Re: Ping: GAS/ia64: spurious dv conflict

["H. J. Lu" <hjl at lucon dot org>]

On Tue, Oct 14, 2003 at 03:08:58PM -0700, Jim Wilson wrote:
> On Tue, 2003-10-14 at 13:04, H. J. Lu wrote:
> > I shouldn't add a mutex relation on a predicated compare. Here is
> > an updated patch.
> 
> The DV handling is by far the most complicated part of the IA-64
> assembler.  It would be a good idea to test this on some real code, such
> as the kernel and glibc.  If you get extra messages you didn't have
> before, or some go away that shouldn't have, then there is a problem
> with your patch.  The kernel and glibc have some hand written IA-64 code
> that can detect problems that gcc can't trigger.  Also, there is the
> chance that this will expose latent kernel/glibc bugs which will have to
> be fixed.
> 
> You put the update_qp_mutex call before the code for .and/.or compares
> which is not correct.  cmp.and can only clear predicate registers, and
> thus cmp.and will never destroy a mutex.  Also, in the .or case, you now
> have a clear_qp_mutex call which is redundant with the update_qp_mutex
> call.  I think the correct place to put the call to update_qp_mutex is
> right before the test for .unc compares.  You can then handle fixing the
> code for .and/.or as a separate problem.
> 
> I am guessing that you put the update_qp_mutex call before the .and/.or
> compares because you need it even when one of the predicates is pr0. 
> The right fix for that is to delete the if statement that checks for p1
> == 0 or p2 == 0, as that code is bogus.  This forces us to make other
> changes to avoid problems though, as we don't want to pass pr0 to
> add_qp_mutex.
> 
> One way to do this is to force the mask for pr0 to be zero.  That is
>       valueT p1mask = (p1 != 0) ? (valueT) 1 << p1 : 0;
>       valueT p2mask = (p2 != 0) ? (valueT) 1 << p2 : 0;
> I think this should work fine for the clear_qp_mutex and
> clear_qp_implies calls, but does not work for the update_qp_mutex and
> add_qp_mutex.  For add_qp_mutex, we need to make sure that we have two
> mask bits.  For update_qp_mutex, if we don't have two mask bits, then we
> can call clear_qp_mutex instead.  Or perhaps use the original mask bits
> there since passing a pr0 mask bit to update_qp_mutex is likely to be
> safe.
> 
> Actually, the add_qp_mutex call for the .and.orcm/.or.andcm case is
> bogus too, since these compares are not guaranteed to create a mutex. 
> So that call can be dropped.  That leaves only the last call and we can
> check for two mask bits there before making the call.
> 
> The comment before the update_qp_mutex function needs fixing.  It refers
> to P1 and P2, but there are no variables of that name.  It should
> instead state that mask must contain two and only two bits, which
> indicate two mutex predicate registers.

Here is the updated patch. I will compile gcc, glibc and kernel to see
if there are any changes in DV warnings.


H.J.