language: en

Reducing Branch Costs via Branch Alignment

Brad Calder and Dirk Grunwald
Department of Computer Science
Campus Box 430
University of Colorado
Boulder, CO 80309-0430 USA
{fcalder,grunwaldg}@cs.colorado.edu

Abstract Several researchers have proposed algorithms for basic block reordering. We call these branch alignment algorithms. The primary emphasis of these algorithms has been on improving instruction cache locality, and the few studies concerned with branch prediction reported small or minimal improvements. As wide-issue architectures become increasingly popular the importance of reducing branch costs will increase, and branch alignment is one mechanism which can effectively reduce these costs. In this paper, we propose an improved branch alignment algorithm that takes into consideration the architectural cost model and the branch prediction architecture when performing the basic block reordering. We show that branch alignment algorithms can improve a broad range of static and dynamic branch prediction architectures. We also show that a programs performance can be improved by approximately 5% even when using recently proposed, highly accurate branch prediction architectures. The programs are compiled by any existing compiler and then transformed via binary transformations. When implementing these algorithms on a Alpha AXP 21604 up to a 16% reduction in total execution time is achieved.

Keywords: Branch Prediction, Profile-based Optimization, Branch Target Buffers, Trace Scheduling.

1 Introduction