Confusing math problem

Chris Angelico rosuav at gmail.com
Fri Feb 22 00:15:17 CET 2013


On Fri, Feb 22, 2013 at 9:33 AM, Dave Angel <davea at davea.name> wrote:
> On 02/21/2013 05:11 PM, Chris Angelico wrote:
>>
>>
>>>  <snip>
>>
>>
>> Note how, in each case, calculating three powers that have the same
>> real-number result gives a one-element set. Three to the sixtieth
>> power can't be perfectly rendered with a 53-bit mantissa, but it's
>> rendered the same way whichever route is used to calculate it.
>>
>
> But you don't know how the floating point math library (note, it's the
> machine's C-library, not Python's that used) actually calculates that.
>
> For example, if they were to calculate 2**64 by squaring the number 6 times,
> that's likely to give a different answer than multiplying by 2 63 times.
> And you don't know how the library does it.  For any integer power up to
> 128, you can do a combination of square and multiply so that the total
> operations are never more than 13, more or less.  But if you then figure a =
> a*a  and b = b/2, and do the same optimization, you might not do them
> exactly in the same order, and therefore might not get exactly the same
> answer.
>
> Even if it's being done in the coprocessor inside the Pentium, we don't have
> a documented algorithm for it.  Professor Kahn helped with the 8087, but I
> know they've tweaked their algorithms over the years (as well as repairing
> bugs).  So it might not be a difference between Python versions, nor between
> OS's, but between processor chips.

I was under the impression that, on most modern FPUs, calculations
were done inside the FPU with more precision than the 53-bit that gets
stored. But in any case, I'd find it _extremely_ surprising if the
calculation actually resulted in something that wasn't one of the two
nearest possible representable values to the correct result. And I'd
call it a CPU/FPU bug.

Of course, as we know, Intel's *never* had an FPU bug before...

ChrisA



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