[Bug tree-optimization/50417] regression: memcpy with known alignment

["gjl at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=50417

--- Comment #18 from Georg-Johann Lay <gjl at gcc dot gnu.org> ---
(In reply to rguenther@suse.de from comment #12)
> On Fri, 8 Jul 2016, olegendo at gcc dot gnu.org wrote:
> 
> > void test (const int *a, int *b)
> > {
> >   a[100] = 1;
> >   b[200] = 2;
> > 
> >   std::memcpy ((char *)b, (char *)a, t);
> > }
> > 
> > where a[100] and b[200] both would result in 32 bit accesses, not 4x1 byte or
> > something, because the base pointer is assumed to be int aligned.
> 
> No, because the access is performed as 'int'.
> 
> >  Why should memcpy be any different?
> 
> Because the memcpy stmt doesn't constitute a memory access but a function
> call.

What about a new command option like -fassume-aligned-xxx that's off per
default?

The user could assert that when she is using memcpy (and friends) with a
pointer of a specific type, then that also asserts that the data behind the
pointer is appropriately aligned and may be accessed accordingly.