Browse Theses

Wide issue superscalar and VLIW processors utilize instruction-level parallelism (ILP) to achieve high performance. However, if insufficient ILP is found, the performance potential of these processors suffers dramatically. Branch instructions, which are one of the major limitations to exploiting ILP, enforce strict ordering conditions in programs to ensure correct execution. Therefore, it is difficult to achieve the desired overlap of instruction execution with branches in the instruction stream. To effectively exploit ILP in the presence of branches requires efficient handling of branches and the dependences they impose.

This dissertation investigates two techniques for exposing and enhancing ILP in the presence of branches, speculative execution and predicated execution. Speculative execution enables an ILP compiler to remove dependences between instructions and prior branches. In this manner, the execution of instructions and predicted future instructions may be overlapped. Compiler-controlled speculative execution is employed using an efficient structure called the superblock. The formation and optimization of superblocks increase ILP along important execution paths by systematically removing constraints due to unimportant paths. In conjunction with superblock optimizations, speculative execution is utilized to remove control dependences in the superblock to aggressively reorder instructions across branches to achieve a high degree of execution overlap.

For many applications, speculative execution alone is not sufficient to achieve high performance. The fundamental limitation is that speculation only removes dependences between branches and other instructions. The branches themselves remain in the code, which causes difficult problems. This motivates the second technique investigated in this dissertation, predicated execution, which is an architectural capability that enables the conditional execution of instructions based on the value of a Boolean source operand. Predicated execution allows a compiler to eliminate branch instructions using this conditional execution support. Additionally, predicated execution provides an efficient interface for the compiler to overlap the execution of multiple paths of control. Predicated execution is exploited in the compiler via a generalized form of a superblock, called the hyperblock. Hyperblocks provide the framework for the compiler to selectively eliminate branches using predicated execution as well as apply speculative execution to exploit ILP.