Optimize access to lexically scoped variables

If we access a lexically scoped variable after the initializer, then we
know it's either initialized or at least undefined, so we don't need to
do the TDZ check anymore.

The ES tests ensure that we don't optimize too much and the newly
revived tst_v4misc test ensures that we do not generate the TDZ check
instruction for certain scenarios.

Change-Id: I6706d1feb22217f323124ee698ebadb70324693b
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

1 files changed, 7 insertions, 2 deletions

diff --git a/src/qml/compiler/qv4compilercontext_p.h b/src/qml/compiler/qv4compilercontext_p.h
index e7847e7072..3df0aa6b3a 100644
--- a/src/qml/compiler/qv4compilercontext_p.h
+++ b/src/qml/compiler/qv4compilercontext_p.h
@@ -167,8 +167,10 @@ struct Context {
         QQmlJS::AST::VariableScope scope = QQmlJS::AST::VariableScope::Var;
         mutable bool canEscape = false;
         QQmlJS::AST::FunctionExpression *function = nullptr;
+        QQmlJS::AST::SourceLocation endOfInitializerLocation;
 
         bool isLexicallyScoped() const { return this->scope != QQmlJS::AST::VariableScope::Var; }
+        bool requiresTDZCheck(const QQmlJS::AST::SourceLocation &accessLocation, bool accessAcrossContextBoundaries) const;
     };
     typedef QMap<QString, Member> MemberMap;
 
@@ -206,6 +208,7 @@ struct Context {
     bool isWithBlock = false;
     bool isCatchBlock = false;
     QString caughtVariable;
+    QQmlJS::AST::SourceLocation lastBlockInitializerLocation;
 
     enum UsesArgumentsObject {
         ArgumentsObjectUnknown,
@@ -313,7 +316,8 @@ struct Context {
         usedVariables.insert(name);
     }
 
-    bool addLocalVar(const QString &name, MemberType contextType, QQmlJS::AST::VariableScope scope, QQmlJS::AST::FunctionExpression *function = nullptr);
+    bool addLocalVar(const QString &name, MemberType contextType, QQmlJS::AST::VariableScope scope, QQmlJS::AST::FunctionExpression *function = nullptr,
+                     const QQmlJS::AST::SourceLocation &endOfInitializer = QQmlJS::AST::SourceLocation());
 
     struct ResolvedName {
         enum Type {
@@ -329,9 +333,10 @@ struct Context {
         bool requiresTDZCheck = false;
         int scope = -1;
         int index = -1;
+        QQmlJS::AST::SourceLocation endOfDeclarationLocation;
         bool isValid() const { return type != Unresolved; }
     };
-    ResolvedName resolveName(const QString &name);
+    ResolvedName resolveName(const QString &name, const QQmlJS::AST::SourceLocation &accessLocation);
     void emitBlockHeader(Compiler::Codegen *codegen);
     void emitBlockFooter(Compiler::Codegen *codegen);