On Mon, Jan 19, 2015 at 6:21 AM, Craig Latta craig@netjam.org wrote:

...

As I understand it, running under NaCl requires reworking the JIT and has real problems doing the self-modifying code involved in inline caches, etc.

 Yep, you rebuild your app to use the NaCl instruction set, and it

translates to physical instructions on demand. The "safety" constraints it places on the code you generate are substantial. If you were even allowed to run the code you want, the performance goal would be demolished.

"Demolished" is probably a too strong word; The Mono JIT they ported to NaCl only slowed down something like 7%. (Not going through the PNaCl layer but just x86 nacl code.) So that is not that bad... However,

 But worse, you've also demolished the "run anywhere" goal from the

outset, by limiting yourself to browsers that support NaCl. And I don't see how to meet this goal with any browser-plugin approach. There just isn't a widely-supported mechanism anymore, after the death of the Netscape-style plugin.

Yes, this is a real issue. In that regard, asm.js has a better chance, but still trying to stay on the JavaScript level, and taking the advantage of JavaScript's JIT is a practical solution, even though it may not give us the real Cog performance.