perf: replace generic __clone by specific methods

baseline 256 length-1 named export: 4_704 ops/sec ±1.59% (0.213ms)
baseline 512 length-1 named export: 2_426 ops/sec ±0.52% (0.412ms)
baseline 1024 length-1 named export: 1_118 ops/sec ±1.23% (0.895ms)
baseline 2048 length-1 named export: 556 ops/sec ±0.77% (1.799ms)
current 256 length-1 named export: 7_073 ops/sec ±33.67% (0.141ms)
current 512 length-1 named export: 4_441 ops/sec ±0.79% (0.225ms)
current 1024 length-1 named export: 2_142 ops/sec ±1.09% (0.467ms)
current 2048 length-1 named export: 943 ops/sec ±2.12% (1.06ms)

packages/babel-parser/src/parser/expression.js

@@ -57,6 +57,7 @@ import {
 import { Errors, SourceTypeModuleErrors } from "./error";
 import type { ParsingError } from "./error";
 import { setInnerComments } from "./comments";
+import { cloneIdentifier } from "./node";
 
 /*::
 import type { SourceType } from "../options";
@@ -1938,7 +1939,7 @@ export default class ExpressionParser extends LValParser {
         prop.value = this.parseMaybeDefault(
           startPos,
           startLoc,
-          prop.key.__clone(),
+          cloneIdentifier(prop.key),
         );
       } else if (this.match(tt.eq) && refExpressionErrors) {
         if (refExpressionErrors.shorthandAssign === -1) {
@@ -1947,10 +1948,10 @@ export default class ExpressionParser extends LValParser {
         prop.value = this.parseMaybeDefault(
           startPos,
           startLoc,
-          prop.key.__clone(),
+          cloneIdentifier(prop.key),
         );
       } else {
-        prop.value = prop.key.__clone();
+        prop.value = cloneIdentifier(prop.key);
       }
       prop.shorthand = true;
 

packages/babel-parser/src/parser/node.js

@@ -27,6 +27,7 @@ class Node implements NodeBase {
   innerComments: Array<Comment>;
   extra: { [key: string]: any };
 
+  // todo(Babel 8): remove this method in Babel 8
   __clone(): this {
     // $FlowIgnore
     const newNode: any = new Node();
@@ -47,6 +48,46 @@ class Node implements NodeBase {
     return newNode;
   }
 }
+const NodePrototype = Node.prototype;
+
+function clonePlaceholder(node: any): any {
+  return cloneIdentifier(node);
+}
+
+export function cloneIdentifier(node: any): any {
+  // We don't need to clone `typeAnnotations` and `optional`: because
+  // cloneIdentifier is only used in object shorthand and named import/export.
+  // Neither of them allow type annotations after the identifier or optional identifier
+  const { type, start, end, loc, range, extra, name } = node;
+  const cloned = Object.create(NodePrototype);
+  cloned.type = type;
+  cloned.start = start;
+  cloned.end = end;
+  cloned.loc = loc;
+  cloned.range = range;
+  cloned.extra = extra;
+  cloned.name = name;
+  if (type === "Placeholder") {
+    cloned.expectedNode = node.expectedNode;
+  }
+  return cloned;
+}
+
+export function cloneStringLiteral(node: any): any {
+  const { type, start, end, loc, range, extra } = node;
+  if (type === "Placeholder") {
+    return clonePlaceholder(node);
+  }
+  const cloned = Object.create(NodePrototype);
+  cloned.type = "StringLiteral";
+  cloned.start = start;
+  cloned.end = end;
+  cloned.loc = loc;
+  cloned.range = range;
+  cloned.extra = extra;
+  cloned.value = node.value;
+  return cloned;
+}
 
 export class NodeUtils extends UtilParser {
   startNode<T: NodeType>(): T {

packages/babel-parser/src/parser/statement.js

@@ -34,6 +34,7 @@ import {
 import type { SourceType } from "../options";
 import { Token } from "../tokenizer";
 import { Position } from "../util/location";
+import { cloneStringLiteral, cloneIdentifier } from "./node";
 
 const loopLabel = { kind: "loop" },
   switchLabel = { kind: "switch" };
@@ -2144,10 +2145,16 @@ export default class StatementParser extends ExpressionParser {
       }
 
       const node = this.startNode();
-      node.local = this.parseModuleExportName();
-      node.exported = this.eatContextual("as")
-        ? this.parseModuleExportName()
-        : node.local.__clone();
+      const isString = this.match(tt.string);
+      const local = this.parseModuleExportName();
+      node.local = local;
+      if (this.eatContextual("as")) {
+        node.exported = this.parseModuleExportName();
+      } else if (isString) {
+        node.exported = cloneStringLiteral(local);
+      } else {
+        node.exported = cloneIdentifier(local);
+      }
       nodes.push(this.finishNode(node, "ExportSpecifier"));
     }
 
@@ -2423,7 +2430,7 @@ export default class StatementParser extends ExpressionParser {
         );
       }
       this.checkReservedWord(imported.name, specifier.start, true, true);
-      specifier.local = imported.__clone();
+      specifier.local = cloneIdentifier(imported);
     }
     this.checkLVal(specifier.local, "import specifier", BIND_LEXICAL);
     node.specifiers.push(this.finishNode(specifier, "ImportSpecifier"));

packages/babel-parser/src/plugins/flow/index.js

@@ -25,6 +25,7 @@ import {
 } from "../../util/scopeflags";
 import type { ExpressionErrors } from "../../parser/util";
 import { Errors, makeErrorTemplates, ErrorCodes } from "../../parser/error";
+import { cloneIdentifier } from "../../parser/node";
 
 const reservedTypes = new Set([
   "_",
@@ -2655,7 +2656,7 @@ export default (superClass: Class<Parser>): Class<Parser> =>
           // `import {type as ,` or `import {type as }`
           specifier.imported = as_ident;
           specifier.importKind = specifierTypeKind;
-          specifier.local = as_ident.__clone();
+          specifier.local = cloneIdentifier(as_ident);
         } else {
           // `import {type as foo`
           specifier.imported = firstIdent;
@@ -2673,7 +2674,7 @@ export default (superClass: Class<Parser>): Class<Parser> =>
           specifier.local = this.parseIdentifier();
         } else {
           isBinding = true;
-          specifier.local = specifier.imported.__clone();
+          specifier.local = cloneIdentifier(specifier.imported);
         }
       } else {
         if (firstIdentIsString) {
@@ -2688,7 +2689,7 @@ export default (superClass: Class<Parser>): Class<Parser> =>
         isBinding = true;
         specifier.imported = firstIdent;
         specifier.importKind = null;
-        specifier.local = specifier.imported.__clone();
+        specifier.local = cloneIdentifier(specifier.imported);
       }
 
       const nodeIsTypeImport = hasTypeImportKind(node);

packages/babel-parser/src/types.js

@@ -98,6 +98,7 @@ export type Identifier = PatternBase & {
   type: "Identifier",
   name: string,
 
+  // @deprecated
   __clone(): Identifier,
 
   // TypeScript only. Used in case of an optional parameter.