Re: [PATCH] Fix PR target/103100 -mstrict-align and memset on not aligned buffers

[Andrew Pinski <pinskia@gmail.com>]

On Wed, Nov 17, 2021 at 1:39 AM Richard Sandiford via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> apinski--- via Gcc-patches <gcc-patches@gcc.gnu.org> writes:
> > From: Andrew Pinski <apinski@marvell.com>
> >
> > The problem here is with -mstrict-align, aarch64_expand_setmem needs
> > to check the alginment of the mode to make sure we can use it for
> > doing the stores.
> >
> > gcc/ChangeLog:
> >
> >       PR target/103100
> >       * config/aarch64/aarch64.c (aarch64_expand_setmem):
> >       Add check for alignment of the mode if STRICT_ALIGNMENT is true.
> > ---
> >  gcc/config/aarch64/aarch64.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/gcc/config/aarch64/aarch64.c b/gcc/config/aarch64/aarch64.c
> > index fdf05505846..2c00583e12c 100644
> > --- a/gcc/config/aarch64/aarch64.c
> > +++ b/gcc/config/aarch64/aarch64.c
> > @@ -23738,7 +23738,9 @@ aarch64_expand_setmem (rtx *operands)
> >        over writing.  */
> >        opt_scalar_int_mode mode_iter;
> >        FOR_EACH_MODE_IN_CLASS (mode_iter, MODE_INT)
> > -     if (GET_MODE_BITSIZE (mode_iter.require ()) <= MIN (n, copy_limit))
> > +     if (GET_MODE_BITSIZE (mode_iter.require ()) <= MIN (n, copy_limit)
> > +         && (!STRICT_ALIGNMENT
> > +             || MEM_ALIGN (dst) >= GET_MODE_ALIGNMENT (mode_iter.require ())))
>
> Sorry for the slow review.  I think instead we should have keep
> track of the alignment of the start byte.  This will be MEM_ALIGN
> for the first iteration but could decrease after writing some bytes.
>
> The net effect should be the same in practice.  It just seems
> more robust.

So looking into this loop further, I think it really needs a rewrite :).
Currently it is not a greedy loop, instead it iterates for each copy
it does and loops over the modes each time too.
Let me rewrite the loop so it is better.

Thanks,
Andrew


>
> Thanks,
> Richard
>
> >         cur_mode = mode_iter.require ();
> >
> >        gcc_assert (cur_mode != BLKmode);