[1] Univ Pittsburgh, Dept Elect & Comp Engn, Pittsburgh, PA 15261 USA
[2] Los Alamos Natl Lab, Performance & Architecture Lab, Los Alamos, NM 87545 USA
[3] IBM Austin Res Lab, Austin, TX 78758 USA
[4] Univ Massachusetts, Dept Comp Sci, Amherst, MA 01003 USA