1. V8 does not perform tail call elimination (AFAIK none of the major JS VMs perform it) because certain legacy JS language features (namely func.arguments and

1. V8 does not perform tail call elimination (AFAIK none of the major JS VMs perform it) because certain legacy JS language features (namely func.arguments and similar) are not TCE-friendly (see for more details http://code.google.com/p/v8/issues/detail?id=457 http://code.google.com/p/v8/issues/detail?id=457). So manually performing TCE indeed reduces number of (relatively) long lived objects: tails of the string created with substr. 2. I am not sure about this question. V8 uses cons-strings internally: so it is important to understand that c = a + b does not result (at least not immediately) in an allocation of a new sequential string and copying characters from a and b to it. That means that concatenating many strings does not cost much (especially if they are large) at the moment of concatenation. Actual concatenation happens later, when you start indexing into this string: V8 will flatten it (convert from cons-string form into sequential form). 3. See 2. If you intermix indexing and concatenation you basically force s to oscillate between cons and sequential forms: cons-string is created, then flattened on indexing, and result is concatenated with the string forming a cons-string which gets flattened... Resulting performance is bad because cons-strings actually become pure overhead (you waste more on their creation than you benefit from them) in this cycle of concatenation and flattening. 4. '*' as a prefix means optimized function and '~' as a prefix means non-optimized function. 5. Everything starting from 'Understanding Numbers' slide is a collection of facts unrelated to the first part of the talk about profiling. To get the idea of how typed arrays affect performance you can read my blog post: http://blog.mrale.ph/post/5436474765/dangers-of-cross-language-benchmark-games http://blog.mrale.ph/post/5436474765/dangers-of-cross-langua... 6. HiddenClass slides try to illustrate the idea of inline caching (http://en.wikipedia.org/wiki/Inline_caching http://en.wikipedia.org/wiki/Inline_caching) without using assembly. It's pseudo-code. HiddenClass is a structure that describes layout of an object. Slides #42-#50 illustrate how VM builds hidden classes hierarchy while executing the code. Slides #51-#52 try to illustrate how you can utilize hidden classes when you compile code. There are couple presentations that discuss hidden classes at the lower and more detailed level: for example Mads Ager's talk at Google IO 2009: http://youtu.be/FrufJFBSoQY http://youtu.be/FrufJFBSoQY 7. No, there were no example (it's impossible to fit that much into a single 40m talk). In V8 ICs have a special megamorphic state in addition to monomorphic state. Megamorphic stubs are not specialized for types IC had seen (unlike monomorphic stubs), instead they rely on a cache of monomorphic stubs (indexed by hidden class of the receiver). 8. Those are functions that force object from fast-mode to dictionary-mode. 9. In non optimized code it is materialized when used in the code. In optimized it is never materialized (but as the slide says: if you "misuse" arguments in a function it will not be optimized). 10. I have not seen any real world code that suffers from this problem. But people ask about scopes and closed-variables quite often that is why it is included.