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

[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 guess that would be much cheaper than a 
division. Anyways I would agree that the overhead should be negligible. 
If someone really cares for that overhead, we might introduce a flag 
such as EIGEN_DONT_CHECK_INTEGER_OVERFLOW.

> Like the standard behavior of operator new and of our own aligned_new
> routines, these checks throw std::bad_alloc on error and don't do
> anything else. In particular, we still don't guarantee that
> aligned_new returns null on error. It feels weird to me to rely
> entirely on exceptions to report these errors, but this is how c++
> works (as long as one doesn't use nothrow-new) and what we've been
> doing. Is this OK?

I don't want to start a philosophical discussion about exceptions vs 
return-type checking, but honestly I guess that the C++ way is much more 
reliable, since about 99% of malloc users hardly ever check for NULL 
pointers ("Come on, how likely is it that I can't get ** MB?").
Furthermore, I guess hardly anyone disables exceptions in C++, unless he 
really knows what he's doing.

Christoph

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Dipl.-Inf. Christoph Hertzberg
Cartesium 0.051
Universität Bremen
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