[Vm-dev] float word order

[David T. Lewis]

On Sun, Apr 19, 2009 at 11:08:14AM -0700, John M McIntosh wrote:
> 
> On 19-Apr-09, at 8:37 AM, David T. Lewis wrote:
> 
> >>The values in a FloatArray are 32-bit floats, packed into 64-bit slots
> >in the object memory. There are no endian issues to worry about. On both
> >32-bit and 64-bit object memories, the values are arranged in the order
> >of an (int *) access. In other words, they are arrays of 32-bit values
> >that just happen to be stuffed onto slots that the object memory thinks
> >are 64-bit words.
> 
> Well that's not quite true, you have to be careful here because might  
> people move data in and out of
> the FloatArray, but let's see..
> 
> 
> MatrixTransform2x3>>at: index put: value
> 	<primitive: 'primitiveAtPut' module: 'FloatArrayPlugin'>
> 	value isFloat
> 		ifTrue:[self basicAt: index put: value asIEEE32BitWord]
> 		ifFalse:[self at: index put: value asFloat].
> 	^value
> 
> 
> CGPoint>>x: aValue
> 	self unsignedLongAt: 1 put:  aValue asFloat asIEEE32BitWord  
> bigEndian: SmalltalkImage current  isBigEndian.

As near as I can tell all accesses to FloatArray and IntegerArray are
on 32 bit boundaries for both 32-bit and 64-bit images, and are not
impacted by host endianness.

I should mention that I have not tried FloatArrayPlugin on 64-bit
images; I should probably have a look at that one of these days.

> Ok, well the reverseBytesInImage logic I'll assume without looking is  
> swapping the bytes in the FloatArray at load time so that accessors
> use SmalltalkImage current isBigEndian to move data in/out in the  
> proper form.

Yes, the bytes in a FloatArray would be swapped at load time if moving
from one endianness to another, but no I don't think that #isBigEndian
is required for accessing the ints or floats on 32 bit boundaries.

Also, a 64-bit image containing FloatArray or IntegerArray instances
should be correctly byte swapped when moved from one endianness to
another, although I have never actually tried it so I can't say for
sure.

Bottom line: This stuff pretty much just works, no special cases to
worry about.

Dave