PowerPC lwsync Instruction

[Joe Buck]

On Thu, Jun 19, 2008 at 03:50:34PM -0500, Joel Sherrill wrote:

> >On Thu, Jun 19, 2008 at 1:36 PM, Joel Sherrill
> ><joel.sherrill@oarcorp.com> wrote:
> >  
> >>Hi,
> >>
> >>I ran into something tracking down a test
> >>failure on psim and now thing there is a
> >>target specific issue that needs addressing.
> >>    
> >
Andrew Pinski wrote:
> >lwsync is sync with the bit 9 set.  So it should be ok as it was a
> >reserved field and was supposed to be ignored on the hardware which
> >did not implement those bits and have it as a sync (but I could be
> >wrong).

Based on

http://gcc.gnu.org/ml/gcc-patches/2006-11/msg01238.html

it appears that it is not ignored, but rather traps, on at least some
hardware.  As a rule (thanks to some bad experiences with the Motorola
68000) processor architects tend to make sure that reserved bits and
unused bits generate a trap, otherwise the software developers start using
those bits for some other purpose and constrain future processor
development.

> I really don't know what to make of this now.  Should
> gcc not generate lwsync for a 603e and psim is right
> Or should psim be fixed to allow bit 9 to be set?

If psim's going to support both older and newer processors,
simulated, it appears it has to depend on the specific processor
being simulated (so psim should make both behaviors possible).