On Fri, Nov 14, 2014 at 06:15:16PM +0000, Joseph Myers wrote:
> On Fri, 14 Nov 2014, Jakub Jelinek wrote:
>
> > On Fri, Nov 14, 2014 at 09:46:14AM +0300, Yury Gribov wrote:
> > > Hi all,
> > >
> > > This patch fixes https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63802 by
> > > only
> > > limiting minimal type alignment with BIGGEST_ALIGNMENT for types with no
> > > __attribute__((aligned)).
> > >
> > > Bootstrapped and regtested on x64. Ok for trunk?
> >
> > The function is primarily used by the C FE for _Alignas, and I have no idea
> > if such a change is desirable for that very much user visible case. Joseph?
>
> If it is true that a type satisfying TYPE_USER_ALIGN will never be
> allocated at an address less-aligned than its TYPE_ALIGN, even if that's
> greater than BIGGEST_ALIGNMENT, then the change seems correct for C11
> _Alignof.
I think it depends on which target and where.
In structs (unless packed) the user aligned fields should be properly
aligned with respect to start of struct and the struct should have user
alignment in that case, automatic vars these days use alloca with
realignment if not handled better by the target, so should be fine too.
For data section vars and for common vars I think it really depends on the
target. Perhaps for TYPE_USER_ALIGN use minimum of the TYPE_ALIGN and
MAX_OFILE_ALIGNMENT ?
For heap objects, it really depends on how it has been allocated, but if
allocated through malloc, the extra alignment is never guaranteed.
So, it really depends in malloc counts or not.
Jakub
