Instruction-Level Parallelism and Its Exploitation - Computer Architecture: A Quantitative Approach - page 158

FIGURE 3.3 Comparison of 2-bit predictors . A noncorrelating predictor for 4096 bits is first,
followed by a noncorrelating 2-bit predictor with unlimited entries and a 2-bit predictor with 2
bits of global history and a total of 1024 entries. Although these data are for an older version
of SPEC, data for more recent SPEC benchmarks would show similar differences in accuracy.
Tournament Predictors: Adaptively Combining Local And
Global Predictors
The primary motivation for correlating branch predictors came from the observation that the
standard 2-bit predictor using only local information failed on some important branches and
that, by adding global information, the performance could be improved. Tournament predictors
take this insight to the next level, by using multiple predictors, usually one based on global
information and one based on local information, and combining them with a selector. Tour-
nament predictors can achieve both beter accuracy at medium sizes (8K-32K bits) and also
make use of very large numbers of prediction bits effectively. Existing tournament predictors
use a 2-bit saturating counter per branch to choose among two different predictors based on
which predictor (local, global, or even some mix) was most effective in recent predictions. As
in a simple 2-bit predictor, the saturating counter requires two mispredictions before changing
the identity of the preferred predictor.