[Openmcl-devel] OpenMCL, Intel, Rosetta

[Gary Byers]

[I have not tried to run anything on a "released" MacIntel system,
and the last DTK update that I saw was in mid-November.  It's possible
that the released system differs from what I've tried to test on.]

A few days ago, Apple released updated Rosetta documentation which
says (in part):

"Rosetta does not support precise exceptions. Any application that
relies on register states being accurate in exception handlers or
signal handlers will not function properly running with Rosetta."

I think that that passage was newly introduced.  I'd been reporting
the fact that exception handlers have invalid values for (some)
registers as a bug, and was still holding out hope that Apple'd
fix it.  I have no idea why it's hard to get this right or whether
this limitation will be lifted in the future.  My personal,
subjective opinion is that things may have rushed/priorities
changed (the original Rosetta emulated a G3 without AltiVec;
the current version seems to support AltiVec), and that getting
this right may have been of lower priority than emulating
AltiVec was.  This is pure speculation on my part.

OpenMCL does expect exception/signal handlers to have accurate
context information.  (It's especially frustrating, because it
seems that Rosetta's able to get -most- register values right;
the PC/srr0 and the MSR/srr1 seem to be wrong, but at least
some of the others seem to be reliably correct.)

If you try to run OpenMCL under Rosetta, the first thing that
you'll find is that it's not possible to reserver ~2GB of memory
on startup (I don't know whether Rosetta's in the way of whether
there are other issues.)   Invoking the lisp with (for instance)

shell> openmcl --heap-reserve 1GB

gets around that.

The released 1.0 image will eventually die (after mapping the
heap image into memory and taking a LONG time to emulate some
cache-flushing operations) because attempts to set up Mach-level
failed and the first exception is unhandled.

A fairly recent bleeding-edge image will try to detect Rosetta
and install POSIX signal handlers instead of counting on Mach
level exception handling in an emulated environment, and it
dies trying to determine what the exception was (it segfaults
trying to read the instruction at the (bogus) PC, the segfault
handler segfaults doing the same thing, and eventually the
stack overflows.)

I'm sure that this limitation affects -some- other applications
(including MCL and at least some other lisp implementations);
if Apple believes that it's acceptable to release Rosetta with
"imprecise" exception handling (I asked someone in Apple DTS
what that meant; they didn't know for sure), I hope that they
can be convinced otherwise.   I'll certainly try to do so.


On Thu, 12 Jan 2006, Gary King wrote:

> I haven't seen mention of this on the mailing list but I have heard
> rumors that Digitool's MCL does not run under Intel Macs. Has anyone
> tried OpenMCL on a DTK or new Macintosh yet?
>
> Thanks,
> -- 
> Gary Warren King
> metabang.com
> http://www.metabang.com/
>
>
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