byteorder.h and strict aliasing

[Andrew Bartlett]

On Thu, 2013-11-14 at 18:42 +0100, Andreas Schneider wrote:
> Hello,
> 
> if you compile Samba with optimization on i686 you get 342 warnings like this:
> 
> lib/util/genrand.c:294:2: warning: dereferencing type-punned pointer will 
> break strict-aliasing rules [-Wstrict-aliasing]
> 
> These warnings normally mean that the compiler can't optimize the code. For 
> the long answer read [1].
> 
> The reason is that we have a define CAREFUL_ALIGNMENT in byteorder.h. There is 
> a comment which states:
> 
> --------------
> The distinction between 386 and other architectures is only there as
> an optimisation. You can take it out completely and it will make no
> difference. The routines (macros) in byteorder.h are totally byteorder
> independent. The 386 optimsation just takes advantage of the fact that
> the x86 processors don't care about alignment, so we don't have to
> align ints on int boundaries etc. If there are other processors out
> there that aren't alignment sensitive then you could also define
> CAREFUL_ALIGNMENT=0 on those processors as well.
> --------------
> 
> Well, to show that this is not an optimization but breaks the compiler 
> optimization I wrote a test with it:

Very nice work!  It's always great to challenge the old assumptions.

Andrew Bartlett
-- 
Andrew Bartlett
http://samba.org/~abartlet/
Authentication Developer, Samba Team           http://samba.org
Samba Developer, Catalyst IT                   http://catalyst.net.nz