Performance concern with let/const

[Luke Hoban]

We've begun deeper investigations of implementation practicalities related to let/const, and two significant performance concerns have been raised.  I think these both merit re-opening discussion of two aspects of the let/const design.


__Temporal dead zones__

For reference on previous discussion of temporal dead zone see [1].

I've expressed concerns with the performance overhead required for temporal dead zones in the past, but we did not at the time have any data to point to regarding the scale of the concern.  

As an experiment, I took the early-boyer test from V8 and changed 'var' to 'let'.  In Chrome preview builds with 'let' support, I saw a consistent ~27% slowdown.  That is, the 'let is the new var' mantra leads to 27% slower code in this example for the same functionality.  

However, we are aware that there are a class of dynamic checks that can be removed by static analysis - in particular intra-procedural use before assignment checks.  We implemented these checks in a Chakra prototype, and even with these, we still see an ~5% slowdown.  

Our belief is that any further removal of these dynamic checks (inter-procedural checks of accesses to closure captured let references) is a much more difficult proposition, if even possible in any reasonable percentage of cases.  

Unless we can be sure that the above perf hit can indeed be easily overcome, I'd like to re-recommend that temporal dead zones for let and const be removed from the ES6 specification.  Both would remain block scoped binding, but would be dynamically observable in 'undefined' state - including that 'const' would be observable as 'undefined' before single assignment.  

In particular - the case against temporal dead zones is as follows:

1. The value of temporal dead zones is to catch a class of programmer errors.  This value is not overly significant (it's far from the most common error that lint-like tools catch today, or that affects large code bases in practice), and I do not believe the need/demand for runtime-enforced protection against this class of errors has been proven.  This feature of let/const is not the primary motivation for either feature (block scoped binding, inlinability and errors on re-assignment to const are the motivating features).

2. The stated goal of 'let' is to replace 'var' in common usage (and if this is not the goal, we should not be adding 'let')

3. Unless the above performance hit can be overcome, and given #2 above, *let will slow down the web by ~5%*.

4. Even if the above performance hit can be (mostly) overcome with net new engine performance work, that is performance work being forced on engine vendors simply to not make the web slower, and comes at the opportunity cost of actually working on making the web *faster*.  

5. We are fairly confident that it is not possible to fully remove the runtime overhead cost associated with temporal dead zones.  That means that, as a rule, 'let' will be slower than 'var'.   And possibly significantly slower in certain coding patterns. Even if that's only 1% slower, I don't think we're going to convince the world to use 'let' if it's primary impact on their code is to make it slower.  (The net value proposition for let simply isn't strong enough to justify this).

6. The only time-proven implementation of let/const (SpiderMonkey) did not implement temporal dead zones.  The impact of this feature on the practical performance of the web is not well enough understood relative to the value proposition of temporal dead zones.


__ Early Errors__

Let and const introduce a few new early errors (though this general concern impacts several other areas of ES6 as well).  Of particular note, assignment to const and re-declaration of 'let' are spec'd as early errors. 

 Assignment to const is meaningfully different than previous early errors, because detecting it requires binding references *before any code runs*.  Chakra today parses the whole script input to report syntax errors, but avoids building and storing ASTs until function bodies are executed [2].  Since it is common for significant amounts of script on typical pages to be downloaded but not ever executed, this can save significant load time performance cost.  

However, if scope chains and variable reference binding for all scopes in the file need to be established before any code executes, significantly more work is required during this load period.  This work cannot be deferred (and potentially avoided entirely if the code is not called), because early errors must be identified before any code executes.

This ultimately means that any script which mentions 'const' will defeat a significant aspect of deferred AST building, and therefore take a load time perf hit.  

More generally - this raises a concern about putting increasingly more aggressive static analysis in early errors.  It may, for example, argue for a 3rd error category, of errors that must be reported before any code in their function body executes.  But more likely, it just argues for allowing any heavy static analysis to be postponed to late errors (or removed entirely and left to lint tools, if the raw overhead is particularly significant).


Luke

[1] https://mail.mozilla.org/pipermail/es-discuss/2011-August/016188.html
[2] http://blogs.msdn.com/b/ie/archive/2012/06/13/advances-in-javascript-performance-in-ie10-and-windows-8.aspx