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, 8 insertions, 1 deletions

diff --git a/src/qml/compiler/qv4compilerscanfunctions.cpp b/src/qml/compiler/qv4compilerscanfunctions.cpp
index 06335b3aa0..5d3a7a6d8c 100644
--- a/src/qml/compiler/qv4compilerscanfunctions.cpp
+++ b/src/qml/compiler/qv4compilerscanfunctions.cpp
@@ -312,6 +312,10 @@ bool ScanFunctions::visit(PatternElement *ast)
     QStringList names;
     ast->boundNames(&names);
 
+    QQmlJS::AST::SourceLocation lastInitializerLocation = ast->lastSourceLocation();
+    if (_context->lastBlockInitializerLocation.isValid())
+        lastInitializerLocation = _context->lastBlockInitializerLocation;
+
     for (const QString &name : qAsConst(names)) {
         if (_context->isStrict && (name == QLatin1String("eval") || name == QLatin1String("arguments")))
             _cg->throwSyntaxError(ast->identifierToken, QStringLiteral("Variable name may not be eval or arguments in strict mode"));
@@ -322,7 +326,8 @@ bool ScanFunctions::visit(PatternElement *ast)
             _cg->throwSyntaxError(ast->identifierToken, QStringLiteral("Missing initializer in const declaration"));
             return false;
         }
-        if (!_context->addLocalVar(name, ast->initializer ? Context::VariableDefinition : Context::VariableDeclaration, ast->scope)) {
+        if (!_context->addLocalVar(name, ast->initializer ? Context::VariableDefinition : Context::VariableDeclaration, ast->scope,
+                                   /*function*/nullptr, lastInitializerLocation)) {
             _cg->throwSyntaxError(ast->identifierToken, QStringLiteral("Identifier %1 has already been declared").arg(name));
             return false;
         }
@@ -485,6 +490,8 @@ void ScanFunctions::endVisit(ForStatement *)
 
 bool ScanFunctions::visit(ForEachStatement *ast) {
     enterEnvironment(ast, ContextType::Block, QStringLiteral("%Foreach"));
+    if (ast->expression)
+        _context->lastBlockInitializerLocation = ast->expression->lastSourceLocation();
     Node::accept(ast->lhs, this);
     Node::accept(ast->expression, this);