Re: [eigen] Checking for integer overflow in size computations

[Christoph Hertzberg <chtz@xxxxxxxxxxxxxxxxxxxxxxxx>]

On 17.10.2011 00:36, Benoit Jacob wrote:
> 2011/10/16 Christoph Hertzberg<chtz@xxxxxxxxxxxxxxxxxxxxxxxx>:
> > On 16.10.2011 22:21, Benoit Jacob wrote:
> > > Note that there is a small performance overhead associated with that.
> > > I tried to minimize it, but there is still 1 integer division in the
> > > size=rows*cols overflow check. I hope that's still negligible compared
> > > to the cost of malloc.
> >
> > I have not looked into your patch yet, but wouldn't a multiplication in
> > int64/int128 also do the job?
>
> I don't know that all platforms have an integer type twice bigger than size_t?
>
> On x86-64 are there 128 bit integers? I didn't know that.
> Also I'm afraid there might exist 32bit platforms without 64bit integers.

Well, naturally I can't say that for every platform, but for x86-64 it 
seems to be possible:
http://stackoverflow.com/questions/1541426/computing-high-64-bits-of-a-64x64-int-product-in-c

If someone really cared, one could implement a backup solution using 
grid multiplication (where most parts can be ignored, and the case that 
both high parts are !=0 is trivial)

Christoph



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Dipl.-Inf. Christoph Hertzberg
Cartesium 0.051
Universität Bremen
Enrique-Schmidt-Straße 5
28359 Bremen

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