Hacker News new | ask | show | jobs
AS3 Bitwise gems - fast integer math (lab.polygonal.de)
10 points by RWilson 6241 days ago
2 comments
mahmud 6241 days ago
Interested parties can also look at how the Tamarin VM implements those bitwise opcodes natively.

http://www.mozilla.org/projects/tamarin/

Flash opcodes into Nanojit IR:

http://hg.mozilla.org/tamarin-central/raw-file/4eb9f961a087/...

Nanojit IR to native x86 translation tables?

http://hg.mozilla.org/tamarin-central/raw-file/4eb9f961a087/...

link
RWilson 6241 days ago
Nice! That's great to hear, since it's always perplexing when native code ISN'T the fastest... Kind of makes you have to ask, "If there is a better way, why isn't that the native way?" But, for Tamarin, that question will be moot.
link
pmjordan 6241 days ago
I don't do flash, but I was under the impression that there's some kind of compilation going on and ActionScript isn't just interpreted. How on earth does a modern compiler not optimise

  a / 64
to

  a >> 6
? I could understand it in a dynamically typed language, where a, and therefore the result, could be non-integer, but ActionScript seems to have static type declarations for variables.
link
_cbsz 6241 days ago
This isn't even "modern" in compiler terms, it's a simple peephole optimization. (grep through the IR, check for "X / 2^n", replace by "X >> n") To get it right when you have

    int a;
    int b = 64;
    a / b
requires dataflow analysis so you can do constant propagation, copy propagation, and dead code elimination. This is more work to get right, but that's not a very good excuse for not at least doing the peephole optimization.
link