Kevin Gadd wrote:
Fancy behavior for out of range indices on Typed Arrays seems like it
could be more trouble than it's worth. Ideally, you want something
that can be cheaply implemented on native targets like x86, if not
implemented for free because it's something the runtime has to do
anyway. Returning undefined seems like it would definitely imply a
performance penalty, or at least make work a lot harder for type
inference/analysis engines, because now you have to prove that all
indices are in range.
We already handle this in TI in SpiderMonkey, and in asm.js AOT
compilation in OdinMonkey. True, we didn't want to change Typed Arrays
incompatibly, so we made the best of it, but IIRC it wasn't a big deal.
Mainly, we have to face the problem of backward compatibility. If we
could make all int arrays return 0 not undefined, ditto for float arrays
viz. NaN, that would be even better for performance.
Clamping is reasonable
No, it makes all negative indexes alias 0. This is bad for SFI-enforcing
compilers (Emscripten, Mandreel, PNaCl).
, but not necessarily what people might expect. Throwing at least does
not suffer from the type information problem that undefined does,
Right, and (Allen points out this may have been missed by the Khronos
editors) typed arrays based on WebIDL index getter/setters require
throwing. No browser impl that I can test does this, though.
though I'm sure it still poses issues for JITs - same 'prove all
indices are in range' problem to eliminate the bounds check. I could
see wrapping being an acceptable choice as well, since that's sort of
'native' semantics.
No, again -- unsafe.
Firefox appears to return undefined for out of range typed array
elements; can someone comment on whether Spidermonkey is easily able
to achieve this and whether it hurts the JIT and/or gathering of type
information? Would some other behavior be faster? Chrome appears to do
this too currently.
Safari too. Differences abound: using Math.pow(2,32) or above as an
index makes an expando in V8 and JSC. JSC allows named expandos (a.foo =
42). Proto-indexed properties shine through for out of bounds indexes.
Working to nail these down to be as efficient and safe as possible...
People really want typed arrays to be fast, so consider that the
context for my comments here. :)
Definitely.
/be
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