[Bug tree-optimization/51017] GCC 4.6 performance regression (vs. 4.4/4.5), PRE increases register pressure

["solar-gcc at openwall dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=51017

--- Comment #19 from Alexander Peslyak <solar-gcc at openwall dot com> ---
(In reply to Alexander Peslyak from comment #17)
> Should we create a new bug for the unnecessary and non-optional use of
> unaligned load instructions for source code like this, or is this considered
> the new intended behavior despite of the major slowdown on such CPUs? 
> (Presumably not only for JtR.  I'd expect this to affect many programs.)

Upon further analysis, I now think that this was my fault, and (presumably) not
common in other programs.  What I had was differing definition vs. declaration,
so a bug.  The lack of alignment specification in the declaration of the struct
essentially told (newer) GCC not to assume alignment - to an extent greater
than e.g. a pointer would.  As far as I can tell, GCC does not currently
produce unaligned load instructions (so assumes that SSE* vectors are properly
aligned) when all it has is a pointer coming from another object file.  I think
that's the common scenario, whereas mine was uncommon (and incorrect).

So let's focus on PRE only.