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.

中文翻译：

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节能按需动态分支预测模型

分支预测器单元 (BPU) 是乱序 (OoO) 处理器中的主要能耗组件之一。对于整数应用程序，我们发现 BPU 消耗了 16% 的处理器能量。在获知提取组是否包含控制指令之前，BPU 与指令高速缓存并行访问。我们发现 85% 的 BPU 查找是针对非分支操作完成的，而在剩余的查找中，42% 是针对可以高精度静态预测的高度偏置的分支进行的。我们评估了分支预测模型的两个变体，该模型结合了动态和静态分支预测，以实现功率受限应用的能量改进。这些模型称为按需分支预测 (ODBP) 和基于路径的按需分支预测 (ODBP-PATH)，是两种新颖的预测技术，它们使用编译器生成的提示来识别可以更准确地静态预测的指令，从而消除不必要的 BPU 查找。ODBP-PATH 是ODBP 的一种实现，它结合了基于程序执行路径的静态和动态分支预测。对于 4 宽 OoO 处理器，ODBP-PATH 在 SPEC Int 2006 基准测试中提供 11% 的平均能量延迟 (ED) 产品改进和 9% 的内核平均节能。