Oh, thank you!
I had thought that the difference between x87 and SSE, and the different
internal precisions, mattered only for intermediate register values during
calculation - and that when a 32-bit float was "at rest", i.e. being stored in
32 bits, it would always have IEEE 754 representation (as described e.g. at
https://en.wikipedia.org/wiki/Single-precision_floating-point_format#IEEE_754_single-precision_binary_floating-point_format:_binary32)
Is this not right? Do I have the wrong mental model here?
The current behaviour does indeed seem a bit off (Poly/ML rev 62a56474f0,
64-bit Linux).
If I take, say, 1.0 and convert it to bytes big-endian, I think I expect to see
sign bit: 0
exponent (8 bits): 127 (exponent is 0, stored unsigned with an offset of 127)
fraction (23 bits): 0 (as the 1 in 1.0 x 2^0 is implicit)
so 0111 1111 1000 0000 0000 0000 0000 0000 or 3F 80 00 00
and that's what PackReal32Big.toBytes 1.0 returns in MLton. In Poly/ML I'm
getting this
> PackReal32Big.toBytes 1.0;
val it = fromList[0wx1F, 0wxC0, 0wx0, 0wx0]: Word8Vector.vector
1F C0 00 00 is the same bit pattern as 3F 80 00 00, but shifted right by one
bit, and no longer a normal IEEE 754 number I think. Is it possible there's an
off-by-one error in the bit lookup, or is this all a symptom of my having the
wrong idea about what's going on?
Thanks,
Chris
On Sat, 6 Feb 2021, at 16:24, David Matthews wrote:
> I've added this to master. It seemed like a good idea although it was a
> bit more complicated than PackReal because Real32.real values are
> "boxed" in 32-bit Poly/ML but tagged in 64-bit.
>
> I'm not exactly clear how useful PackRealN operations are for general
> data interchange. Currently they just store and load the bytes that
> make up the number but how those are interpreted will depend on the
> platform. For example it seems that the X87 format used on X86/32 is
> different from the SSE format used on X86/64.
>
> David
>
> On 02/02/2021 09:25, Chris Cannam wrote:
> > Hello! I find I could do with the PackReal32{Big,Little} structures, 32-bit
> > floats being often more amenable to serialisation and used in some storage
> > formats.
> >
> > Would there be any appetite for adding these?
> >
> > Thanks,
> >
> >
> > Chris
> > _______________________________________________
> > polyml mailing list
> > [email protected]
> > http://lists.inf.ed.ac.uk/mailman/listinfo/polyml
> >
>
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