(span);
+    pub fn add_redeclaration(&mut self, symbol_id: SymbolId, span: Span) {
+        if let Some(redeclaration_id) = self.redeclarations[symbol_id] {
+            self.redeclaration_spans[redeclaration_id].push(span);
+        } else {
+            let redeclaration_id = self.redeclaration_spans.push(vec![span]);
+            self.redeclarations[symbol_id] = Some(redeclaration_id);
+        };
     }
 
     pub fn create_reference(&mut self, reference: Reference) -> ReferenceId {
@@ -200,7 +216,7 @@ impl SymbolTable {
         self.scope_ids.reserve(additional_symbols);
         self.declarations.reserve(additional_symbols);
         self.resolved_references.reserve(additional_symbols);
-        self.redeclare_variables.reserve(additional_symbols);
+        self.redeclarations.reserve(additional_symbols);
 
         self.references.reserve(additional_references);
     }

crates/oxc_syntax/src/symbol.rs

@@ -23,11 +23,29 @@ impl Idx for SymbolId {
     }
 }
 
+#[derive(Debug, Clone, Copy, Eq, PartialEq, Ord, PartialOrd, Hash)]
+#[cfg_attr(feature = "serialize", derive(Serialize))]
+pub struct RedeclarationId(NonZeroU32);
+
+impl Idx for RedeclarationId {
+    #[allow(clippy::cast_possible_truncation)]
+    fn from_usize(idx: usize) -> Self {
+        // SAFETY: + 1 is always non-zero.
+
+        unsafe { Self(NonZeroU32::new_unchecked(idx as u32 + 1)) }
+    }
+
+    fn index(self) -> usize {
+        self.0.get() as usize - 1
+    }
+}
+
 #[cfg(feature = "serialize")]
 #[wasm_bindgen::prelude::wasm_bindgen(typescript_custom_section)]
 const TS_APPEND_CONTENT: &'static str = r#"
 export type SymbolId = number;
 export type SymbolFlags = unknown;
+export type RedeclarationId = unknown;
 "#;
 
 bitflags! {