user-guided speculative precomputation? (my wacky ia64 idea)

[Daniel Jacobowitz]

On Tue, Apr 06, 2004 at 09:52:51PM -0400, James Morrison wrote:
> 
> Duraid Madina <duraid@octopus.com.au> writes:
> > This #pragma would be added to code such as (taken from
> > http://www.intel.com/technology/itj/2002/volume06issue01/art03_specprecomp/p06_xeon.htm
> > )
> > 
> > {
> > 	n = NodeArray[0];
> > 	while(n && remaining)
> > 	{
> > 		doSomeWork();
> > #pragma delinquent_load
> > 		n->i = n->next->j + n->next->k + n->next->l;
> > 		n = n->next;
> > 		remaining--;
> > 	}
> > }
> > 
> 
>  I'm not sure if I should be scared by this example or put it up to lazyness.
> However, n is going to be compared each time we go through the loop even
> though it is only supposed to be tested once.  If n is ever 0 at the top of
> the loop, after executing the loop, then a segfault would have already occured.
> I don't suppose it's possible to optimize this, is it?

Eh, you're incorrect.  Note the n = n->next at the bottom of the loop.

In general, if the circumstance you're describing had actually occured
in this example - yes, I believe that at least tree-ssa or lno could
optimize this.  There would be a non-null marker on the dereferenced
pointer, which would reach a PHI at the top of the loop.

-- 
Daniel Jacobowitz
MontaVista Software                         Debian GNU/Linux Developer