Re: IOS floating point performance
Re: IOS floating point performance
- Subject: Re: IOS floating point performance
- From: David Rowland <email@hidden>
- Date: Thu, 08 Aug 2013 13:45:39 -0700
Fascinating (Spock - Star Trek, 1967)
I added just this to the loop
for (int jj = 0; jj<100;++jj)
{
T += (float)jj/1000.00;
……..
The time for 100 iterations is 0.033 sec. about ten times slower than before. I guess some optimizations remain.
Still, that works out at 330 microseconds for one iteration, pretty good.
D
On Aug 8, 2013, at 1:14 PM, Thomas Wetmore <email@hidden> wrote:
> David,
>
> Why don't you increment T by a little bit in each iteration, say by jj/1000., to prove that no optimization is occurring? I would do it but I develop for Mac OSX only.
>
> Tom Wetmore
>
> On Aug 8, 2013, at 3:59 PM, David Rowland <email@hidden> wrote:
>
>> I ran it in Debug mode which should turn off most optimizations. I ran the loop 100 times and then 200 times. The latter took almost exactly twice the time as the former. The results are saved in instance variables of the C++ class this belongs to.
>>
>>
>> On Aug 8, 2013, at 12:06 PM, Sandy McGuffog <email@hidden> wrote:
>>
>>> Be careful using that code as a test; a good optimizing compiler could pick up that sin is a library function without side effects, and no result is saved, and optimize that loop to two calls to adjustValueRadians.
>>>
>>> Sandy
>>>
>>> On Aug 8, 2013, at 8:17 PM, Thomas Wetmore <email@hidden> wrote:
>>>
>>>> David,
>>>>
>>>> Those are lightening speeds. So I agree with you wholeheartedly -- there is no sense in working on a custom table-driven approach. The current approach must already be table-based with speeds like that.
>>>>
>>>> Tom Wetmore
>>>>
>>>>
>>>> On Aug 8, 2013, at 1:26 PM, David Rowland <email@hidden> wrote:
>>>>
>>>>> I wrote an app that calculates the positions of Sun and Moon and other information as well. The heart of the Moon calculation is this. I added a loop around it and called a stopwatch at the beginning and end.
>>>>>
>>>>> startTime();
>>>>> for (int jj = 0; jj<100;++jj)
>>>>> {
>>>>> //in radians
>>>>> double lambda = 3.81040282295402 + 8399.70910754626 * T
>>>>> + 0.109781209950443 * sin(adjustValueRadians(2.35619449019234 + 8328.69146829639 * T))
>>>>> - 0.022165681500328 * sin(adjustValueRadians(4.5256387504213 - 7214.06294691607 * T))
>>>>>
>>>>> One hundred times through this loop, on my iPhone 5, took about 0.0028 seconds. Two hundred times took about 0.0056 sec.
>>>>>
>>>>> I infer that one pass takes about 0.000028 seconds, or 28.0 microseconds.
>>>>>
>>>>> The functions are probably very carefully written and could not be improved by table lookups, vector libraries, etc. That is barking up the wrong tree.
>>>>>
>>>>> David
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