Re: movmem pattern and missed alignment

[Michael Matz <matz at suse dot de>]

Hi,

On Mon, 8 Oct 2018, Paul Koning wrote:

> >> extern int *i, *j;
> >> extern int iv[40], jv[40];
> >> 
> >> void f1(void)
> >> {
> >>    __builtin_memcpy (i, j, 32);
> >> }
> >> 
> >> void f2(void)
> >> {
> >>    __builtin_memcpy (iv, jv, 32);
> >> }
> > 
> > Yes, memcpy doesn't require anything bigger than byte alignment and
> > GCC infers alignemnt
> > only from actual memory references or from declarations (like iv /
> > jv).  For i and j there
> > are no dereferences and thus you get alignment of 1.
> > 
> > Richard.
> 
> Ok, but why is that not a bug?  The whole point of passing alignment to 
> the movmem pattern is to let it generate code that takes advantage of 
> the alignment.  So we get a missed optimization.

Only if you somewhere visibly add accesses to *i and *j.  Without them you 
only have the "accesses" via memcpy, and as Richi says, those don't imply 
any alignment requirements.  The i and j pointers might validly be char* 
pointers in disguise and hence be in fact only 1-aligned.  I.e. there's 
nothing in your small example program from which GCC can infer that those 
two global pointers are in fact 2-aligned.


Ciao,
Michael.