Re: SMS scheduling

Canqun Yang
Sat Oct 2 12:25:00 GMT 2004

Mostafa Hagog <>:

> Canqun Yang <> wrote on 28/09/2004 
> > Hi, all
> >
> > I tested the Swing Modulo Scheduling of GCC. For
> > simple program, the numerical caculation of PI, it
> > achieves significant speedup on IA64. But, for 
> > bit complex programs, the SMS can hardly work out a
> > successful schedule.
> >
> > The algorithms implemented by Ayal and Mostafa are
> > correct. It seems that the SMS itself is wrong. The
> > schedule priority order calculated by SMS is much
> > different from normal MinDist algorithm.
> SMS prioritizes the nodes using critical patch based
> heuristic - the main idea is that instructions that 
> on the critical path are less flexible in means of
> scheduling.
> Why do you think that the priority order should be
> according to MinDist algorithm (what is the minimum
> distance in this case)? Can you provide an example
> that supports this?

Yes, I'll give an example after my holiday.
> > Besides this,
> > SMS is not sensitive to II. Is SMS really wrong?
> The node ordering step in SMS is not sensitive to II
> (as mentioned before it is based on critical path
> heuristic).  However, the scheduling step is sensitive
> to II - failing to schedule the nodes of a loop within
> II cycles will lead to try scheduling the nodes again
> within II + 1 cycles.
The node order calculated by SMS is not suitable. 
Although, the scheduling step does increase the II 
value to try a successful schedule, it is useless. In 
fact, the scheduler almost always fail at the same insn 
while the II is increasing. 

Canqun Yang
Creative Compiler Research Group.
National University of Defense Technology, China.

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