From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Edelsohn To: Paul Koning Cc: law@cygnus.com, crux@Pool.Informatik.RWTH-Aachen.DE, egcs@cygnus.com Subject: Re: Some Haifa scheduler bugs Date: Wed, 03 Sep 1997 11:50:00 -0000 Message-id: <9709031849.AA23508@rios1.watson.ibm.com> References: <9709031838.AA27038@kona.> X-SW-Source: 1997-09/msg00105.html >>>>> Paul Koning writes: Paul> "law@cygnus.com" wrote: >> However, for execution tests that fail, I wonder how many (if any) are >> caused by -fsched-spec-load-dangerous. If I remember right that >> relies on the OS to support non-faulting loads from zero. Paul> If that switch indeed depends on not faulting references to zero, it Paul> probably isn't worth having. Or if it's kept it needs a warning that Paul> in many OS's it will fail. The reason is that faulting on references Paul> to zero is universally considered a Good Thing and many OS's go out of Paul> their way to provide that feature. I disagree with your justification that faulting on dereferncing address zero is considered a Good Thing. This depends on your priorities and the correctness of your program. I do not want to get into a general discussion about programming styles and kernel design, but speculatively dereferencing zero is very important for good performance on some systems; some operating systems go out of their way specifically to make page zero valid, as opposed to making it a VM error. This needs to be set in the configuation file on a system-by-system basis and not universally enabled or disabled. David