Experimental Physics and
Industrial Control System

"Jeff Hill" <[email protected]> · Fri, 17 Jun 2005 18:16:21 -0600

Same answer.

This isn't so much of a CPU efficiency issue as a programming efficiency
issue. If it takes time to write the code to do the check it's less likely
to be checked and strange and mysterious bugs are more likely to find a
place to roost.

> -----Original Message-----
> From: Eric Norum [mailto:[email protected]]
> Sent: Friday, June 17, 2005 3:32 PM
> To: Benjamin Franksen
> Cc: [email protected]
> Subject: Re: Fundamental Types document / unsigned integers
> 
> 
> On Jun 17, 2005, at 4:21 PM, Benjamin Franksen wrote:
> > Regarding Jeff's argument as for the advantages of programming with
> > numbers that are garanteed to be non-negative. I would argue that this
> > is also mostly valid in C/C++, where arrays are always indexed starting
> > with zero. Many languages allow upper /and/ lower index bounds to be
> > arbitrary (signed) integers, or even any other data type, provided the
> > programmer can specify a one-to-one mapping onto a bounded interval of
> > integers. Thus, non-negativity seems to be a somewhat arbitrary
> > guarantee (why not, for instance, strict positivity?). Furthermore,
> > with regard to the efficiency question (only one range-check for upper
> > bound instead of two for upper and lower), in  C/C++ you are always
> > free to apply the zero-cost type cast from signed to unsigned, thereby
> > mapping negative numbers to large positive ones, and then range-test
> > only for the upper bound. This will fail for /exactly/ the cases where
> > the original check failed, as long as you don't rely on the upper half
> > of the possible range, something you (Jeff) suggested is to be avoided
> > anyway.
> 
> The inefficiency is only in typing a few extra characters on your
> keyboard.   Any modern optimizing compiler is going to optimize
> 	if ((i < 0) || (i >= j))
> (where i and j are signed integer values) into a single test without
> you needing to help it by performing the cast.
> 
> --
> Eric Norum <[email protected]>
> Advanced Photon Source
> Argonne National Laboratory
> (630) 252-4793

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System