Branch predictors are widely used to boost the performance of
microprocessors. However, this comes at the expense of power because
accurate branch prediction requires simultaneous access to several large
tables on every fetch. Consumed power can be drastically reduced by
operating the predictor under sub-nomimal voltage levels (undervolting)
using a separate voltage domain. Faulty behavior resulting from
undervolting the predictor arrays impacts performance due to additional
mispredictions but does not compromise system reliability or functional
correctness. In this work, we explore how two well established branch
predictors (Tournament and L-Tage) behave when aggressively undervolted
below minimum fault-free supply voltage (V-min). Our results based on
fault injection and performance simulations show that both predictors
significantly reduce their power consumption by more than 63% and can
deliver a peak 6.4% energy savings in the overall system, without
observable performance degradation However, energy consumption can
increase for both predictors due to extra mispredictions, if
undervolting becomes too aggressive
