From: "Simon Kröger" <SimonKroeger@...>
Date: 2006-01-06T06:17:56+09:00
Subject: Re: [SUMMARY] Numeric Maze (#60)

Lou Vanek wrote:

> The shift operator comes in handy and can save a lot of time.
> These are equivalent operations:
> 
>  >time ruby -e '1_000_000.times { ||  2**31 }'
> 
> real    0m33.225s
> user    0m32.452s
> sys     0m0.015s
>  >time ruby -e '1_000_000.times { ||  1<<31 }'
> 
> real    0m3.469s
> user    0m3.421s
> sys     0m0.000s
>  >time ruby -e '1_000_000.times { || 
> 2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2 }'
> 
> real    0m13.839s
> user    0m13.656s
> sys     0m0.015s
> 
> 
> Kero wrote:

This is an implementation detail,

the ** operator of Fixnum calls rb_big_pow converting self to a Bignum 
regardless of the value. '<<' checks the size of the result and uses 
real bitshifting if possible.

While this was interresting to me it's not the explanation of the effect 
above because the value 2**31 is always a bignum, but the implementation 
of the ** operator in Bignum isn't very fast (while shifting the 
internal representation of the bignum is faster)

To prove my point that this isn't realy because bitshifts are faster try 
this:

1_000_000.times { ||  2.0**30 }

At least on my machine this is faster than any of your three examples 
above. (if you choose smaller values '<<' will be the fastest because it 
doesn't convert to bignums as stated above)

cheers

Simon