Manuel,
There is no "undefined behavior" here. K&R defined it
very clearly: Arrays are pointers, and square brackets are
syntactic sugar for pointer arithmetic and dereferencing:
cam_xyz[i][j] vs. *(cam_xyz + (i)*3 + (j))
Dave Coffin 2/27/2015
On Fri, Feb 27, 2015 at 07:30:31PM +0100, Manuel López-Ibáñez wrote:
> On 27 February 2015 at 19:12, Manuel López-Ibáñez <lopezibanez@xxxxxxxxx> wrote:
> >> This sounds like a compiler bug to me. A statement
> >> of the form:
> >>
> >> for (i=0; i < 12; i++) {
> >> <code that doesn't change "i">
> >> }
> >>
> >> should never generate an infinite loop.
> >
> > If there is undefined behavior, then not only can generate an infinite
> > loop but many kinds of nasty effects. The reasons are difficult to
> > explain, but you just have to think that at some moment the code has
> > to be converted to something like:
> >
> > if(x) goto Y
> > else goto Z
> >
> > and some optimization may decide that x is always false or always true
> > (because of something that goes inside the for-loop that cannot happen
> > ever has started a possibly long chain of transformations that
> > ultimately leads to that conclusion) Voilá: infinite loop.
>
> And the above should also explain why this kind of arguments:
>
> * It was working before
> * It only happens with a particular optimization
> * It only happens with a particular version of GCC
> * It works in every machine and every compiler I have tested it
> * "I ran my last full test (over 2000 raw photos) with
> GCC 4.9.1" and it doesn't happen
>
> are not useful. Maybe by chance, at some step in the long chain of
> events, the UB did not have any bad effect, but something seemingly
> unrelated changed within that chain and suddenly you get an infinite
> loop, and this does not mean that the compiler had a bug before or has
> a bug now, only that the chain of events was and is valid as long as
> your program does not have UB.
>
> For those interested in UB, I will strongly recommend reading:
> http://www.eskimo.com/~scs/readings/undef.950321.html
>
> Cheers,
>
> Manuel.
