1392223 - Do not generate bytecode for absolutely-unreachable code

Status: NEW

(Core :: JavaScript Engine, enhancement, P3)

Description

[Tooru Fujisawa [:arai] (away until 08-17)]

code:
  `dis(function (o) {return 10; eval(o)})`
actual result:
  bytecode for `eval(o)` is generated
expected result:
  don't generate code for unreachanbel code

see also: https://dxr.mozilla.org/mozilla-central/rev/7dddbd85047c6dc73ddbe1e423cd643a217845b3/devtools/client/debugger/new/parser-worker.js#17334
(that seems to be an optimization(?) for V8)

currently we're emitting warning for unreachable code.
we could use the same information in bytecode emitter.
we should make sure declaration etc keeps working.

Comment 1

[Tooru Fujisawa [:arai] (away until 08-17)]

might be related to bug 1341943.
but in this case it's explicit JS statements, not implicit return.

Comment 2

[Tooru Fujisawa [:arai] (away until 08-17)]

(In reply to Tooru Fujisawa [:arai] from comment #0)

see also:
https://dxr.mozilla.org/mozilla-central/rev/
7dddbd85047c6dc73ddbe1e423cd643a217845b3/devtools/client/debugger/new/parser-
worker.js#17334
(that seems to be an optimization(?) for V8)

The link doesn't work, but the code is the following:

	module.exports = function toFastproperties(o) {
		function Sub() {}
		Sub.prototype = o;
		var receiver = new Sub(); // create an instance
		function ic() { return typeof receiver.foo; } // perform access
		ic(); 
		ic();
		return o;
		eval("o" + o); // ensure no dead code elimination
	}