Stage 1-1: Implement Function.prototype.toString revision proposal.

Tag #960

13 files changed, 330 insertions, 154 deletions

diff --git a/js/src/frontend/BytecodeCompiler.cpp b/js/src/frontend/BytecodeCompiler.cpp
index 76afe80b1..b5be5f5ac 100644
--- a/js/src/frontend/BytecodeCompiler.cpp
+++ b/js/src/frontend/BytecodeCompiler.cpp
@@ -77,7 +77,7 @@ class MOZ_STACK_CLASS BytecodeCompiler
     bool canLazilyParse();
     bool createParser();
     bool createSourceAndParser(Maybe<uint32_t> parameterListEnd = Nothing());
-    bool createScript();
+    bool createScript(uint32_t preludeStart = 0);
     bool emplaceEmitter(Maybe<BytecodeEmitter>& emitter, SharedContext* sharedContext);
     bool handleParseFailure(const Directives& newDirectives);
     bool deoptimizeArgumentsInEnclosingScripts(JSContext* cx, HandleObject environment);
@@ -242,10 +242,11 @@ BytecodeCompiler::createSourceAndParser(Maybe<uint32_t> parameterListEnd /* = No
 }
 
 bool
-BytecodeCompiler::createScript()
+BytecodeCompiler::createScript(uint32_t preludeStart /* = 0 */)
 {
     script = JSScript::Create(cx, options,
-                              sourceObject, /* sourceStart = */ 0, sourceBuffer.length());
+                              sourceObject, /* sourceStart = */ 0, sourceBuffer.length(),
+                              preludeStart);
     return script != nullptr;
 }
 
@@ -456,7 +457,7 @@ BytecodeCompiler::compileStandaloneFunction(MutableHandleFunction fun,
     if (fn->pn_funbox->function()->isInterpreted()) {
         MOZ_ASSERT(fun == fn->pn_funbox->function());
 
-        if (!createScript())
+        if (!createScript(fn->pn_funbox->preludeStart))
             return false;
 
         Maybe<BytecodeEmitter> emitter;
@@ -650,7 +651,8 @@ frontend::CompileLazyFunction(JSContext* cx, Handle<LazyScript*> lazy, const cha
     MOZ_ASSERT(sourceObject);
 
     Rooted<JSScript*> script(cx, JSScript::Create(cx, options, sourceObject,
-                                                  lazy->begin(), lazy->end()));
+                                                  lazy->begin(), lazy->end(),
+                                                  lazy->preludeStart()));
     if (!script)
         return false;
 
diff --git a/js/src/frontend/BytecodeCompiler.h b/js/src/frontend/BytecodeCompiler.h
index 72e967639..0bc1ab2ab 100644
--- a/js/src/frontend/BytecodeCompiler.h
+++ b/js/src/frontend/BytecodeCompiler.h
@@ -109,6 +109,8 @@ IsIdentifier(JSLinearString* str);
  * As above, but taking chars + length.
  */
 bool
+IsIdentifier(const char* chars, size_t length);
+bool
 IsIdentifier(const char16_t* chars, size_t length);
 
 /* True if str is a keyword. Defined in TokenStream.cpp. */
diff --git a/js/src/frontend/BytecodeEmitter.cpp b/js/src/frontend/BytecodeEmitter.cpp
index 4eb7bf880..c5e62ae34 100644
--- a/js/src/frontend/BytecodeEmitter.cpp
+++ b/js/src/frontend/BytecodeEmitter.cpp
@@ -7834,7 +7834,8 @@ BytecodeEmitter::emitFunction(ParseNode* pn, bool needsProto)
 
             Rooted<JSObject*> sourceObject(cx, script->sourceObject());
             Rooted<JSScript*> script(cx, JSScript::Create(cx, options, sourceObject,
-                                                          funbox->bufStart, funbox->bufEnd));
+                                                          funbox->bufStart, funbox->bufEnd,
+                                                          funbox->preludeStart));
             if (!script)
                 return false;
 
diff --git a/js/src/frontend/Parser.cpp b/js/src/frontend/Parser.cpp
index 209265a58..1ba725a82 100644
--- a/js/src/frontend/Parser.cpp
+++ b/js/src/frontend/Parser.cpp
@@ -441,7 +441,8 @@ UsedNameTracker::rewind(RewindToken token)
 }
 
 FunctionBox::FunctionBox(ExclusiveContext* cx, LifoAlloc& alloc, ObjectBox* traceListHead,
-                         JSFunction* fun, Directives directives, bool extraWarnings,
+                         JSFunction* fun, uint32_t preludeStart,
+                         Directives directives, bool extraWarnings,
                          GeneratorKind generatorKind, FunctionAsyncKind asyncKind)
   : ObjectBox(fun, traceListHead),
     SharedContext(cx, Kind::ObjectBox, directives, extraWarnings),
@@ -454,6 +455,7 @@ FunctionBox::FunctionBox(ExclusiveContext* cx, LifoAlloc& alloc, ObjectBox* trac
     bufEnd(0),
     startLine(1),
     startColumn(0),
+    preludeStart(preludeStart),
     length(0),
     generatorKindBits_(GeneratorKindAsBits(generatorKind)),
     asyncKindBits_(AsyncKindAsBits(asyncKind)),
@@ -738,7 +740,8 @@ Parser<ParseHandler>::newObjectBox(JSObject* obj)
 
 template <typename ParseHandler>
 FunctionBox*
-Parser<ParseHandler>::newFunctionBox(Node fn, JSFunction* fun, Directives inheritedDirectives,
+Parser<ParseHandler>::newFunctionBox(Node fn, JSFunction* fun, uint32_t preludeStart,
+                                     Directives inheritedDirectives,
                                      GeneratorKind generatorKind, FunctionAsyncKind asyncKind,
                                      bool tryAnnexB)
 {
@@ -753,8 +756,9 @@ Parser<ParseHandler>::newFunctionBox(Node fn, JSFunction* fun, Directives inheri
      * function.
      */
     FunctionBox* funbox =
-        alloc.new_<FunctionBox>(context, alloc, traceListHead, fun, inheritedDirectives,
-                                options().extraWarningsOption, generatorKind, asyncKind);
+        alloc.new_<FunctionBox>(context, alloc, traceListHead, fun, preludeStart,
+                                inheritedDirectives, options().extraWarningsOption,
+                                generatorKind, asyncKind);
     if (!funbox) {
         ReportOutOfMemory(context);
         return nullptr;
@@ -2214,6 +2218,7 @@ Parser<SyntaxParseHandler>::finishFunction()
     LazyScript* lazy = LazyScript::Create(context, fun, pc->closedOverBindingsForLazy(),
                                           pc->innerFunctionsForLazy, versionNumber(),
                                           funbox->bufStart, funbox->bufEnd,
+                                          funbox->preludeStart,
                                           funbox->startLine, funbox->startColumn);
     if (!lazy)
         return false;
@@ -2267,6 +2272,33 @@ Parser<FullParseHandler>::standaloneFunction(HandleFunction fun,
 {
     MOZ_ASSERT(checkOptionsCalled);
 
+    // Skip prelude.
+    TokenKind tt;
+    if (!tokenStream.getToken(&tt))
+        return null();
+    if (asyncKind == AsyncFunction) {
+        MOZ_ASSERT(tt == TOK_ASYNC);
+        if (!tokenStream.getToken(&tt))
+            return null();
+    }
+    MOZ_ASSERT(tt == TOK_FUNCTION);
+
+    if (!tokenStream.getToken(&tt))
+        return null();
+    if (generatorKind == StarGenerator && asyncKind == SyncFunction) {
+        MOZ_ASSERT(tt == TOK_MUL);
+        if (!tokenStream.getToken(&tt))
+            return null();
+    }
+
+    // Skip function name, if present.
+    if (tt == TOK_NAME || tt == TOK_YIELD) {
+        MOZ_ASSERT(tokenStream.currentName() == fun->explicitName());
+    } else {
+        MOZ_ASSERT(fun->explicitName() == nullptr);
+        tokenStream.ungetToken();
+    }
+
     Node fn = handler.newFunctionDefinition();
     if (!fn)
         return null();
@@ -2276,8 +2308,8 @@ Parser<FullParseHandler>::standaloneFunction(HandleFunction fun,
         return null();
     fn->pn_body = argsbody;
 
-    FunctionBox* funbox = newFunctionBox(fn, fun, inheritedDirectives, generatorKind,
-                                         asyncKind, /* tryAnnexB = */ false);
+    FunctionBox* funbox = newFunctionBox(fn, fun, /* preludeStart = */ 0, inheritedDirectives,
+                                         generatorKind, asyncKind, /* tryAnnexB = */ false);
     if (!funbox)
         return null();
     funbox->initStandaloneFunction(enclosingScope);
@@ -2295,7 +2327,6 @@ Parser<FullParseHandler>::standaloneFunction(HandleFunction fun,
         return null();
     }
 
-    TokenKind tt;
     if (!tokenStream.getToken(&tt, TokenStream::Operand))
         return null();
     if (tt != TOK_EOF) {
@@ -2991,8 +3022,8 @@ Parser<ParseHandler>::checkFunctionDefinition(HandleAtom funAtom, Node pn, Funct
 
 template <>
 bool
-Parser<FullParseHandler>::skipLazyInnerFunction(ParseNode* pn, FunctionSyntaxKind kind,
-                                                bool tryAnnexB)
+Parser<FullParseHandler>::skipLazyInnerFunction(ParseNode* pn, uint32_t preludeStart,
+                                                FunctionSyntaxKind kind, bool tryAnnexB)
 {
     // When a lazily-parsed function is called, we only fully parse (and emit)
     // that function, not any of its nested children. The initial syntax-only
@@ -3001,7 +3032,7 @@ Parser<FullParseHandler>::skipLazyInnerFunction(ParseNode* pn, FunctionSyntaxKin
 
     RootedFunction fun(context, handler.nextLazyInnerFunction());
     MOZ_ASSERT(!fun->isLegacyGenerator());
-    FunctionBox* funbox = newFunctionBox(pn, fun, Directives(/* strict = */ false),
+    FunctionBox* funbox = newFunctionBox(pn, fun, preludeStart, Directives(/* strict = */ false),
                                          fun->generatorKind(), fun->asyncKind(), tryAnnexB);
     if (!funbox)
         return false;
@@ -3031,8 +3062,8 @@ Parser<FullParseHandler>::skipLazyInnerFunction(ParseNode* pn, FunctionSyntaxKin
 
 template <>
 bool
-Parser<SyntaxParseHandler>::skipLazyInnerFunction(Node pn, FunctionSyntaxKind kind,
-                                                  bool tryAnnexB)
+Parser<SyntaxParseHandler>::skipLazyInnerFunction(Node pn, uint32_t preludeStart,
+                                                  FunctionSyntaxKind kind, bool tryAnnexB)
 {
     MOZ_CRASH("Cannot skip lazy inner functions when syntax parsing");
 }
@@ -3108,7 +3139,8 @@ Parser<ParseHandler>::templateLiteral(YieldHandling yieldHandling)
 
 template <typename ParseHandler>
 typename ParseHandler::Node
-Parser<ParseHandler>::functionDefinition(InHandling inHandling, YieldHandling yieldHandling,
+Parser<ParseHandler>::functionDefinition(uint32_t preludeStart, InHandling inHandling,
+                                         YieldHandling yieldHandling,
                                          HandleAtom funName, FunctionSyntaxKind kind,
                                          GeneratorKind generatorKind, FunctionAsyncKind asyncKind,
                                          InvokedPrediction invoked)
@@ -3132,7 +3164,7 @@ Parser<ParseHandler>::functionDefinition(InHandling inHandling, YieldHandling yi
     // functions, which are also lazy. Instead, their free variables and
     // source extents are recorded and may be skipped.
     if (handler.canSkipLazyInnerFunctions()) {
-        if (!skipLazyInnerFunction(pn, kind, tryAnnexB))
+        if (!skipLazyInnerFunction(pn, preludeStart, kind, tryAnnexB))
             return null();
         return pn;
     }
@@ -3165,8 +3197,9 @@ Parser<ParseHandler>::functionDefinition(InHandling inHandling, YieldHandling yi
     // reparse a function due to failed syntax parsing and encountering new
     // "use foo" directives.
     while (true) {
-        if (trySyntaxParseInnerFunction(pn, fun, inHandling, yieldHandling, kind, generatorKind,
-                                        asyncKind, tryAnnexB, directives, &newDirectives))
+        if (trySyntaxParseInnerFunction(pn, fun, preludeStart, inHandling, yieldHandling, kind,
+                                        generatorKind, asyncKind, tryAnnexB, directives,
+                                        &newDirectives))
         {
             break;
         }
@@ -3193,6 +3226,7 @@ Parser<ParseHandler>::functionDefinition(InHandling inHandling, YieldHandling yi
 template <>
 bool
 Parser<FullParseHandler>::trySyntaxParseInnerFunction(ParseNode* pn, HandleFunction fun,
+                                                      uint32_t preludeStart,
                                                       InHandling inHandling,
                                                       YieldHandling yieldHandling,
                                                       FunctionSyntaxKind kind,
@@ -3226,14 +3260,15 @@ Parser<FullParseHandler>::trySyntaxParseInnerFunction(ParseNode* pn, HandleFunct
         // Make a FunctionBox before we enter the syntax parser, because |pn|
         // still expects a FunctionBox to be attached to it during BCE, and
         // the syntax parser cannot attach one to it.
-        FunctionBox* funbox = newFunctionBox(pn, fun, inheritedDirectives, generatorKind,
-                                             asyncKind, tryAnnexB);
+        FunctionBox* funbox = newFunctionBox(pn, fun, preludeStart, inheritedDirectives,
+                                             generatorKind, asyncKind, tryAnnexB);
         if (!funbox)
             return false;
         funbox->initWithEnclosingParseContext(pc, kind);
 
-        if (!parser->innerFunction(SyntaxParseHandler::NodeGeneric, pc, funbox, inHandling,
-                                   yieldHandling, kind, inheritedDirectives, newDirectives))
+        if (!parser->innerFunction(SyntaxParseHandler::NodeGeneric, pc, funbox, preludeStart,
+                                   inHandling, yieldHandling, kind,
+                                   inheritedDirectives, newDirectives))
         {
             if (parser->hadAbortedSyntaxParse()) {
                 // Try again with a full parse. UsedNameTracker needs to be
@@ -3259,13 +3294,14 @@ Parser<FullParseHandler>::trySyntaxParseInnerFunction(ParseNode* pn, HandleFunct
     } while (false);
 
     // We failed to do a syntax parse above, so do the full parse.
-    return innerFunction(pn, pc, fun, inHandling, yieldHandling, kind, generatorKind, asyncKind,
-                         tryAnnexB, inheritedDirectives, newDirectives);
+    return innerFunction(pn, pc, fun, preludeStart, inHandling, yieldHandling, kind,
+                         generatorKind, asyncKind, tryAnnexB, inheritedDirectives, newDirectives);
 }
 
 template <>
 bool
 Parser<SyntaxParseHandler>::trySyntaxParseInnerFunction(Node pn, HandleFunction fun,
+                                                        uint32_t preludeStart,
                                                         InHandling inHandling,
                                                         YieldHandling yieldHandling,
                                                         FunctionSyntaxKind kind,
@@ -3276,13 +3312,14 @@ Parser<SyntaxParseHandler>::trySyntaxParseInnerFunction(Node pn, HandleFunction
                                                         Directives* newDirectives)
 {
     // This is already a syntax parser, so just parse the inner function.
-    return innerFunction(pn, pc, fun, inHandling, yieldHandling, kind, generatorKind, asyncKind,
-                         tryAnnexB, inheritedDirectives, newDirectives);
+    return innerFunction(pn, pc, fun, preludeStart, inHandling, yieldHandling, kind,
+                          generatorKind, asyncKind, tryAnnexB, inheritedDirectives, newDirectives);
 }
 
 template <typename ParseHandler>
 bool
 Parser<ParseHandler>::innerFunction(Node pn, ParseContext* outerpc, FunctionBox* funbox,
+                                    uint32_t preludeStart,
                                     InHandling inHandling, YieldHandling yieldHandling,
                                     FunctionSyntaxKind kind, Directives inheritedDirectives,
                                     Directives* newDirectives)
@@ -3306,6 +3343,7 @@ Parser<ParseHandler>::innerFunction(Node pn, ParseContext* outerpc, FunctionBox*
 template <typename ParseHandler>
 bool
 Parser<ParseHandler>::innerFunction(Node pn, ParseContext* outerpc, HandleFunction fun,
+                                    uint32_t preludeStart,
                                     InHandling inHandling, YieldHandling yieldHandling,
                                     FunctionSyntaxKind kind,
                                     GeneratorKind generatorKind, FunctionAsyncKind asyncKind,
@@ -3317,14 +3355,14 @@ Parser<ParseHandler>::innerFunction(Node pn, ParseContext* outerpc, HandleFuncti
     // parser. In that case, outerpc is a ParseContext from the full parser
     // instead of the current top of the stack of the syntax parser.
 
-    FunctionBox* funbox = newFunctionBox(pn, fun, inheritedDirectives, generatorKind,
-                                         asyncKind, tryAnnexB);
+    FunctionBox* funbox = newFunctionBox(pn, fun, preludeStart, inheritedDirectives,
+                                         generatorKind, asyncKind, tryAnnexB);
     if (!funbox)
         return false;
     funbox->initWithEnclosingParseContext(outerpc, kind);
 
-    return innerFunction(pn, outerpc, funbox, inHandling, yieldHandling, kind, inheritedDirectives,
-                         newDirectives);
+    return innerFunction(pn, outerpc, funbox, preludeStart, inHandling, yieldHandling, kind,
+                         inheritedDirectives, newDirectives);
 }
 
 template <typename ParseHandler>
@@ -3359,8 +3397,8 @@ Parser<FullParseHandler>::standaloneLazyFunction(HandleFunction fun, bool strict
         return null();
 
     Directives directives(strict);
-    FunctionBox* funbox = newFunctionBox(pn, fun, directives, generatorKind, asyncKind,
-                                         /* tryAnnexB = */ false);
+    FunctionBox* funbox = newFunctionBox(pn, fun, /* preludeStart = */ 0, directives,
+                                         generatorKind, asyncKind, /* tryAnnexB = */ false);
     if (!funbox)
         return null();
     funbox->initFromLazyFunction();
@@ -3529,8 +3567,8 @@ Parser<ParseHandler>::functionFormalParametersAndBody(InHandling inHandling,
 
 template <typename ParseHandler>
 typename ParseHandler::Node
-Parser<ParseHandler>::functionStmt(YieldHandling yieldHandling, DefaultHandling defaultHandling,
-                                   FunctionAsyncKind asyncKind)
+Parser<ParseHandler>::functionStmt(uint32_t preludeStart, YieldHandling yieldHandling,
+                                   DefaultHandling defaultHandling, FunctionAsyncKind asyncKind)
 {
     MOZ_ASSERT(tokenStream.isCurrentTokenType(TOK_FUNCTION));
 
@@ -3579,8 +3617,8 @@ Parser<ParseHandler>::functionStmt(YieldHandling yieldHandling, DefaultHandling
     }
 
     YieldHandling newYieldHandling = GetYieldHandling(generatorKind, asyncKind);
-    Node fun = functionDefinition(InAllowed, newYieldHandling, name, Statement, generatorKind,
-                                  asyncKind, PredictUninvoked);
+    Node fun = functionDefinition(preludeStart, InAllowed, newYieldHandling, name, Statement,
+                                  generatorKind, asyncKind, PredictUninvoked);
     if (!fun)
         return null();
 
@@ -3597,7 +3635,8 @@ Parser<ParseHandler>::functionStmt(YieldHandling yieldHandling, DefaultHandling
 
 template <typename ParseHandler>
 typename ParseHandler::Node
-Parser<ParseHandler>::functionExpr(InvokedPrediction invoked, FunctionAsyncKind asyncKind)
+Parser<ParseHandler>::functionExpr(uint32_t preludeStart, InvokedPrediction invoked,
+                                   FunctionAsyncKind asyncKind)
 {
     MOZ_ASSERT(tokenStream.isCurrentTokenType(TOK_FUNCTION));
 
@@ -3628,8 +3667,8 @@ Parser<ParseHandler>::functionExpr(InvokedPrediction invoked, FunctionAsyncKind
         tokenStream.ungetToken();
     }
 
-    return functionDefinition(InAllowed, yieldHandling, name, Expression, generatorKind,
-                              asyncKind, invoked);
+    return functionDefinition(preludeStart, InAllowed, yieldHandling, name, Expression,
+                              generatorKind, asyncKind, invoked);
 }
 
 /*
@@ -5074,7 +5113,7 @@ Parser<FullParseHandler>::exportDeclaration()
       }
 
       case TOK_FUNCTION:
-        kid = functionStmt(YieldIsKeyword, NameRequired);
+        kid = functionStmt(pos().begin, YieldIsKeyword, NameRequired);
         if (!kid)
             return null();
 
@@ -5114,7 +5153,7 @@ Parser<FullParseHandler>::exportDeclaration()
         ParseNode* nameNode = nullptr;
         switch (tt) {
           case TOK_FUNCTION:
-            kid = functionStmt(YieldIsKeyword, AllowDefaultName);
+            kid = functionStmt(pos().begin, YieldIsKeyword, AllowDefaultName);
             if (!kid)
                 return null();
             break;
@@ -5131,7 +5170,7 @@ Parser<FullParseHandler>::exportDeclaration()
 
                 if (nextSameLine == TOK_FUNCTION) {
                     tokenStream.consumeKnownToken(nextSameLine);
-                    kid = functionStmt(YieldIsName, AllowDefaultName, AsyncFunction);
+                    kid = functionStmt(pos().begin, YieldIsName, AllowDefaultName, AsyncFunction);
                     if (!kid)
                         return null();
                     break;
@@ -5232,7 +5271,7 @@ Parser<ParseHandler>::consequentOrAlternative(YieldHandling yieldHandling)
         // will report the strict mode error.
         if (!pc->sc()->strict()) {
             tokenStream.consumeKnownToken(next, TokenStream::Operand);
-            return functionStmt(yieldHandling, NameRequired);
+            return functionStmt(pos().begin, yieldHandling, NameRequired);
         }
     }
 
@@ -6182,7 +6221,7 @@ Parser<ParseHandler>::labeledItem(YieldHandling yieldHandling)
             return null();
         }
 
-        return functionStmt(yieldHandling, NameRequired);
+        return functionStmt(pos().begin, yieldHandling, NameRequired);
     }
 
     tokenStream.ungetToken();
@@ -6649,6 +6688,10 @@ Parser<ParseHandler>::classDefinition(YieldHandling yieldHandling,
             tokenStream.ungetToken();
         }
 
+        uint32_t nameOffset;
+        if (!tokenStream.peekOffset(&nameOffset))
+            return null();
+            
         PropertyType propType;
         Node propName = propertyName(yieldHandling, classMethods, &propType, &propAtom);
         if (!propName)
@@ -6701,7 +6744,7 @@ Parser<ParseHandler>::classDefinition(YieldHandling yieldHandling,
             if (!tokenStream.isCurrentTokenType(TOK_RB))
                 funName = propAtom;
         }
-        Node fn = methodDefinition(propType, funName);
+        Node fn = methodDefinition(nameOffset, propType, funName);
         if (!fn)
             return null();
 
@@ -7094,8 +7137,9 @@ Parser<ParseHandler>::statementListItem(YieldHandling yieldHandling,
             if (!tokenStream.peekTokenSameLine(&nextSameLine))
                 return null();
             if (nextSameLine == TOK_FUNCTION) {
+                uint32_t preludeStart = pos().begin;
                 tokenStream.consumeKnownToken(TOK_FUNCTION);
-                return functionStmt(yieldHandling, NameRequired, AsyncFunction);
+                return functionStmt(preludeStart, yieldHandling, NameRequired, AsyncFunction);
             }
         }
 
@@ -7174,7 +7218,7 @@ Parser<ParseHandler>::statementListItem(YieldHandling yieldHandling,
 
       //   HoistableDeclaration[?Yield, ~Default]
       case TOK_FUNCTION:
-        return functionStmt(yieldHandling, NameRequired);
+        return functionStmt(pos().begin, yieldHandling, NameRequired);
 
       //   ClassDeclaration[?Yield, ~Default]
       case TOK_CLASS:
@@ -7677,8 +7721,10 @@ Parser<ParseHandler>::assignExpr(InHandling inHandling, YieldHandling yieldHandl
 
         tokenStream.seek(start);
 
-        if (!tokenStream.peekToken(&next, TokenStream::Operand))
+        if (!tokenStream.getToken(&next, TokenStream::Operand))
             return null();
+        uint32_t preludeStart = pos().begin;
+        tokenStream.ungetToken();
 
         GeneratorKind generatorKind = NotGenerator;
         FunctionAsyncKind asyncKind = SyncFunction;
@@ -7702,7 +7748,7 @@ Parser<ParseHandler>::assignExpr(InHandling inHandling, YieldHandling yieldHandl
             }
         }
 
-        Node arrowFunc = functionDefinition(inHandling, yieldHandling, nullptr,
+        Node arrowFunc = functionDefinition(preludeStart, inHandling, yieldHandling, nullptr,
                                             Arrow, generatorKind, asyncKind);
         if (!arrowFunc)
             return null();
@@ -8054,8 +8100,8 @@ Parser<ParseHandler>::generatorComprehensionLambda(unsigned begin)
 
     // Create box for fun->object early to root it.
     Directives directives(/* strict = */ outerpc->sc()->strict());
-    FunctionBox* genFunbox = newFunctionBox(genfn, fun, directives, StarGenerator, SyncFunction,
-                                            /* tryAnnexB = */ false);
+    FunctionBox* genFunbox = newFunctionBox(genfn, fun, /* preludeStart = */ 0, directives,
+                                            StarGenerator, SyncFunction, /* tryAnnexB = */ false);
     if (!genFunbox)
         return null();
     genFunbox->isGenexpLambda = true;
@@ -8087,12 +8133,14 @@ Parser<ParseHandler>::generatorComprehensionLambda(unsigned begin)
 
     MUST_MATCH_TOKEN(TOK_RP, JSMSG_PAREN_IN_PAREN);
 
+    uint32_t end = pos().end;
     handler.setBeginPosition(comp, begin);
-    handler.setEndPosition(comp, pos().end);
+    handler.setEndPosition(comp, end);
+    genFunbox->bufEnd = end;
     handler.addStatementToList(body, comp);
-    handler.setEndPosition(body, pos().end);
+    handler.setEndPosition(body, end);
     handler.setBeginPosition(genfn, begin);
-    handler.setEndPosition(genfn, pos().end);
+    handler.setEndPosition(genfn, end);
 
     Node generator = newDotGeneratorName();
     if (!generator)
@@ -9158,6 +9206,8 @@ Parser<ParseHandler>::objectLiteral(YieldHandling yieldHandling, PossibleError*
         if (tt == TOK_RC)
             break;
 
+        TokenPos namePos = pos();
+        
         tokenStream.ungetToken();
 
         PropertyType propType;
@@ -9309,7 +9359,7 @@ Parser<ParseHandler>::objectLiteral(YieldHandling yieldHandling, PossibleError*
                 }
             }
 
-            Node fn = methodDefinition(propType, funName);
+            Node fn = methodDefinition(namePos.begin, propType, funName);
             if (!fn)
                 return null();
 
@@ -9336,13 +9386,15 @@ Parser<ParseHandler>::objectLiteral(YieldHandling yieldHandling, PossibleError*
 
 template <typename ParseHandler>
 typename ParseHandler::Node
-Parser<ParseHandler>::methodDefinition(PropertyType propType, HandleAtom funName)
+Parser<ParseHandler>::methodDefinition(uint32_t preludeStart, PropertyType propType,
+                                       HandleAtom funName)
 {
     FunctionSyntaxKind kind = FunctionSyntaxKindFromPropertyType(propType);
     GeneratorKind generatorKind = GeneratorKindFromPropertyType(propType);
     FunctionAsyncKind asyncKind = AsyncKindFromPropertyType(propType);
     YieldHandling yieldHandling = GetYieldHandling(generatorKind, asyncKind);
-    return functionDefinition(InAllowed, yieldHandling, funName, kind, generatorKind, asyncKind);
+    return functionDefinition(preludeStart, InAllowed, yieldHandling, funName, kind,
+                              generatorKind, asyncKind);
 }
 
 template <typename ParseHandler>
@@ -9404,7 +9456,7 @@ Parser<ParseHandler>::primaryExpr(YieldHandling yieldHandling, TripledotHandling
 
     switch (tt) {
       case TOK_FUNCTION:
-        return functionExpr(invoked);
+        return functionExpr(pos().begin, invoked);
 
       case TOK_CLASS:
         return classDefinition(yieldHandling, ClassExpression, NameRequired);
@@ -9471,8 +9523,9 @@ Parser<ParseHandler>::primaryExpr(YieldHandling yieldHandling, TripledotHandling
                 return null();
 
             if (nextSameLine == TOK_FUNCTION) {
+                uint32_t preludeStart = pos().begin;
                 tokenStream.consumeKnownToken(TOK_FUNCTION);
-                return functionExpr(PredictUninvoked, AsyncFunction);
+                return functionExpr(preludeStart, PredictUninvoked, AsyncFunction);
             }
         }
 
diff --git a/js/src/frontend/Parser.h b/js/src/frontend/Parser.h
index 12642fad8..090931f5b 100644
--- a/js/src/frontend/Parser.h
+++ b/js/src/frontend/Parser.h
@@ -954,7 +954,8 @@ class Parser final : private JS::AutoGCRooter, public StrictModeGetter
      * cx->tempLifoAlloc.
      */
     ObjectBox* newObjectBox(JSObject* obj);
-    FunctionBox* newFunctionBox(Node fn, JSFunction* fun, Directives directives,
+    FunctionBox* newFunctionBox(Node fn, JSFunction* fun, uint32_t preludeStart,
+                                Directives directives,
                                 GeneratorKind generatorKind, FunctionAsyncKind asyncKind,
                                 bool tryAnnexB);
 
@@ -1034,8 +1035,9 @@ class Parser final : private JS::AutoGCRooter, public StrictModeGetter
 
     // Parse an inner function given an enclosing ParseContext and a
     // FunctionBox for the inner function.
-    bool innerFunction(Node pn, ParseContext* outerpc, FunctionBox* funbox, InHandling inHandling,
-                       YieldHandling yieldHandling, FunctionSyntaxKind kind,
+    bool innerFunction(Node pn, ParseContext* outerpc, FunctionBox* funbox, uint32_t preludeStart,
+                       InHandling inHandling, YieldHandling yieldHandling,
+                       FunctionSyntaxKind kind,
                        Directives inheritedDirectives, Directives* newDirectives);
 
     // Parse a function's formal parameters and its body assuming its function
@@ -1088,9 +1090,10 @@ class Parser final : private JS::AutoGCRooter, public StrictModeGetter
      * Some parsers have two versions:  an always-inlined version (with an 'i'
      * suffix) and a never-inlined version (with an 'n' suffix).
      */
-    Node functionStmt(YieldHandling yieldHandling, DefaultHandling defaultHandling,
+    Node functionStmt(uint32_t preludeStart,
+                      YieldHandling yieldHandling, DefaultHandling defaultHandling,
                       FunctionAsyncKind asyncKind = SyncFunction);
-    Node functionExpr(InvokedPrediction invoked = PredictUninvoked,
+    Node functionExpr(uint32_t preludeStart, InvokedPrediction invoked = PredictUninvoked,
                       FunctionAsyncKind asyncKind = SyncFunction);
 
     Node statementList(YieldHandling yieldHandling);
@@ -1222,7 +1225,7 @@ class Parser final : private JS::AutoGCRooter, public StrictModeGetter
     bool tryNewTarget(Node& newTarget);
     bool checkAndMarkSuperScope();
 
-    Node methodDefinition(PropertyType propType, HandleAtom funName);
+    Node methodDefinition(uint32_t preludeStart, PropertyType propType, HandleAtom funName);
 
     /*
      * Additional JS parsers.
@@ -1230,7 +1233,8 @@ class Parser final : private JS::AutoGCRooter, public StrictModeGetter
     bool functionArguments(YieldHandling yieldHandling, FunctionSyntaxKind kind,
                            Node funcpn);
 
-    Node functionDefinition(InHandling inHandling, YieldHandling yieldHandling, HandleAtom name,
+    Node functionDefinition(uint32_t preludeStart,
+                            InHandling inHandling, YieldHandling yieldHandling, HandleAtom name,
                             FunctionSyntaxKind kind,
                             GeneratorKind generatorKind, FunctionAsyncKind asyncKind,
                             InvokedPrediction invoked = PredictUninvoked);
@@ -1321,14 +1325,16 @@ class Parser final : private JS::AutoGCRooter, public StrictModeGetter
 
     bool checkFunctionDefinition(HandleAtom funAtom, Node pn, FunctionSyntaxKind kind,
                                  GeneratorKind generatorKind, bool* tryAnnexB);
-    bool skipLazyInnerFunction(Node pn, FunctionSyntaxKind kind, bool tryAnnexB);
-    bool innerFunction(Node pn, ParseContext* outerpc, HandleFunction fun,
+    bool skipLazyInnerFunction(Node pn, uint32_t preludeStart, FunctionSyntaxKind kind,
+                               bool tryAnnexB);
+    bool innerFunction(Node pn, ParseContext* outerpc, HandleFunction fun, uint32_t preludeStart,
                        InHandling inHandling, YieldHandling yieldHandling,
                        FunctionSyntaxKind kind,
                        GeneratorKind generatorKind, FunctionAsyncKind asyncKind, bool tryAnnexB,
                        Directives inheritedDirectives, Directives* newDirectives);
-    bool trySyntaxParseInnerFunction(Node pn, HandleFunction fun, InHandling inHandling,
-                                     YieldHandling yieldHandling, FunctionSyntaxKind kind,
+    bool trySyntaxParseInnerFunction(Node pn, HandleFunction fun, uint32_t preludeStart,
+                                     InHandling inHandling, YieldHandling yieldHandling,
+                                     FunctionSyntaxKind kind,
                                      GeneratorKind generatorKind, FunctionAsyncKind asyncKind,
                                      bool tryAnnexB,
                                      Directives inheritedDirectives, Directives* newDirectives);
diff --git a/js/src/frontend/SharedContext.h b/js/src/frontend/SharedContext.h
index a6ac542f6..b20417d5d 100644
--- a/js/src/frontend/SharedContext.h
+++ b/js/src/frontend/SharedContext.h
@@ -450,6 +450,7 @@ class FunctionBox : public ObjectBox, public SharedContext
     uint32_t        bufEnd;
     uint32_t        startLine;
     uint32_t        startColumn;
+    uint32_t        preludeStart;
     uint16_t        length;
 
     uint8_t         generatorKindBits_;     /* The GeneratorKind of this function. */
@@ -476,8 +477,8 @@ class FunctionBox : public ObjectBox, public SharedContext
     FunctionContextFlags funCxFlags;
 
     FunctionBox(ExclusiveContext* cx, LifoAlloc& alloc, ObjectBox* traceListHead, JSFunction* fun,
-                Directives directives, bool extraWarnings, GeneratorKind generatorKind,
-                FunctionAsyncKind asyncKind);
+                uint32_t preludeStart, Directives directives, bool extraWarnings,
+                GeneratorKind generatorKind, FunctionAsyncKind asyncKind);
 
     MutableHandle<LexicalScope::Data*> namedLambdaBindings() {
         MOZ_ASSERT(context->compartment()->runtimeFromAnyThread()->keepAtoms());
diff --git a/js/src/frontend/TokenStream.cpp b/js/src/frontend/TokenStream.cpp
index 179a7c244..b040d2998 100644
--- a/js/src/frontend/TokenStream.cpp
+++ b/js/src/frontend/TokenStream.cpp
@@ -172,6 +172,12 @@ frontend::IsIdentifier(JSLinearString* str)
 }
 
 bool
+frontend::IsIdentifier(const char* chars, size_t length)
+{
+    return ::IsIdentifier(chars, length);
+}
+
+bool
 frontend::IsIdentifier(const char16_t* chars, size_t length)
 {
     return ::IsIdentifier(chars, length);
diff --git a/js/src/frontend/TokenStream.h b/js/src/frontend/TokenStream.h
index 5d6b4b795..77eea3d81 100644
--- a/js/src/frontend/TokenStream.h
+++ b/js/src/frontend/TokenStream.h
@@ -570,6 +570,13 @@ class MOZ_STACK_CLASS TokenStream
         return true;
     }
 
+    MOZ_MUST_USE bool peekOffset(uint32_t* offset, Modifier modifier = None) {
+        TokenPos pos;
+        if (!peekTokenPos(&pos, modifier))
+            return false;
+        *offset = pos.begin;
+        return true;
+    }
     // This is like peekToken(), with one exception:  if there is an EOL
     // between the end of the current token and the start of the next token, it
     // return true and store TOK_EOL in |*ttp|.  In that case, no token with
diff --git a/js/src/jsapi.cpp b/js/src/jsapi.cpp
index 37d023bd4..9ee29ffe4 100644
--- a/js/src/jsapi.cpp
+++ b/js/src/jsapi.cpp
@@ -4250,7 +4250,7 @@ JS_GetFunctionScript(JSContext* cx, HandleFunction fun)
  */
 static bool
 CompileFunction(JSContext* cx, const ReadOnlyCompileOptions& optionsArg,
-                const char* name,
+                HandleAtom name, bool isInvalidName,
                 SourceBufferHolder& srcBuf, uint32_t parameterListEnd,
                 HandleObject enclosingEnv, HandleScope enclosingScope,
                 MutableHandleFunction fun)
@@ -4261,13 +4261,8 @@ CompileFunction(JSContext* cx, const ReadOnlyCompileOptions& optionsArg,
     assertSameCompartment(cx, enclosingEnv);
     RootedAtom funAtom(cx);
 
-    if (name) {
-        funAtom = Atomize(cx, name, strlen(name));
-        if (!funAtom)
-            return false;
-    }
-
-    fun.set(NewScriptedFunction(cx, 0, JSFunction::INTERPRETED_NORMAL, funAtom,
+    fun.set(NewScriptedFunction(cx, 0, JSFunction::INTERPRETED_NORMAL,
+                                isInvalidName ? nullptr : name,
                                 /* proto = */ nullptr,
                                 gc::AllocKind::FUNCTION, TenuredObject,
                                 enclosingEnv));
@@ -4285,11 +4280,17 @@ CompileFunction(JSContext* cx, const ReadOnlyCompileOptions& optionsArg,
         return false;
     }
 
+    // When function name is not a valid identifier, the generated function
+    // source in srcBuf doesn't have a function name.  Set it here.
+    if (isInvalidName)
+        fun->setAtom(name);
+
     return true;
 }
 
 static MOZ_MUST_USE bool
-BuildFunctionString(unsigned nargs, const char* const* argnames,
+BuildFunctionString(const char* name, size_t nameLen,
+                    unsigned nargs, const char* const* argnames,
                     const SourceBufferHolder& srcBuf, StringBuffer* out,
                     uint32_t* parameterListEnd)
 {
@@ -4298,6 +4299,12 @@ BuildFunctionString(unsigned nargs, const char* const* argnames,
 
     if (!out->ensureTwoByteChars())
         return false;
+    if (!out->append("function "))
+        return false;
+    if (name) {
+        if (!out->append(name, nameLen))
+            return false;
+    }
     if (!out->append("("))
         return false;
     for (unsigned i = 0; i < nargs; i++) {
@@ -4334,15 +4341,32 @@ JS::CompileFunction(JSContext* cx, AutoObjectVector& envChain,
     if (!CreateNonSyntacticEnvironmentChain(cx, envChain, &env, &scope))
         return false;
 
+    size_t nameLen = 0;
+    bool isInvalidName = false;
+    RootedAtom nameAtom(cx);
+    if (name) {
+        nameLen = strlen(name);
+        nameAtom = Atomize(cx, name, nameLen);
+        if (!nameAtom)
+            return false;
+
+        // If name is not valid identifier
+        if (!js::frontend::IsIdentifier(name, nameLen))
+            isInvalidName = true;
+    }
+
     uint32_t parameterListEnd;
     StringBuffer funStr(cx);
-    if (!BuildFunctionString(nargs, argnames, srcBuf, &funStr, &parameterListEnd))
+    if (!BuildFunctionString(isInvalidName ? nullptr : name, nameLen, nargs, argnames, srcBuf,
+                             &funStr, &parameterListEnd)) {
         return false;
+    }
 
     size_t newLen = funStr.length();
     SourceBufferHolder newSrcBuf(funStr.stealChars(), newLen, SourceBufferHolder::GiveOwnership);
 
-    return CompileFunction(cx, options, name, newSrcBuf, parameterListEnd, env, scope, fun);
+    return CompileFunction(cx, options, nameAtom, isInvalidName, newSrcBuf, parameterListEnd, env,
+                           scope, fun);
 }
 
 JS_PUBLIC_API(bool)
diff --git a/js/src/jsfun.cpp b/js/src/jsfun.cpp
index bcb0da80b..4412362c2 100644
--- a/js/src/jsfun.cpp
+++ b/js/src/jsfun.cpp
@@ -815,8 +815,10 @@ CreateFunctionPrototype(JSContext* cx, JSProtoKey key)
 
     RootedFunction functionProto(cx, &functionProto_->as<JSFunction>());
 
-    const char* rawSource = "() {\n}";
+    const char* rawSource = "function () {\n}";
     size_t sourceLen = strlen(rawSource);
+    size_t begin = 9;
+    MOZ_ASSERT(rawSource[begin] == '(');
     mozilla::UniquePtr<char16_t[], JS::FreePolicy> source(InflateString(cx, rawSource, &sourceLen));
     if (!source)
         return nullptr;
@@ -838,8 +840,9 @@ CreateFunctionPrototype(JSContext* cx, JSProtoKey key)
     RootedScript script(cx, JSScript::Create(cx,
                                              options,
                                              sourceObject,
-                                             0,
-                                             ss->length()));
+                                             begin,
+                                             ss->length(),
+                                             0));
     if (!script || !JSScript::initFunctionPrototype(cx, script, functionProto))
         return nullptr;
 
@@ -1019,53 +1022,62 @@ js::FunctionToString(JSContext* cx, HandleFunction fun, bool prettyPrint)
         }
     }
 
-    if (fun->isAsync()) {
-        if (!out.append("async "))
-            return nullptr;
-    }
-
-    bool funIsMethodOrNonArrowLambda = (fun->isLambda() && !fun->isArrow()) || fun->isMethod() ||
-                                        fun->isGetter() || fun->isSetter();
+    bool funIsNonArrowLambda = fun->isLambda() && !fun->isArrow();
     bool haveSource = fun->isInterpreted() && !fun->isSelfHostedBuiltin();
 
-    // If we're not in pretty mode, put parentheses around lambda functions and methods.
-    if (haveSource && !prettyPrint && funIsMethodOrNonArrowLambda) {
+    // If we're not in pretty mode, put parentheses around lambda functions
+    // so that eval returns lambda, not function statement.
+    if (haveSource && !prettyPrint && funIsNonArrowLambda) {
         if (!out.append("("))
             return nullptr;
     }
-    if (!fun->isArrow()) {
-        bool ok;
-        if (fun->isStarGenerator() && !fun->isAsync())
-            ok = out.append("function* ");
-        else
-            ok = out.append("function ");
-        if (!ok)
-            return nullptr;
-    }
-    if (fun->explicitName()) {
-        if (!out.append(fun->explicitName()))
-            return nullptr;
-    }
 
     if (haveSource && !script->scriptSource()->hasSourceData() &&
         !JSScript::loadSource(cx, script->scriptSource(), &haveSource))
     {
         return nullptr;
     }
+
+    auto AppendPrelude = [&out, &fun]() {
+        if (fun->isAsync()) {
+            if (!out.append("async "))
+                return false;
+        }
+
+        if (!fun->isArrow()) {
+            if (!out.append("function"))
+                return false;
+
+            if (fun->isStarGenerator()) {
+                if (!out.append('*'))
+                    return false;
+            }
+        }
+
+        if (fun->explicitName()) {
+            if (!out.append(' '))
+                return false;
+            if (!out.append(fun->explicitName()))
+                return false;
+        }
+        return true;
+    };
+
     if (haveSource) {
-        Rooted<JSFlatString*> src(cx, script->sourceData(cx));
+        Rooted<JSFlatString*> src(cx, script->sourceDataWithPrelude(cx));
         if (!src)
             return nullptr;
 
         if (!out.append(src))
             return nullptr;
 
-        if (!prettyPrint && funIsMethodOrNonArrowLambda) {
+        if (!prettyPrint && funIsNonArrowLambda) {
             if (!out.append(")"))
                 return nullptr;
         }
     } else if (fun->isInterpreted() && !fun->isSelfHostedBuiltin()) {
-        if (!out.append("() {\n    ") ||
+        if (!AppendPrelude() ||
+            !out.append("() {\n    ") ||
             !out.append("[sourceless code]") ||
             !out.append("\n}"))
         {
@@ -1076,13 +1088,15 @@ js::FunctionToString(JSContext* cx, HandleFunction fun, bool prettyPrint)
 
         bool derived = fun->infallibleIsDefaultClassConstructor(cx);
         if (derived && fun->isDerivedClassConstructor()) {
-            if (!out.append("(...args) {\n    ") ||
+            if (!AppendPrelude() ||
+                !out.append("(...args) {\n    ") ||
                 !out.append("super(...args);\n}"))
             {
                 return nullptr;
             }
         } else {
-            if (!out.append("() {\n    "))
+            if (!AppendPrelude() ||
+                !out.append("() {\n    "))
                 return nullptr;
 
             if (!derived) {
@@ -1669,7 +1683,18 @@ FunctionConstructor(JSContext* cx, const CallArgs& args, GeneratorKind generator
 
     StringBuffer sb(cx);
 
-    if (!sb.append('('))
+    if (isAsync) {
+        if (!sb.append("async "))
+            return false;
+    }
+    if (!sb.append("function"))
+         return false;
+    if (isStarGenerator && !isAsync) {
+        if (!sb.append('*'))
+            return false;
+    }
+
+    if (!sb.append(" anonymous("))
         return false;
 
     if (args.length() > 1) {
@@ -1696,6 +1721,9 @@ FunctionConstructor(JSContext* cx, const CallArgs& args, GeneratorKind generator
         }
     }
 
+    if (!sb.append('\n'))
+        return false;
+
     // Remember the position of ")".
     Maybe<uint32_t> parameterListEnd = Some(uint32_t(sb.length()));
     MOZ_ASSERT(FunctionConstructorMedialSigils[0] == ')');
diff --git a/js/src/jsscript.cpp b/js/src/jsscript.cpp
index 9f914943e..33ae56d6f 100644
--- a/js/src/jsscript.cpp
+++ b/js/src/jsscript.cpp
@@ -235,6 +235,7 @@ XDRRelazificationInfo(XDRState<mode>* xdr, HandleFunction fun, HandleScript scri
     {
         uint32_t begin = script->sourceStart();
         uint32_t end = script->sourceEnd();
+        uint32_t preludeStart = script->preludeStart();
         uint32_t lineno = script->lineno();
         uint32_t column = script->column();
 
@@ -242,6 +243,7 @@ XDRRelazificationInfo(XDRState<mode>* xdr, HandleFunction fun, HandleScript scri
             packedFields = lazy->packedFields();
             MOZ_ASSERT(begin == lazy->begin());
             MOZ_ASSERT(end == lazy->end());
+            MOZ_ASSERT(preludeStart == lazy->preludeStart());
             MOZ_ASSERT(lineno == lazy->lineno());
             MOZ_ASSERT(column == lazy->column());
             // We can assert we have no inner functions because we don't
@@ -255,7 +257,7 @@ XDRRelazificationInfo(XDRState<mode>* xdr, HandleFunction fun, HandleScript scri
 
         if (mode == XDR_DECODE) {
             lazy.set(LazyScript::Create(cx, fun, script, enclosingScope, script,
-                                        packedFields, begin, end, lineno, column));
+                                        packedFields, begin, end, preludeStart, lineno, column));
 
             // As opposed to XDRLazyScript, we need to restore the runtime bits
             // of the script, as we are trying to match the fact this function
@@ -517,7 +519,7 @@ js::XDRScript(XDRState<mode>* xdr, HandleScope scriptEnclosingScope, HandleScrip
             sourceObject = &enclosingScript->sourceObject()->as<ScriptSourceObject>();
         }
 
-        script = JSScript::Create(cx, options, sourceObject, 0, 0);
+        script = JSScript::Create(cx, options, sourceObject, 0, 0, 0);
         if (!script)
             return false;
 
@@ -600,6 +602,8 @@ js::XDRScript(XDRState<mode>* xdr, HandleScope scriptEnclosingScope, HandleScrip
         return false;
     if (!xdr->codeUint32(&script->sourceEnd_))
         return false;
+    if (!xdr->codeUint32(&script->preludeStart_))
+        return false;
 
     if (!xdr->codeUint32(&lineno) ||
         !xdr->codeUint32(&column) ||
@@ -930,6 +934,7 @@ js::XDRLazyScript(XDRState<mode>* xdr, HandleScope enclosingScope, HandleScript
     {
         uint32_t begin;
         uint32_t end;
+        uint32_t preludeStart;
         uint32_t lineno;
         uint32_t column;
         uint64_t packedFields;
@@ -943,12 +948,14 @@ js::XDRLazyScript(XDRState<mode>* xdr, HandleScope enclosingScope, HandleScript
 
             begin = lazy->begin();
             end = lazy->end();
+            preludeStart = lazy->preludeStart();
             lineno = lazy->lineno();
             column = lazy->column();
             packedFields = lazy->packedFields();
         }
 
         if (!xdr->codeUint32(&begin) || !xdr->codeUint32(&end) ||
+            !xdr->codeUint32(&preludeStart) ||
             !xdr->codeUint32(&lineno) || !xdr->codeUint32(&column) ||
             !xdr->codeUint64(&packedFields))
         {
@@ -957,7 +964,7 @@ js::XDRLazyScript(XDRState<mode>* xdr, HandleScope enclosingScope, HandleScript
 
         if (mode == XDR_DECODE) {
             lazy.set(LazyScript::Create(cx, fun, nullptr, enclosingScope, enclosingScript,
-                                        packedFields, begin, end, lineno, column));
+                                        packedFields, begin, end, preludeStart, lineno, column));
             if (!lazy)
                 return false;
             fun->initLazyScript(lazy);
@@ -1430,6 +1437,13 @@ JSScript::sourceData(JSContext* cx)
     return scriptSource()->substring(cx, sourceStart(), sourceEnd());
 }
 
+JSFlatString*
+JSScript::sourceDataWithPrelude(JSContext* cx)
+{
+    MOZ_ASSERT(scriptSource()->hasSourceData());
+    return scriptSource()->substring(cx, preludeStart(), sourceEnd());
+}
+
 UncompressedSourceCache::AutoHoldEntry::AutoHoldEntry()
   : cache_(nullptr), sourceChunk_()
 {
@@ -2428,7 +2442,8 @@ JSScript::initCompartment(ExclusiveContext* cx)
 
 /* static */ JSScript*
 JSScript::Create(ExclusiveContext* cx, const ReadOnlyCompileOptions& options,
-                 HandleObject sourceObject, uint32_t bufStart, uint32_t bufEnd)
+                 HandleObject sourceObject, uint32_t bufStart, uint32_t bufEnd,
+                 uint32_t preludeStart)
 {
     MOZ_ASSERT(bufStart <= bufEnd);
 
@@ -2450,6 +2465,7 @@ JSScript::Create(ExclusiveContext* cx, const ReadOnlyCompileOptions& options,
     script->setSourceObject(sourceObject);
     script->sourceStart_ = bufStart;
     script->sourceEnd_ = bufEnd;
+    script->preludeStart_ = preludeStart;
 
     return script;
 }
@@ -3382,7 +3398,8 @@ CreateEmptyScriptForClone(JSContext* cx, HandleScript src)
            .setNoScriptRval(src->noScriptRval())
            .setVersion(src->getVersion());
 
-    return JSScript::Create(cx, options, sourceObject, src->sourceStart(), src->sourceEnd());
+    return JSScript::Create(cx, options, sourceObject, src->sourceStart(), src->sourceEnd(),
+                            src->preludeStart());
 }
 
 JSScript*
@@ -3932,7 +3949,8 @@ JSScript::formalLivesInArgumentsObject(unsigned argSlot)
 }
 
 LazyScript::LazyScript(JSFunction* fun, void* table, uint64_t packedFields,
-                       uint32_t begin, uint32_t end, uint32_t lineno, uint32_t column)
+                       uint32_t begin, uint32_t end, 
+                       uint32_t preludeStart, uint32_t lineno, uint32_t column)
   : script_(nullptr),
     function_(fun),
     enclosingScope_(nullptr),
@@ -3941,6 +3959,7 @@ LazyScript::LazyScript(JSFunction* fun, void* table, uint64_t packedFields,
     packedFields_(packedFields),
     begin_(begin),
     end_(end),
+    preludeStart_(preludeStart),
     lineno_(lineno),
     column_(column)
 {
@@ -3990,7 +4009,7 @@ LazyScript::maybeForwardedScriptSource() const
 /* static */ LazyScript*
 LazyScript::CreateRaw(ExclusiveContext* cx, HandleFunction fun,
                       uint64_t packedFields, uint32_t begin, uint32_t end,
-                      uint32_t lineno, uint32_t column)
+                      uint32_t preludeStart, uint32_t lineno, uint32_t column)
 {
     union {
         PackedView p;
@@ -4018,7 +4037,8 @@ LazyScript::CreateRaw(ExclusiveContext* cx, HandleFunction fun,
 
     cx->compartment()->scheduleDelazificationForDebugger();
 
-    return new (res) LazyScript(fun, table.forget(), packed, begin, end, lineno, column);
+    return new (res) LazyScript(fun, table.forget(), packed, begin, end,
+                                preludeStart, lineno, column);
 }
 
 /* static */ LazyScript*
@@ -4026,7 +4046,8 @@ LazyScript::Create(ExclusiveContext* cx, HandleFunction fun,
                    const frontend::AtomVector& closedOverBindings,
                    Handle<GCVector<JSFunction*, 8>> innerFunctions,
                    JSVersion version,
-                   uint32_t begin, uint32_t end, uint32_t lineno, uint32_t column)
+                   uint32_t begin, uint32_t end,
+                   uint32_t preludeStart, uint32_t lineno, uint32_t column)
 {
     union {
         PackedView p;
@@ -4049,7 +4070,8 @@ LazyScript::Create(ExclusiveContext* cx, HandleFunction fun,
     p.isDerivedClassConstructor = false;
     p.needsHomeObject = false;
 
-    LazyScript* res = LazyScript::CreateRaw(cx, fun, packedFields, begin, end, lineno, column);
+    LazyScript* res = LazyScript::CreateRaw(cx, fun, packedFields, begin, end, preludeStart,
+                                            lineno, column);
     if (!res)
         return nullptr;
 
@@ -4070,7 +4092,7 @@ LazyScript::Create(ExclusiveContext* cx, HandleFunction fun,
                    HandleScript script, HandleScope enclosingScope,
                    HandleScript enclosingScript,
                    uint64_t packedFields, uint32_t begin, uint32_t end,
-                   uint32_t lineno, uint32_t column)
+                   uint32_t preludeStart, uint32_t lineno, uint32_t column)
 {
     // Dummy atom which is not a valid property name.
     RootedAtom dummyAtom(cx, cx->names().comma);
@@ -4079,7 +4101,8 @@ LazyScript::Create(ExclusiveContext* cx, HandleFunction fun,
     // holding this lazy script.
     HandleFunction dummyFun = fun;
 
-    LazyScript* res = LazyScript::CreateRaw(cx, fun, packedFields, begin, end, lineno, column);
+    LazyScript* res = LazyScript::CreateRaw(cx, fun, packedFields, begin, end, preludeStart,
+                                            lineno, column);
     if (!res)
         return nullptr;
 
diff --git a/js/src/jsscript.h b/js/src/jsscript.h
index 87da79901..bb8635581 100644
--- a/js/src/jsscript.h
+++ b/js/src/jsscript.h
@@ -575,10 +575,6 @@ class ScriptSource
         introductionOffset_ = offset;
         hasIntroductionOffset_ = true;
     }
-
-    uint32_t parameterListEnd() const {
-        return parameterListEnd_;
-    }
 };
 
 class ScriptSourceHolder
@@ -857,9 +853,19 @@ class JSScript : public js::gc::TenuredCell
 
     uint32_t        bodyScopeIndex_; /* index into the scopes array of the body scope */
 
-    /* Range of characters in scriptSource which contains this script's source. */
+    // Range of characters in scriptSource which contains this script's source.
+    // each field points the following location.
+    //
+    //   function * f(a, b) { return a + b; }
+    //   ^          ^                        ^
+    //   |          |                        |
+    //   |          sourceStart_             sourceEnd_
+    //   |
+    //   preludeStart_
+    //
     uint32_t        sourceStart_;
     uint32_t        sourceEnd_;
+    uint32_t        preludeStart_;
 
     // Number of times the script has been called or has had backedges taken.
     // When running in ion, also increased for any inlined scripts. Reset if
@@ -1020,7 +1026,7 @@ class JSScript : public js::gc::TenuredCell
     // instead of private to suppress -Wunused-private-field compiler warnings.
   protected:
 #if JS_BITS_PER_WORD == 32
-    // Currently no padding is needed.
+    uint32_t padding;
 #endif
 
     //
@@ -1031,7 +1037,7 @@ class JSScript : public js::gc::TenuredCell
     static JSScript* Create(js::ExclusiveContext* cx,
                             const JS::ReadOnlyCompileOptions& options,
                             js::HandleObject sourceObject, uint32_t sourceStart,
-                            uint32_t sourceEnd);
+                            uint32_t sourceEnd, uint32_t preludeStart);
 
     void initCompartment(js::ExclusiveContext* cx);
 
@@ -1178,6 +1184,10 @@ class JSScript : public js::gc::TenuredCell
         return sourceEnd_;
     }
 
+    size_t preludeStart() const {
+        return preludeStart_;
+    }
+
     bool noScriptRval() const {
         return noScriptRval_;
     }
@@ -1501,7 +1511,8 @@ class JSScript : public js::gc::TenuredCell
     bool mayReadFrameArgsDirectly();
 
     JSFlatString* sourceData(JSContext* cx);
-
+    JSFlatString* sourceDataWithPrelude(JSContext* cx);
+    
     static bool loadSource(JSContext* cx, js::ScriptSource* ss, bool* worked);
 
     void setSourceObject(JSObject* object);
@@ -1920,7 +1931,8 @@ class LazyScript : public gc::TenuredCell
     // instead of private to suppress -Wunused-private-field compiler warnings.
   protected:
 #if JS_BITS_PER_WORD == 32
-    uint32_t padding;
+    // uint32_t padding;
+    // Currently no padding is needed.
 #endif
 
   private:
@@ -1960,20 +1972,25 @@ class LazyScript : public gc::TenuredCell
     };
 
     // Source location for the script.
+    // See the comment in JSScript for the details.
     uint32_t begin_;
     uint32_t end_;
+    uint32_t preludeStart_;
+    // Line and column of |begin_| position, that is the position where we
+    // start parsing.
     uint32_t lineno_;
     uint32_t column_;
 
     LazyScript(JSFunction* fun, void* table, uint64_t packedFields,
-               uint32_t begin, uint32_t end, uint32_t lineno, uint32_t column);
+               uint32_t begin, uint32_t end, uint32_t preludeStart,
+               uint32_t lineno, uint32_t column);
 
     // Create a LazyScript without initializing the closedOverBindings and the
     // innerFunctions. To be GC-safe, the caller must initialize both vectors
     // with valid atoms and functions.
     static LazyScript* CreateRaw(ExclusiveContext* cx, HandleFunction fun,
                                  uint64_t packedData, uint32_t begin, uint32_t end,
-                                 uint32_t lineno, uint32_t column);
+                                 uint32_t preludeStart, uint32_t lineno, uint32_t column);
 
   public:
     static const uint32_t NumClosedOverBindingsLimit = 1 << NumClosedOverBindingsBits;
@@ -1985,7 +2002,7 @@ class LazyScript : public gc::TenuredCell
                               const frontend::AtomVector& closedOverBindings,
                               Handle<GCVector<JSFunction*, 8>> innerFunctions,
                               JSVersion version, uint32_t begin, uint32_t end,
-                              uint32_t lineno, uint32_t column);
+                              uint32_t preludeStart, uint32_t lineno, uint32_t column);
 
     // Create a LazyScript and initialize the closedOverBindings and the
     // innerFunctions with dummy values to be replaced in a later initialization
@@ -2000,7 +2017,7 @@ class LazyScript : public gc::TenuredCell
                               HandleScript script, HandleScope enclosingScope,
                               HandleScript enclosingScript,
                               uint64_t packedData, uint32_t begin, uint32_t end,
-                              uint32_t lineno, uint32_t column);
+                              uint32_t preludeStart, uint32_t lineno, uint32_t column);
 
     void initRuntimeFields(uint64_t packedFields);
 
@@ -2173,6 +2190,9 @@ class LazyScript : public gc::TenuredCell
     uint32_t end() const {
         return end_;
     }
+    uint32_t preludeStart() const {
+        return preludeStart_;
+    }
     uint32_t lineno() const {
         return lineno_;
     }
@@ -2199,7 +2219,8 @@ class LazyScript : public gc::TenuredCell
 };
 
 /* If this fails, add/remove padding within LazyScript. */
-JS_STATIC_ASSERT(sizeof(LazyScript) % js::gc::CellSize == 0);
+static_assert(sizeof(LazyScript) % js::gc::CellSize == 0,
+              "Size of LazyScript must be an integral multiple of js::gc::CellSize");
 
 struct ScriptAndCounts
 {
diff --git a/js/src/wasm/AsmJS.cpp b/js/src/wasm/AsmJS.cpp
index 2237d1d7f..2b6690363 100644
--- a/js/src/wasm/AsmJS.cpp
+++ b/js/src/wasm/AsmJS.cpp
@@ -318,6 +318,7 @@ struct js::AsmJSMetadata : Metadata, AsmJSMetadataCacheablePod
     // Function constructor, this will be the first character in the function
     // source. Otherwise, it will be the opening parenthesis of the arguments
     // list.
+    uint32_t                preludeStart;
     uint32_t                srcStart;
     uint32_t                srcBodyStart;
     bool                    strict;
@@ -1758,6 +1759,7 @@ class MOZ_STACK_CLASS ModuleValidator
         if (!asmJSMetadata_)
             return false;
 
+        asmJSMetadata_->preludeStart = moduleFunctionNode_->pn_funbox->preludeStart;
         asmJSMetadata_->srcStart = moduleFunctionNode_->pn_body->pn_pos.begin;
         asmJSMetadata_->srcBodyStart = parser_.tokenStream.currentToken().pos.end;
         asmJSMetadata_->strict = parser_.pc->sc()->strict() &&
@@ -7049,6 +7051,7 @@ ParseFunction(ModuleValidator& m, ParseNode** fnOut, unsigned* line)
     TokenStream& tokenStream = m.tokenStream();
 
     tokenStream.consumeKnownToken(TOK_FUNCTION, TokenStream::Operand);
+    uint32_t preludeStart = tokenStream.currentToken().pos.begin;
     *line = tokenStream.srcCoords.lineNum(tokenStream.currentToken().pos.end);
 
     TokenKind tk;
@@ -7071,7 +7074,7 @@ ParseFunction(ModuleValidator& m, ParseNode** fnOut, unsigned* line)
 
     ParseContext* outerpc = m.parser().pc;
     Directives directives(outerpc);
-    FunctionBox* funbox = m.parser().newFunctionBox(fn, fun, directives, NotGenerator,
+    FunctionBox* funbox = m.parser().newFunctionBox(fn, fun, preludeStart, directives, NotGenerator,
                                                     SyncFunction, /* tryAnnexB = */ false);
     if (!funbox)
         return false;
@@ -8054,7 +8057,7 @@ HandleInstantiationFailure(JSContext* cx, CallArgs args, const AsmJSMetadata& me
         return false;
     }
 
-    uint32_t begin = metadata.srcStart;
+    uint32_t begin = metadata.preludeStart;
     uint32_t end = metadata.srcEndAfterCurly();
     Rooted<JSFlatString*> src(cx, source->substringDontDeflate(cx, begin, end));
     if (!src)
@@ -8085,7 +8088,7 @@ HandleInstantiationFailure(JSContext* cx, CallArgs args, const AsmJSMetadata& me
     SourceBufferHolder::Ownership ownership = stableChars.maybeGiveOwnershipToCaller()
                                               ? SourceBufferHolder::GiveOwnership
                                               : SourceBufferHolder::NoOwnership;
-    SourceBufferHolder srcBuf(chars, stableChars.twoByteRange().length(), ownership);
+    SourceBufferHolder srcBuf(chars, end - begin, ownership);
     if (!frontend::CompileStandaloneFunction(cx, &fun, options, srcBuf, Nothing()))
         return false;
 
@@ -8537,6 +8540,7 @@ LookupAsmJSModuleInCache(ExclusiveContext* cx, AsmJSParser& parser, bool* loaded
         return true;
 
     // See AsmJSMetadata comment as well as ModuleValidator::init().
+    asmJSMetadata->preludeStart = parser.pc->functionBox()->preludeStart;
     asmJSMetadata->srcStart = parser.pc->functionBox()->functionNode->pn_body->pn_pos.begin;
     asmJSMetadata->srcBodyStart = parser.tokenStream.currentToken().pos.end;
     asmJSMetadata->strict = parser.pc->sc()->strict() && !parser.pc->sc()->hasExplicitUseStrict();
@@ -8834,7 +8838,7 @@ js::AsmJSModuleToString(JSContext* cx, HandleFunction fun, bool addParenToLambda
     MOZ_ASSERT(IsAsmJSModule(fun));
 
     const AsmJSMetadata& metadata = AsmJSModuleFunctionToModule(fun).metadata().asAsmJS();
-    uint32_t begin = metadata.srcStart;
+    uint32_t begin = metadata.preludeStart;
     uint32_t end = metadata.srcEndAfterCurly();
     ScriptSource* source = metadata.scriptSource.get();
 
@@ -8843,17 +8847,15 @@ js::AsmJSModuleToString(JSContext* cx, HandleFunction fun, bool addParenToLambda
     if (addParenToLambda && fun->isLambda() && !out.append("("))
         return nullptr;
 
-    if (!out.append("function "))
-        return nullptr;
-
-    if (fun->explicitName() && !out.append(fun->explicitName()))
-        return nullptr;
-
     bool haveSource = source->hasSourceData();
     if (!haveSource && !JSScript::loadSource(cx, source, &haveSource))
         return nullptr;
 
     if (!haveSource) {
+        if (!out.append("function "))
+            return nullptr;
+        if (fun->explicitName() && !out.append(fun->explicitName()))
+            return nullptr;
         if (!out.append("() {\n    [sourceless code]\n}"))
             return nullptr;
     } else {