Re: Some Haifa scheduler bugs

[David Edelsohn <dje at watson dot ibm dot com>]

>>>>> 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