Comment #51 on issue 2599 by [email protected]: Implement "use asm"
http://code.google.com/p/v8/issues/detail?id=2599
I think that the key of asm.js is that it enforces static types. Not only
in the javascript "Number" but low level int's and floats with the enforced
size. This means that theoretically code should never have to
be "de-optimized" as it should always use the same types.
Now, AFAIK this is already a benefit as once compiled that compiled routine
will be good for the lifetime of the program. However, by taking into
account the knowledge that the code is asm.js you can take more time
optimizing as you know it will be useful, and you can likely optimize
earlier because you don't need to take time to track what types are used
(you know the types before the code runs once).
I believe that asm.js also ensures that all functions called are static
(except for the FFI) so that inlining is easier as you don't have to worry
about the inlined function changing.
So some things to take away is that type guarantees are pretty much "free"
in that code doesn't get deoptimized but in "use asm" blocks you can make
stricter assumptions about types more safely.
The function staticness is also automatically beneficial for the same
reason.
So v8 already benefits quite a bit from asm.js and the final tweaks would
be minor (they shouldn't change inner loops). One of the largest benefits
would be removing the deoptimization checks, but that is very dangerous
because if it can't be guaranteed that the assumptions are always valid you
could possibly get undefined behaviour.
So we get back to the general agreement that this bug is more about
emitting the proper warnings, rather than treating asm.js as special by the
jit (and whatever other components).
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