[Bug c/66230] Using optimizations causes program to segfault

[gpnuma at centaurean dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66230

--- Comment #11 from gpnuma at centaurean dot com ---
(In reply to Markus Trippelsdorf from comment #10)
> (In reply to gpnuma from comment #8)
> > Thanks Markus I didn't think these alignment issues were actually the
> > problem, it goes a long way.
> > 
> > By doing memmoves instead of pointer cast allocations I got rid of the
> > segfault, but of course things are much slower... this "undefined behaviour"
> > is really treacherous !!
> > 
> > Is there any way to ensure proper alignment so I don't fall into this trap
> > and still benefit from maximum speed ?
> 
> I'm afraid there is no general recipe that would ensure proper alignment.
> But using memcpy hasn't necessary to be "much slower".
> And trading undefined behavior for a little more speed isn't a good idea in
> general.

Thanks, actually the code with __builtin_memmove is 30% slower compiled with
gcc 4.9.2 or 4.8 than it is with pointer cast allocations in 4.8 (4.9 can't say
because of the segfault).

However after testing with gcc 5.1 I had the pleasant surprise to see that it's
performing at the same speed as before, which means 30% faster than gcc 4.9.

30% faster is huge, you've obviously done a great job in the optimization
stages for 5.1 !