Implement optional chaining

This change implements optional chaining (https://github.com/tc39/proposal-optional-chaining) by adding a new type of optional lookup with an offset to the end of a chain.
If `undefined` or `null` is encountered during an access marked as optional, we jump to that end offset.

Features:
- Full support for all kinds of optional chain
- With some codegen overhead but zero overhead during normal non-optional FieldMemberExpression resolution
- Properly retains this contexts and does not need to resolve anything twice (this has been an issue previously)
- No extra AST structures, just flags for existing ones

[ChangeLog][QtQml] Added support for optional chaining (https://github.com/tc39/proposal-optional-chaining)

Fixes: QTBUG-77926
Change-Id: I9a41cdc4ca272066c79c72b9b22206498a546843
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

1 files changed, 14 insertions, 0 deletions

diff --git a/src/qml/jit/qv4baselinejit.cpp b/src/qml/jit/qv4baselinejit.cpp
index 22cbfd1536..f660e363ea 100644
--- a/src/qml/jit/qv4baselinejit.cpp
+++ b/src/qml/jit/qv4baselinejit.cpp
@@ -283,6 +283,13 @@ void BaselineJIT::generate_LoadProperty(int name)
     BASELINEJIT_GENERATE_RUNTIME_CALL(LoadProperty, CallResultDestination::InAccumulator);
 }
 
+void BaselineJIT::generate_LoadOptionalProperty(int name, int offset)
+{
+    labels.insert(as->jumpEqNull(absoluteOffset(offset)));
+
+    generate_LoadProperty(name);
+}
+
 void BaselineJIT::generate_GetLookup(int index)
 {
     STORE_IP();
@@ -295,6 +302,13 @@ void BaselineJIT::generate_GetLookup(int index)
     BASELINEJIT_GENERATE_RUNTIME_CALL(GetLookup, CallResultDestination::InAccumulator);
 }
 
+void BaselineJIT::generate_GetOptionalLookup(int index, int offset)
+{
+    labels.insert(as->jumpEqNull(absoluteOffset(offset)));
+
+    generate_GetLookup(index);
+}
+
 void BaselineJIT::generate_StoreProperty(int name, int base)
 {
     STORE_IP();