Re: long double data type
Re: long double data type
- Subject: Re: long double data type
- From: Stephen Davis <email@hidden>
- Date: Tue, 15 Jun 2010 14:23:16 -0700
On Jun 15, 2010, at 2:13 PM, Luigi Castelli wrote:
> Thank you for your replies Ian and Stephen.
> This is exactly what I am after...
>
>> "On Intel macs, the type long double corresponds to IEEE-754 double
>> extended precision. A double extended number is represented in 80 bits,
>> and has a precision of 64 significant bits, roughly like 19 significant
>> decimal digits. 15 bits are used to encode the exponent, which gives an
>> exponent range from -16383 to 16384, inclusive."
>
> Yes, that's what I learnt too, however I have also come across information similar to what Ian mentioned.
>
>> There are no 80 bit floating point data types.
>
> However from what "man float" says, the long double data type defines exactly an 80 bit type.
>
> The two pieces of information seem to contradict each other.
The fact that long double is 80 bits is an implementation detail, not a predefined type. On PowerPC, long double = double unless you force it to be bigger in which case it is emulated in software with 2 doubles (simplistic statement!).
> I am trying to avoid software emulation for bigger types, if I can get away with native types.
>
> To summarize: if it is true that computations can be performed natively with a maximum accuracy of 80 bits (64 bit effective resolution),
>
> AND
>
> it is all IEE compliant because computations are performed in the SSE unit, I will go for it.
They are not IEEE-compliant b/c there is no 80-bit compliance definition. Single and double precision floats are compliant, long doubles are not.
stephen
> However I am still not sure that's the case...
>
> - Luigi
>
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> --- On Tue, 6/15/10, Stephen Davis <email@hidden> wrote:
>
>> From: Stephen Davis <email@hidden>
>> Subject: Re: long double data type
>> To: "Luigi Castelli" <email@hidden>
>> Cc: email@hidden
>> Date: Tuesday, June 15, 2010, 1:15 PM
>> On Jun 15, 2010, at 10:44 AM, Luigi
>> Castelli wrote:
>>
>>> Hi there,
>>>
>>> this is the second time that I get a similar
>> response:
>>>
>>>> Are you sure that your DSP approach is sound?
>>>>
>>>> If 64bits are not enough there is probably
>> something wrong
>>>> with your algorithm.
>>>
>>> I really don't want to sound arrogant because it is
>> not what I am about, however I ASSURE you that I performed
>> all the necessary tests, both mathematical and auditive to
>> corroborate my conclusion. My approach is sound, I just
>> don't have enough bits for the audio quality I am after. I
>> understand it might sound weird to hear that 64 bits are not
>> enough.
>>> I might have reached the upper limit of the current
>> technology, but doubting my approach is not helping. I would
>> ask advice on the approach if I wasn't sure it was the right
>> one.
>>>
>>> I would love to get back on track with the questions I
>> have asked.
>>> I apologize in advance if I offended somebody. It was
>> not my intention.
>>>
>>> Thank you for your understanding.
>>>
>>> - Luigi
>>
>> Tahome, personally I found your comment a bit rude and a
>> non-answer to the original question. To paraphrase the
>> old adage, "If you don't have anything useful to add, don't
>> say anything." Let's keep the responses in the helpful
>> category, not in the flame-bait category.
>>
>> From "man float":
>>
>> "On Intel macs, the type long double corresponds to
>> IEEE-754 double extended precision. A double extended
>> number is represented in 80 bits, and has a precision of 64
>> significant bits, roughly like 19 significant decimal
>> digits. 15 bits are used to encode the exponent, which
>> gives an exponent range from -16383 to 16384, inclusive."
>>
>> This is a direct mapping to the x86 (technically x87) FPU
>> hardware which has always been 80 bits. Note that, on
>> Mac OS X (intel) systems, single and double precision
>> floating point math is normally executed in the SSE unit,
>> not the x87 FPU.
>>
>> hth,
>> stephen
>>
>>>
>>>
>>> --- On Tue, 6/15/10, tahome izwah <email@hidden>
>> wrote:
>>>
>>>> From: tahome izwah <email@hidden>
>>>> Subject: Re: long double data type
>>>> To: email@hidden
>>>> Date: Tuesday, June 15, 2010, 8:33 AM
>>>> I know that I am not really
>>>> contributing, but...
>>>>
>>>>> (Yes, I did my tests and 64 bits of precision
>> just
>>>> doesn't cut it)
>>>>
>>>> Are you sure that your DSP approach is sound?
>>>>
>>>> If 64bits are not enough there is probably
>> something wrong
>>>> with your algorithm.
>>>>
>>>> Just my 2 centibits
>>>> --th
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>>>
>>>
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>
>
>
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