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Topic: integer Vs floating point efficiency?
Replies: 13   Last Post: Dec 5, 2012 8:41 PM

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Gordon Sande

Posts: 119
Registered: 5/13/10
Re: integer Vs floating point efficiency?
Posted: Dec 3, 2012 9:33 AM
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On 2012-12-03 06:03:24 -0400, Rui Maciel said:

> Gordon Sande wrote:
>

>> On 2012-12-01 08:41:21 -0400, Rui Maciel said:
>>
>> Instruction timings etc are to be found in the processor's data sheets.
>> Ask the manufacturer of one. The web sites will have copies or request
>> pages.

>
> I've followed your advice, and I'm in the process of digesting AMD and
> Intel's docs that cover this subject. If anyone is interested, the
> documents are available at:
>
> http://support.amd.com/us/psearch/Pages/psearch.aspx?type=2.1%3b2.12&product=2.1.2&contentType=Tech%20Doc%20Processor
>
> http://www.intel.com/content/www/us/en/processors/architectures-software-
> developer-manuals.html
>
>
> Unfortunately, the information isn't compiled or easily accessible. To make
> matters worse, I've stumbled on a claim that Intel's docs have been
> "incomplete and full of errors", which isn't very reassuring.
>
> Nevertheless, some info on this subject has been compiled by third parties,
> such as the following article:
>
> http://www.agner.org/optimize/instruction_tables.pdf
>
>
> As should be expected, latency varies with the make and model of the
> processor, and instruction latency alone doesn't say everything there is to
> say (or, possibly, anything) about the inherent efficiency of integer Vs
> floating point data types. In addition, there are more than a way to skin
> some numerical cats with the AMD64 platform, which adds to the complexity of
> this issue.
>
> Nevertheless, from what I gathered so far it appears that, in general,
> integer operations do present a small, but possibly entirely meaningless,
> efficiency advantage.
>
>
> Rui Maciel


Modern processors have the joys of pipelines so timing depends on what
happened before
as well as the joys of hierarchical memories. Sometimes the processor
times are completely
swamped by memory costs with extreme cases of requiring extra
processing to reduce
memory traffic. Early lierature was for simple machines with uniform
memory faster than
the processor so is completley misleading for current technology of
processors faster than
memory amd memory of varying speed.

Micro optimization is rarely of great importance as the effects of
large scale algorithm
issues dominate in virtually all situations. If you had one of the
situations where
instruction timing was an issue you probaly would not have asked the
question. Is the old
story of the price of yachts. If you have to ask then you probably can
not afford one!







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