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[Bug c/66230] Using optimizations causes program to segfault
- From: "gpnuma at centaurean dot com" <gcc-bugzilla at gcc dot gnu dot org>
- To: gcc-bugs at gcc dot gnu dot org
- Date: Thu, 21 May 2015 15:53:00 +0000
- Subject: [Bug c/66230] Using optimizations causes program to segfault
- Auto-submitted: auto-generated
- References: <bug-66230-4 at http dot gcc dot gnu dot org/bugzilla/>
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 !