The branch predictor unit (BPU) is among the main energy consuming components in out-of-order (OoO) processors. For integer applications, we find 16 percent of the processor energy is consumed by the BPU. BPU is accessed in parallel with the instruction cache before it is known if a fetch group contains control instructions. We find 85 percent of BPU lookups are done for non-branch operations, and of the remaining lookups, 42 percent are done for highly biased branches that can be predicted statically with high accuracy. We evaluate two variants of a branch prediction model that combines dynamic and static branch prediction to achieve energy improvements for power-constrained applications. These models, named on-demand branch prediction (ODBP) and path-based on-demand branch prediction (ODBP-PATH), are two novel prediction techniques that eliminate unnecessary BPU lookups using compiler generated hints to identify instructions that can be more accurately predicted statically. ODBP-PATH is an implementation of ODBP that combines static and dynamic branch prediction based on the program path of execution. For a 4-wide OoO processor, ODBP-PATH delivers 11 percent average energy-delay (ED) product improvement, and 9 percent core average energy saving on the SPEC Int 2006 benchmarks.

We introduce the Coarse-Grain Out-of-Order (CG-OoO) general-purpose processor designed to achieve close to In-Order (InO) processor energy while maintaining Out-of-Order (OoO) performance. CG-OoO is an energy-performance-proportional architecture. Block-level code processing is at the heart of this architecture; CG-OoO speculates, fetches, schedules, and commits code at block-level granularity. It eliminates unnecessary accesses to energy-consuming tables and turns large tables into smaller, distributed tables that are cheaper to access. CG-OoO leverages compiler-level code optimizations to deliver efficient static code and exploits dynamic block-level and instruction-level parallelism. CG-OoO introduces Skipahead, a complexity effective, limited out-of-order instruction scheduling model. Through the energy efficiency techniques applied to the compiler and processor pipeline stages, CG-OoO closes 62% of the average energy gap between the InO and OoO baseline processors at the same area and nearly the same performance as the OoO. This makes CG-OoO 1.8× more efficient than the OoO on the energy-delay product inverse metric. CG-OoO meets the OoO nominal performance while trading off the peak scheduling performance for superior energy efficiency.