Observability of NaN distinctions — is this a concern?

Brandon Benvie bbenvie at mozilla.com
Wed Mar 20 13:15:56 PDT 2013

On 3/20/2013 12:56 PM, Allen Wirfs-Brock wrote:
> If you're specifically talking about reading/writing TypedArray 
> elements (really ArrayBuffers) you have to take into account the 
> possibility that you can have different types overlaying the same 
> buffer storage.  Hence a NaN bit pattern might be written as 2 Uint32 
> values and then retrieved as a Float64 value.  In that case, there is 
> no Float64 write to perform the normalization so it must be done on 
> all reads.  Such normalization is especially important if object 
> pointers are represented using  NaN-boxing.
> Allen
Exactly, you can't control it on the write end. Writing two uint32's is 
precisely how you do NaN-tagging using typed arrays (since you can't 
write an uint64). The way to normalize is to coerce to a single 
canonical NaN on read, if anything.

(an example of NaN-tagging using typed arrays: 

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