1271646 - NaN value canonicalized inconsistently

Status: RESOLVED DUPLICATE of bug 1140152

(Core :: JavaScript Engine, defect)

Description

[Mike Pennisi [:jugglinmike]]

Version: 3dc1902afe42afd9c94390d130995abe95480b05 (Thu May 5 17:45:58 2016 +0100)
OS: Ubuntu Linux
Architecture: x64

    var myNaN = -NaN;
    var source = new Float32Array([myNaN]);
    var sliced = source.slice();
    var sourceBytes = new Uint8Array(source.buffer);
    var slicedBytes = new Uint8Array(sliced.buffer);

    print(sourceBytes.join(' '));
    print(slicedBytes.join(' '));

Expected: the same bit pattern, printed twice
Actual: two distinct bit patterns

    0 0 192 255
    0 0 192 127

This behavior is observable using Float32Array and Float64Array. It seems to be
limited to the `%TypedArray%.prototype.slice` method--the expected behavior
occurs when Set (for example) is used instead, as in:

    subject[0] = myNaN;

ES2015 24.1.1.6 SetValueInBuffer

> If value is NaN, rawValue may be set to any implementation chosen IEEE
> 754-2008 binary64 format Not-a-Number encoding. An implementation must always
> choose the same encoding for each implementation distinguishable NaN value.

http://www.ecma-international.org/ecma-262/6.0/#sec-setvalueinbuffer

I discovered this while writing tests for Test262--SpiderMonkey is currently
failing one of the following tests currently under review:

https://github.com/tc39/test262/pull/623

Comment 1

[Lars T Hansen [:lth]]

I don't think this is a bug.

slice() is required to canonicalize because it performs a sequence of GetValueFromBuffer and SetValueInBuffer; the former method *must* canonicalize.  But SetValueInBuffer is not required to canonicalize - it can just store the bit pattern (the wording is "may").  So source.buffer contains -NaN because the value was not canonicalized and sliced.buffer contains NaN, the canonical representation.

Comment 2

[Daniel Ehrenberg]

See step 15 of https://tc39.github.io/ecma262/#sec-%typedarray%.prototype.slice . If the type of the source and destination are the same, implementations are required to copy byte-by-byte, rather than through Get/Set.