[Python-Dev] Re: marshal / unmarshal
tim.peters at gmail.com
Mon Apr 11 17:27:43 CEST 2005
>> The 754 standard doesn't say anything about how the difference between
>> signaling and quiet NaNs is represented. So it's possible that a qNaN
>> on one box would "look like" an sNaN on a different box, and vice
>> versa. But since most people run with all FPU traps disabled, and
>> Python doesn't expose a way to read the FPU status flags, they
>> couldn't tell the difference.
> OK. Do you have any intuition as to whether 754 implementations
> actually *do* differ on this point?
Not anymore -- hasn't been part of my job, or a hobby, for over a
decade. There were differences a decade+ ago. All NaNs have all
exponent bits set, and at least one mantissa bit set, and every bit
pattern of that form represents a NaN. That's all the standard says.
The most popular way to distinguish quiet from signaling NaNs keyed
off the most-significant mantissa bit: set for a qNaN, clear for an
sNaN. It's possible that all 754 HW does that now.
There's at least still that Pentium hardware adds a third not-a-number
possibility: in addition to 754's quiet and signaling NaNs, it also
has "indeterminate" values. Here w/ native Windows Python 2.4 on a
>>> inf = 1e300 * 1e300
>>> inf - inf # indeterminate
>>> - _ # but the negation of IND is a quiet NaN
Do the same thing under Cygwin Python on the same box and it prints "NaN" twice.
Do people care about this? I don't know. It seems unlikely -- in
effect, IND just gives a special string name to a single one of the
many bit patterns that represent a quiet NaN. OTOH, Pentium hardware
still preserves this distinction, and MS library docs do too. IND
isn't part of the 754 standard (although, IIRC, it was part of a
pre-standard draft, which Intel implemented and is now stuck with).
>> Copying bytes works perfectly for all other cases (signed zeroes,
>> non-zero finites, infinities), because their representations are
>> wholly defined, although it's possible that a subnormal on one box
>> will be treated like a zero (with the same sign) on a
>> partially-conforming box.
> I'd find struggling to care about that pretty hard.
>>> The question, of course, is how to tell.
>> Store a few small doubles at module initialization time and stare at
> ./configure time, surely?
Unsure. Not all Python platforms _have_ "./configure time". Module
initialization code is harder to screw up for that reason (the code is
in an obvious place then, self-contained, and doesn't require any
relevant knowledge of any platform porter unless/until it breaks).
>> their bits. That's enough to settle whether a 754 format is in use,
>> and, if it is, whether it's big-endian or little-endian.
> Do you have a pointer to code that does this?
No. Pemberton's enquire.c contains enough code to do it. Given how
few distinct architectures still exist, it's probably enough to store
just double x = 1.5 and stare at it.
>>>  Exaggeration, I realize -- but how many non 754 systems are out
>>> there? How many will see Python 2.5?
>> No idea here. The existing pack routines strive to do a good job of
>> _creating_ an IEEE-754-format representation regardless of platform
>> representation. I assume that code would still be present, so
>> "oddball" platforms would be left no worse off than they are now.
> Well, yes, given the above. The text this footnote was attached to
> was asking if just assuming 754 float formats would inconvenience
I think I'm still missing your intent here. If you're asking whether
Python can blindly assume that 745 is in use, I'd say that's
undesirable but defensible if necessary.
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