feat(codegen): keep arrow function PIFEs

[sapphi-red]

PIFE is the abbreviation of "Possibly-Invoked Function Expressions". It is a function expression wrapped with a parenthesized expression.
PIFEs annotate functions that are likely to be invoked eagerly. When v8 encounters such expressions, it compiles them eagerly (rather than compiling it later). See v8's blog post for more details.

The blog post only mentions regular FunctionExpressions, but ArrowFunctions are also supported. The cases that will be eagerly compiled are:

• when ! comes before function literals (code) (e.g. !function(){})
• when a function expression is wrapped with parenthesis (code) (e.g. (function () {}), (async function () {}))
• when an arrow function is wrapped with parenthesis (code1, code2, code3) (e.g. console.log((() => {})))
• when () or `` (tagged templates) comes after function literals (only in some cases) (code1, code2) (e.g. ~function(){}(), ~function(){}`` )
• when explicit compile hints are used (code1, code2) (e.g. //# allFunctionsCalledOnLoad)

Keeping PIFEs as-is in the code generation will unlock optimizations like rolldown/rolldown#5319.

Support by other engines

• SpiderMonkey (Firefox)
  • when a function expression / arrow function expression is wrapped with parenthesis (code1, code2, code3)
• JavaScriptCore (Safari)
  • probably not supported

Note: this PR only implements arrow function PIFEs for now.

[graphite-app]

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[codspeed-hq]

CodSpeed Instrumentation Performance Report

Merging #12353 will not alter performance

_{Comparing feat/keep-pifes (0920e98) with main (998c67b)}

Summary

✅ 34 untouched benchmarks

[sapphi-red]

@oxc-project/core Does this approach make sense? If it does, I'll add pife field to FunctionExpression, too.

[overlookmotel]

I'm wondering if the additional pife field is necessary.

I assume this alternative approach would also work:

• Don't change AST or parser.
• Alter codegen to print parentheses for a ParenthesizedExpression when it contains a Function or ArrowFunctionExpression.
• Make sure minifier doesn't strip off the parentheses.

Current state of play appears to be:

// PIFEs retained in codegen, functions unwrapped in minifier. Good!
(() => alert(1))();
(() => { alert(2); })();

// PIFEs retained in codegen and minifier,
// but not optimal minification - could unwrap functions
!function() { alert(3); }();
!function() { alert(4); }(), function() { alert(5); }();

// PIFE lost. Bad!
f1 = (function() { alert(6); });
f2 = (() => alert(7));
f3 = (() => { alert(8); });

Playground

So, unless I'm missing something, it looks like the last 3 cases (ParenthesizedExpressions) are the only one we need to correct.

Personally I think it might be preferable not to introduce a pife property for this. It's a duplication of information which is already in AST - which personally I'm keen to avoid, because info in 2 places can get out of sync during transformations.

[Boshen]

Make sure minifier doesn't strip off the parentheses.

Parentheses are not supposed to have meanings ... minifier is free to discard it, or it will break.

Parser also has a preserveParentheses: false mode :-)

[Boshen]

@oxc-project/core Does this approach make sense? If it does, I'll add pife field to FunctionExpression, too.

Let me think about this.

[sapphi-red]

Yeah, I mainly went with this approach considering preserveParentheses: false.

[Boshen]

My thoughts:

• PIFE is a semantic feature
• A semantic feature should be encoded in the AST
• PIFE requires parenthesis, theoretically they can use any other syntax
• preserveParentheses: false omits parens from the AST
• DCE and minifier passes removes parens from the AST

[Boshen]

Merge activity

• Jul 20, 1:12 PM UTC: The merge label '0-merge' was detected. This PR will be added to the Graphite merge queue once it meets the requirements.
• Jul 20, 1:12 PM UTC: Boshen added this pull request to the Graphite merge queue.
• Jul 20, 1:20 PM UTC: Merged by the Graphite merge queue.