On Mar 20, 2013, at 12:34 PM, Kevin Reid wrote:
> On Wed, Mar 20, 2013 at 12:10 PM, Allen Wirfs-Brock <al...@wirfs-brock.com>
> wrote:
> On Mar 20, 2013, at 10:09 AM, Kevin Reid wrote:
>
> > Depending on what you mean by “ECMAScript code”, this may be false given
> > the Typed Arrays extension, which allows direct access to the bit-patterns
> > of float values (the Typed Arrays spec permits, but does not require,
> > replacing a NaN value with any other NaN value on read or write).
>
> This is not how it is specified in the ES6 spec. See 15.13.5.1.3 steps 7.b &
> 8.b and 15.13.5.1.4 steps 7.a & 8.a. Normalization of NaN values is required
> on retrieval and permitted on stores form/to ArrayBuffers.
>
> I see. I was reading the Khronos version and hadn't realized it was included
> in ES6.
>
> The ES spec. requirement (which isn't new to ES6) still applies. If they
> expose observably different NaN values to any ECMAScript code they aren't
> conforming to the spec.
>
> Then it seems to me that the wording of the spec, while not
> self-contradictory, makes it unnecessarily unobvious how to correctly
> implement it. Consider these two cases (which I think are exhaustive):
>
> 1. The implementation uses exactly one bit pattern for JS values which are
> NaN. In this case, normalization is required on reads and is a no-op for
> writes.
>
> 2. The implementation represents JS values which are NaN using arbitrary NaN
> bit patterns (and SameValue considers them all equal). In this case,
> normalization is unnecessary for reads and necessary for writes (else, as my
> example code shows, the difference is observable which contradicts 8.1.5).
>
> Thus, normalization on write is either a no-op or necessary, so should be
> mandatory, and normalization on read is unobservable in either case, so need
> not be mandatory.
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
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