3 ms·

> It can’t allocate normal JavaScript objects or access normal JavaScript properties. No strings. Only arithmetic and typed arrays (a single one actually with m

by aeflash 14y ago

> It can’t allocate normal JavaScript objects or access normal JavaScript properties. No strings. Only arithmetic and typed arrays (a single one actually with multiple views). I think he is missing a core concept here. You can create and modify strings, just at a lower level. An array of ints can be used as a char array, which in turn can be used as a string. You just have to interface with it at a lower level, probably implementing your own C-style strlen/strtok functions. It will be more complicated, especially if you need to accommodate unicode, but it is doable. Granted, there is no advantage to abandoning the built-in string manipulation functions except speed, but you would only use asm.js when speed absolutely matters. At a more fundamental level, when you have access to memory through a stack and/or heap, and can create arrays of ints with which you can do normal arithmetic and load/store operations, you really can do anything. After all, that is the entirety of what actual assembler code has to work with. You just have to make your own abstractions.

mraleph 14y ago
When I say "no strings" I mean JavaScript built-in primitive string values. I will amend the paragraph to make it explicit. It was added because I talked to people who think that just adding "use asm" on top of their node.js module will make it go as fast as C++ without realizing that asm.js is a very limited subset with its own rules. You can surely represent a JS string as a Uint16Array or Uint8Array, but copying things in and out of emulated heap into real heap will cause penalty. You will also have to reimplement a lot of functionality that host JS engine provides. Trying to implement say a regexp engine in asm.js that would be as performant as V8's built-in on would be a challenge. Yes, you can do anything. I don't dispute that. But you have to do it manually and sometimes you are replicating functionality that already exists in the host engine e.g. you might implement GC, but it will less efficient than builtin one.

by aeflash 14y ago

mraleph 14y ago

When I say "no strings" I mean JavaScript built-in primitive string values. I will amend the paragraph to make it explicit. It was added because I talked to people who think that just adding "use asm" on top of their node.js module will make it go as fast as C++ without realizing that asm.js is a very limited subset with its own rules. You can surely represent a JS string as a Uint16Array or Uint8Array, but copying things in and out of emulated heap into real heap will cause penalty. You will also have to reimplement a lot of functionality that host JS engine provides. Trying to implement say a regexp engine in asm.js that would be as performant as V8's built-in on would be a challenge. Yes, you can do anything. I don't dispute that. But you have to do it manually and sometimes you are replicating functionality that already exists in the host engine e.g. you might implement GC, but it will less efficient than builtin one.