Re: [RFA] PPC ABI compliance fix

[Elena Zannoni <ezannoni at redhat dot com>]

Kevin Buettner writes:
 > On Mar 22,  2:28pm, Elena Zannoni wrote:
 > 
 > > Back in November, gcc changed the way structures <= 8 bytes are
 > > returned on the PPC. The change was made to be compliant with the
 > > SVR4 ABI.
 > > 
 > > http://gcc.gnu.org/ml/gcc-patches/2001-11/msg01468.html
 > > 
 > > The abi specifies that such structures are passed in r3 and r4.
 > > Bigger structures are passed in memory.
 > > 
 > > Gcc was passing every structure in memory.  The change was made for
 > > embedded targets, but not for natives (PowerPC Linux, NetBSD, and
 > > FreeBSD).
 > > 
 > > Of course this change breaks binary compatibility with older gcc's.
 > > Not sure what to do about that, if anything, the case it covers is a
 > > corner case, anyway.
 > 
 > Hmm...  I have a hunch that, eventually, the natives will need to
 > change too.
 > 

Gcc is explicitly disabling ABI compliance for the natives. I think
Aldy asked to the gcc mailing list if he could change at least Linux,
and there was a loud 'No'.

 > For now though, your change is okay.
 > 
 > (Alternately, we could take a really hard nosed stance and implement
 > the ABI exactly as written and just put up with the failures.)
 > 

Ironically enough the gcc implementation is not correct, yet. It
screws up where exactly in r3 and r4 it puts the structure if the
structure is strictly less than 8 bytes. :-( 
For instances 5 characters are stored like

R3        R4
. . . h   e l l o

Instead of 

R3        R4
h e l l   o . . .


I thought Aldy was taking care of this, but he has gone on vacation.

Elena


 > Kevin