commit faedbc8c4b0b89c02f58711358c8f7b9a91364ce
author drousso@apple.com <drousso@apple.com@268f45cc-cd09-0410-ab3c-d52691b4dbfc> Wed Apr 15 07:02:55 2020 +0000
committer drousso@apple.com <drousso@apple.com@268f45cc-cd09-0410-ab3c-d52691b4dbfc> Wed Apr 15 07:02:55 2020 +0000
tree 3c93bf89422c359c48d663e67e7a2545b2d76b89
parent 618a75ee48fde4ee762ac97aa4d31edbcdd772a1

[ESNext] Implement logical assignment operators
https://bugs.webkit.org/show_bug.cgi?id=209716

Reviewed by Ross Kirsling.

JSTests:

* stress/logical-assignment-operator-and.js: Added.
* stress/logical-assignment-operator-nullish.js: Added.
* stress/logical-assignment-operator-or.js: Added.

* test262/config.yaml:
* test262/expectations.yaml:
Right now, test262 expects an early error to be thrown if the lhs is not simple, which does
not match what we do with other read-modify assignment operators. This is likely to change
in the future to match existing behavior (throw a `ReferenceError`) [1].

[1]: <https://github.com/tc39/ecma262/issues/257#issuecomment-502878708>

Source/JavaScriptCore:

Implement the logical assignment operators proposal, which is now Stage 3. It introduces
three new assignment operators which will only store the result of the rhs in the lhs if the
lhs meets the given condition:
 - `??=`, for if the lhs is nullish (`null` or `undefined`)
 - `||=`, for if the lhs is falsy
 - `&&=`, for if the lhs is truthy

This short circuiting can be beneficial as it can avoid a redundant store when used in the
common JavaScript programming pattern of "defaulting" a parameter.

```js
    function foo(x) {
        x = x || 42;
    }
```

If `x` is a truthy value, it would result in the rhs `x` being stored back into the lhs `x`.
In some situations, this can have negative unintended side-effects, such as for `innerHTML`.

Logical assignment operators, however, are defined such that they only store if the rhs is
to actually be needed/used, skipping the redundant store and simply returning lhs otherwise.

In the case of readonly references, this means that an error is only thrown when the
assignment occurs, meaning that if the lhs already satisfies the condition it will be used
and returned with no error.

* parser/ParserTokens.h:
* parser/Lexer.cpp:
(JSC::Lexer<T>::lexWithoutClearingLineTerminator):
* parser/Parser.cpp:
(JSC::Parser<LexerType>::parseAssignmentExpression):

* parser/ASTBuilder.h:
(JSC::ASTBuilder::makeAssignNode):
* parser/Nodes.h:
* parser/NodeConstructors.h:
(JSC::ShortCircuitReadModifyResolveNode::ShortCircuitReadModifyResolveNode): Added.
(JSC::ShortCircuitReadModifyBracketNode::ShortCircuitReadModifyBracketNode): Added.
(JSC::ShortCircuitReadModifyDotNode::ShortCircuitReadModifyDotNode): Added.
* bytecompiler/NodesCodegen.cpp:
(JSC::emitShortCircuitAssignment): Added.
(JSC::ShortCircuitReadModifyResolveNode::emitBytecode): Added.
(JSC::ShortCircuitReadModifyDotNode::emitBytecode): Added.
(JSC::ShortCircuitReadModifyBracketNode::emitBytecode): Added.

* runtime/OptionsList.h:
Add a `useLogicalAssignmentOperators` setting for controlling this feature.

Tools:

* Scripts/run-jsc-stress-tests:


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@260119 268f45cc-cd09-0410-ab3c-d52691b4dbfc