PowerPC64 alignment of double in structs

Alan Modra amodra at bigpond.net.au
Thu Sep 18 14:50:56 EST 2003

On Wed, Sep 17, 2003 at 11:26:14PM +0200, Benjamin Herrenschmidt wrote:
> On Wed, 2003-09-17 at 18:11, Alan Modra wrote:
> > http://gcc.gnu.org/ml/gcc-patches/2003-09/msg01003.html is a proposal
> > for fixing struct layout rules on powerpc64-linux-gcc to comply with
> > the PowerPC64 Linux ABI.  The ABI specifies 4 byte alignment for
> > doubles, which is a little surprising for someone without an AIX
> > background, and isn't ideal for speed.  An alternative would be to
> > change the ABI and gcc (and presumably xlc) to natural alignment.
> For what it's worth on ABI matters, my opinion is too enforce strict
> alignement (same goes for Altivec). I'd go further saying that not
> enforcing alignement by default is completely insane.

The insanity is due to trying to be compatible with AIX, to aid
migration of apps from AIX to Linux.  The trouble is, we now have
problems migrating apps from other flavours of Linux to PowerPC64

An example:  I went looking for places in glibc that might break if
we changed alignment of doubles.  I didn't find anything that would
cause a problem if we changed.  ie. old binaries could be linked with
a new glibc and vice versa without trouble.  However, I did find one
place that's broken currently, and needs natural alignment of doubles
to work..  In malloc/obstack.c:

/* Determine default alignment.  */
struct fooalign {char x; double d;};
  ((PTR_INT_TYPE) ((char *) &((struct fooalign *) 0)->d - (char *) 0))

Oops, obstacks only aligned to 4 bytes.

Alan Modra
IBM OzLabs - Linux Technology Centre

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