PowerPC64 alignment of double in structs

Alan Modra amodra at bigpond.net.au
Thu Sep 18 11:35:23 EST 2003

On Wed, Sep 17, 2003 at 04:55:13PM -0500, Steve Munroe 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.
> >
> > I'm interested in opinions..
> Unfortunately this change might break backward compatibility with existing
> libraries.

Yes, that's clear.  Did you read the gcc mailing list thread?  We're
ABI incompatible *now* with xlc, and the rules xlc uses are not easy
to follow.

gcc-3.4 is an appropriate time to make an ABI change, because in
fixing a bug with the way function args are passed, gcc already has
a small ABI incompatibility with older gcc code.

Alan Modra
IBM OzLabs - Linux Technology Centre

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