Thoughts on IEEE P754

Sam Ruby rubys at intertwingly.net
Fri Aug 22 16:26:59 PDT 2008


On Fri, Aug 22, 2008 at 7:07 PM, Waldemar Horwat <waldemar at google.com> wrote:
> Sam Ruby wrote:
>> I find it easier to talk about real examples than abstractions.  I've
>> done the following quickly, so forgive me if I get some detail wrong.
>>
>> A binary floating point number has 52 bits of fraction, and by
>> assuming an implicit leading one, they get an additional bit.  This
>> means that 1.1 is stored as (for brevity, I'll use hex)
>>
>> [1].1999999999999
>>
>> A conversion of that to decimal128 would be equivalent to computing
>>
>> 4953959590107545m / 4503599627370496m
>>
>> Which would produce
>>
>> 1.099999999999999866773237044981215
>
> That's incorrect.  The correct answer is 1.100000000000000088817841970012523, which is closer mathematically to 1.1.

Apparently, I rounded incorrectly.  Rounding the first number
correctly produces:

1.199999999999A
4953959590107546m / 4503599627370496m
1.100000000000000088817841970012523

>> Repeating that for 1.2 produces
>>
>> 0x13333333333333L
>> 5404319552844595m / 4503599627370496m
>> 1.199999999999999955591079014993738
>
> This one is correct.

One ramification of requiring up-conversion to decimal128 of mixed
mode operations would mean that the following would be true

  (1.1 < 1.1m) && (1.2m < 1.2)

>    Waldemar
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