On Sun, Mar 31, 2013 at 4:16 PM, Sam Lin <[email protected]> wrote:
> Hi, > > 1. Numbers: > About the tagging technique, Objects and small integer are both to be > set as 32 bits. > Further, they use the bottom bit as the tagged value. if the bit is > set, then this means that 31 bits are object pointers > (Does this mean each bit points to the object accordingly?). > Otherwise, then that 31 bits represent integer values. > I think my understanding so far is correct. Let me know if I am wrong > here. > However, I dont really understand why v8 does this and how this tagging > technique can improve the performance. > Not having to box numbers is a performance advantage. If you don't box them, you need to recognize them somehow, hence the tagging. > > 2. Arrays: > I went through the slides, there are two types of ways that how v8 > handles elements: > 1. Fast elements: Linear storage for compact key sets. > This is understandable that this one should be very fast. But I > dont know what does "key sets" mean here? > "Set" in the mathematical sense: the set of all keys. > 2. Dictionary elements: Hash table storage. > The speaker(Google i/o 2012) mentions this one is much more > compact than the first one. > I thought this one is slower because the v8 switches to this mode > because the array is sparse. > Why the speaker says this one is more compact? > Dictionary mode allows a compact representation of arrays with sparse key sets. > 3. As the speaker gave an example, > var A = new Array(); > A[0] = 77; > A[1] = 88; > A[2] = 0.5; > A[3] = true; > This will cause the hidden class changes a couple of times which > cost resources. > > The better way suggested by v8 is: > var A= [77,88,0.5,true]; > He said this is a hint to v8. But could any one explain this more > specifically on how v8 take this single allocation? > When various object types are listed in the array literal, V8 can see right away what kind of stuff will be stored in the array. Also, for the record, it is generally advisable not to mix types of objects stored in an array, especially if some of them are numbers. Arrays containing only numbers enable some optimizations that make working with them faster. > > Some questions here. Thanks for your reply in advance. > > > > Regards- > > > -- > -- > v8-users mailing list > [email protected] > http://groups.google.com/group/v8-users > --- > You received this message because you are subscribed to the Google Groups > "v8-users" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected]. > For more options, visit https://groups.google.com/groups/opt_out. > > > -- -- v8-users mailing list [email protected] http://groups.google.com/group/v8-users --- You received this message because you are subscribed to the Google Groups "v8-users" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. For more options, visit https://groups.google.com/groups/opt_out.
