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Re: [Public WebGL] NaN handling in Typed Array spec



Kenneth Russell:
> A bug was recently filed against WebKit's implementation of Typed
> Arrays (https://bugs.webkit.org/show_bug.cgi?id=53598). The basic
> issue is that the Web IDL specification defines the bit pattern
> for the not-a-number (NaN) value. Ordinarily, it is not possible
> for ECMAScript programs to examine this bit pattern, but with the
> introduction of the Typed Array specification, it is possible to use
> a Float32Array to store NaN and then read back the bytes using, for
> example, a Uint8Array.

The IDL float type is exactly an IEEE 754 single.  When a JS Number
value is converted into an IDL float (by assigning to a property
corresponding to an IDL attribtue of type float, or by passing as an
argument it to a method whose corresponding IDL operation argument is of
type float), then a particular bit pattern is chosen for that IDL float
value, yes.

If it is defined that a Float32Array stores in the array exactly
whatever value comes out of the “convert a JS value to an IDL float
value” algorithm (which is what you would get without any special
wording, if you just refer to the ‘float value’ argument of
Flaot32Array::set()), and if a Uint8Array can be overlaid on that memory
to read the bit pattern, then that’s the bit pattern you should get.

If you want to define Float32Array such that assigning the JS NaN value
to one of its array index properties instead stores a different NaN
bit pattern in the array, that’s fine, but you just need to define that.
The IDL float value that you get from converting the JS NaN need not be
the same value that gets stored in the array – the value that is stored
is whatever the definition of the Float32Array::set() operation says is
stored.

> Some ECMAScript engines use multiple representations for NaN
> internally, and forcing them to be canonicalized into a single bit
> pattern would impose a significant performance penalty on all stores
> into Float32Arrays. It is absolutely essential for WebGL programs that
> loads from and stores into Float32Arrays remain as performant as
> possible.
> 
> I would like to add a small, normative section to the Typed Array
> specification indicating that the bit pattern for NaN values stored
> using Float32Array, Float64Array and DataView is not specified, and
> that implementations may utilize any of the legal NaN bit patterns
> defined by the IEEE-754 specification. I do not believe that doing so
> would introduce any significant ambiguity into the spec; this is a
> small corner case.

A requirement like that wouldn’t violate Web IDL, so it’s OK from that
perspective.

-- 
Cameron McCormack ≝ http://mcc.id.au/
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