| This thesis introduces a program attribute called value locality and proposes speculative execution under the weak dependence model. The weak dependence model lays a theoretical foundation for exploiting value locality and other program attributes by speculatively relaxing and deferring the detection and enforcement of control- and data-flow dependences between instructions to expose more instruction-level parallelism without violating program correctness. Value locality is a program attribute that describes the likelihood of the recurrence of a previously-seen value within a storage location inside a computer system. Most modern processors already exploit value locality through the use of control speculation (i.e. branch prediction), which seeks to predict the future values of condition code bits and branch-target addresses based on previously-seen values. Experimental results indicate that value locality exists for condition codes and branch target addresses, and for general-purpose and floating-point registers as well. Furthermore, value locality exists not only in the data flow portion of a processor, but also in the control logic, where both register and memory dependences between instructions tend to remain static and are relatively easily predictable.;To exploit value locality, several dynamic prediction mechanisms are proposed. These mechanisms increase instruction-level parallelism by speculatively collapsing explicit and implicit dependences between instructions and folding away execution pipeline stages under the pipeline contraction framework. Detailed evaluation of value prediction for load instructions only, as well as all instructions that write general-purpose or floating-point registers shows significant potential for performance improvements. Further experimental results indicate that both register and memory dependence relationships between instructions are easily predictable. These discoveries result in significant potential for further performance improvements, particularly in conjunction with wide-issue and deeply-pipelined superscalar processors that employ aggressive techniques like accurate dynamic branch prediction and instruction fetching via trace cache to overcome control- and data-flow restrictions on parallel execution of serial programs. Finally, this thesis introduces a new microarchitectural paradigm called Superflow that supersedes historical limits on instruction flow, register dataflow, and memory dataflow and demonstrates potential performance improvements of 2-4x over the current state-of-the-art microprocessors. |
