The inner most loop iteration counter: a new dimension in branch history

The most efficient branch predictors proposed in academic literature exploit both global branch history and local branch history. However, local history branch predictor components introduce major design challenges, particularly for the management of speculative histories. Therefore, most effective hardware designs use only global history components and very limited forms of local histories such as a loop predictor.

The wormhole (WH) branch predictor was recently introduced to exploit branch outcome correlation in multidimensional loops. For some branches encapsulated in a multidimensional loop, their outcomes are correlated with those of the same branch in neighbor iterations, but in the previous outer loop iteration. Unfortunately, the practical implementation of the WH predictor is even more challenging than the implementation of local history predictors.

In this paper, we introduce practical predictor components to exploit this branch outcome correlation in multidimensional loops: the IMLI-based predictor components. The iteration index of the inner most loop in an application can be efficiently monitored at instruction fetch time using the Inner Most Loop Iteration (IMLI) counter. The outcomes of some branches are strongly correlated with the value of this IMLI counter. A single PC+IMLI counter indexed table, the IMLI-SIC table, added to a neural component of any recent predictor (TAGE-based or perceptron-inspired) captures this correlation. Moreover, using the IMLI counter, one can efficiently manage the very long local histories of branches that are targeted by the WH predictor. A second IMLI-based component, IMLI-OH, allows for tracking the same set of hard-to-predict branches as WH.

Managing the speculative states of the IMLI-based predictor components is quite simple. Our experiments show that augmenting a state-of-the-art global history predictor with IMLI components outperforms previous state-of-the-art academic predictors leveraging local and global history at much lower hardware complexity (i.e., smaller storage budget, smaller number of tables and simpler management of speculative states).