[js] Analyzer fails to optimize constant conditional inside abstract when StringBuf is used

[barisyild]

The analyzer optimizer fails to fold a constant conditional expression inside an abstract type when StringBuf is used in the same function. The condition this == 0 where this is known to be 0 at compile-time should be evaluated to true, but instead generates a runtime conditional 0 == 0 ? 65 : 66.

Removing the new StringBuf().addChar(10); line causes the analyzer to correctly optimize the conditional.

Minimal Reproduction

abstract W(Int) from Int {
	public inline function test() {
		trace(this == 0 ? 65 : 66);
		new StringBuf().addChar(10);
	}
}

class Test {
	static function main() {
		final w:W = 0;
		w.test();
	}
}

Actual Output (JS)

class Test {
	static main() {
		console.log("Test.hx:3:", 0 == 0 ? 65 : 66);  // Not optimized!
		let _this_b = "";
		_this_b += String.fromCodePoint(10);
	}
}

Expected Output (JS)

class Test {
	static main() {
		console.log("Test.hx:3:", 65);  // Should be constant-folded
		let _this_b = "";
		_this_b += String.fromCodePoint(10);
	}
}

Notes

• Removing new StringBuf().addChar(10); causes the analyzer to correctly optimize to 65
• The same code outside an abstract is optimized correctly
• The value of w is known at compile-time (w:W = 0)

[Simn]

The actual culprit here is String.fromCharCode which has untyped, which always causes the analyzer to give up. The solution is to implement untyped __define_feature__ differently.

[tobil4sk]

The actual culprit here is String.fromCharCode which has untyped, which always causes the analyzer to give up.

Could the @:pure meta on String.fromCharCode not help at all here?

[barisyild]

The actual culprit here is String.fromCharCode which has untyped, which always causes the analyzer to give up. The solution is to implement untyped define_feature differently.

So how does it work outside of the abstract?

https://try.haxe.org/#eB31680E

[barisyild]

Interestingly, while std cannot be optimized, untyped or direct JavaScript calls can be optimized. Doesn't std already do that?

direct ✅: https://try.haxe.org/#D35ED5de
untyped ✅: https://try.haxe.org/#9384A72B
std ❗: https://try.haxe.org/#47Ef0Eee