Hi, On Mon, 16 Feb 2009, Jeff Law wrote:
> > If the initial alternative selection was done cleverly (like chose the > > alternatives allowing the largest register sets which don't > > immediately create conflicting demands for a pseudo register) the > > opportunities for making an uncolorable graph colorable by chosing > > another alternative will be very small. This can only happen if that > > new alternative somehow allows for the uncolorable node a completely > > new set of register (like say float instead of integer regs), which > > would mean also selecting other alternatives for all instructions > > where this pseudo also is used. > > > > So it's not impossible, but I think it would happen relatively seldom > > that changing the alternatives improves the situation. > > > Of course. However, we might want to pick a narrower class if it has a > smaller cost. The mn103 targets come to mind. In general you're better > off with d0-d3/a0-a3 as they're the cheapest (cost & space). However, > you've got some extended registers which can be used just like > d0-d3/a0-a3, but which are more expensive (but still cheaper than > memory). I'd rather model this as a set of preferrable colors in the node. If they're still free when coloring the node, good, if not, too bad, but there are still others to chose from. This is more or less equivalent to chosing a different alternative, but more explicit for the coloring problem and with less ripple-down effects. Ciao, Michael.
