Re: [R] x*x*x*... vs x^n

From: Prof Brian Ripley <ripley_at_stats.ox.ac.uk>
Date: Wed 29 Jun 2005 - 23:39:08 EST

On Wed, 29 Jun 2005, Duncan Murdoch wrote:

> On 6/29/2005 7:32 AM, Robin Hankin wrote:

>> I have been wondering if there one can speed up calculating small powers
>> of numbers such as x^8 using multiplication.
>>
>> In addition, one can be a bit clever and calculate x^8 using only 3
>> multiplies.
>>
>> look at this:
>>
>>
>> > f1 <- function(x){x*x*x*x*x*x*x*x}
>> > f2 <- function(x){x^8}
>> > f3 <- function(x){x2 <- x*x;x4 <- x2*x2;return(x4*x4)}
>>
>> [so f1() and f2() and f3() are algebraically identical]
>>
>>
>> > a <- rnorm(1000000)
>> > system.time(ignore <- f1(a))
>> [1] 0.50 0.17 2.88 0.00 0.00
>>
>> > system.time(ignore <- f2(a))
>> [1] 0.31 0.03 1.40 0.00 0.00
>>
>> > system.time(ignore <- f3(a))
>> [1] 0.10 0.07 0.18 0.00 0.00
>>
>>
>> [these figures show little variance from trial to trial]
>>
>>
>> I was expecting f2() and f3() to be about the same.
>> I was not expecting a factor of 3 there!
>>
>> anyone got any comments?
>
> If you look in src/main/arithmetic.c, you'll see that R does a general
> real-valued power (using C's pow() function) whenever either one of the
> args is real (except for a few special cases, e.g. non-numbers, or
> powers of 2 or 0.5). There is an internal R function equivalent to your
> f3, but it is not used in the situation of real^integer (and in any
> case, x^8 is real^real).
>
> I suppose if you wanted to submit a patch someone would take a look, but
> the question is whether there is really any calculation whose execution
> time would be materially affected by this. Most computations are not
> dominated by integer power calculations, so is this really worth the
> trouble?

As Luke Tierney frequently points out, selecting special cases can take more time than you save. The assembler code used by modern OSes will have worked out that compromise pretty effectively: even for real^real it spots simple cases of the exponent.

Also, it depends on the CPU: I get on Athlon 2600

> system.time(ignore <- f1(a), gcFirst=T)
[1] 0.08 0.05 0.14 0.00 0.00
> system.time(ignore <- f2(a), gcFirst=T)
[1] 0.20 0.01 0.20 0.00 0.00
> system.time(ignore <- f3(a), gcFirst=T)
[1] 0.03 0.02 0.05 0.00 0.00

and Opteron 248

> system.time(ignore <- f1(a), gcFirst=T)
[1] 0.08 0.06 0.14 0.00 0.00
> system.time(ignore <- f2(a), gcFirst=T)
[1] 0.19 0.01 0.20 0.00 0.00
> system.time(ignore <- f3(a), gcFirst=T)
[1] 0.04 0.01 0.05 0.00 0.00

Note

  1. the use of gcFirst=T
  2. these need to be run several times to tune the gc() behaviour. After which f1(a) caused a couple of gc()s and the others none.
  3. the Opteron is in general a much faster machine so these are all surprisingly slow.

Occasionally the C-level pow() is slow: that happened in one MinGW update and for R Windows users we replaced it. Even there I was unsure that it would make enough difference on a real problem, but eventually found one where it did (MDS with Minkowski distances). It was because of that real problem that I added the special case for 0.5, after careful timing (and hearing Luke's comment alluded to above).

-- 
Brian D. Ripley,                  ripley@stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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Received on Wed Jun 29 23:44:54 2005

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