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Re: PowerPC lwsync Instruction
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).