Section: New Results

Elimination of Parallel Copies Using Code Motion on Data Dependence Graphs

Participants : Florian Brandner, Quentin Colombet.

Traditional approaches to copy elimination during register allocation are based on interference graphs and register coalescing. Variables are represented as nodes in a graph, which are coalesced, if they can be assigned the same register. However, decoupled approaches strive to avoid interference graphs and thus often resort to local recoloring.

A common assumption of existing coalescing and recoloring approaches is that the original ordering of the instructions in the program is not changed. We developed an extension of a local recoloring technique called Parallel Copy Motion. We perform code motion on data dependence graphs in order to eliminate useless copies and reorder instructions, while at the same time a valid register assignment is preserved. Our results show that even after traditional register allocation with coalescing our technique is able to eliminate an additional 3% (up to 9%) of the remaining copies and reduce the weighted costs of register copies by up to 25% for the SPECINT 2000 benchmarks. In comparison to Parallel Copy Motion, our technique removes 11% (up to 20%) more copies and up to 39% more of the copy costs.

These results have been accepted for publication at SAC'12 [10] and, in a longer version, in the journal Computer Languages, Systems, and Structures [5] .