Register allocation with two phase approach does optimal coalescing after the spilling. Sometime Live range splitting makes the coalescing non optimal. The splitted Live range are connected by move instruction. Thus the Live range splitting and more specifically aggressive Live range splitting increases the number of move instruction and making the coalescing non optimal.
The optimal coalescing should take the frequency of the regions into consideration. The more the frequency of the region the cost associated with the splitted live ranges will increase. If such cost increasing then we should do aggressive coalescing in this case and remove the move instruction by associating the same color or hard registers associated with move instruction for the splitting live ranges. I am think of adding such heuristics in the GCC Top Down Region based register allocator for the optimal coalescing, thereby reducing the move instruction connected with the splitted Live ranges in the more frequently code. Thoughts please ? Thanks & Regards Ajit
