Method and apparatus to control the use of hierarchical branch predictors based on the effectiveness of their results

An apparatus comprising:

• a main-branch target buffer (BTB);
• a micro-BTB separate from and smaller than the main-BTB, and configured to produce prediction information associated with a branching instruction;
• a main-BTB confidence counter configured to measure a correctness of the prediction information produced by the main-BTB;
• a main-BTB misprediction rate counter configured to measure a rate of mispredictions produced by the micro-BTB;
• and
• a micro-BTB enablement circuit configured to enable a usage of the micro-BTB's prediction information, based, at least in part, upon the values of the main-BTB confidence counter and the main-BTB misprediction rate counter;
• wherein both the main-BTB confidence counter and the main-BTB misprediction rate counter update in response to a commitment of a branch instruction.

A system comprising:

• a front-end logic section configured to fetch and predict a series of instructions to be executed;
• and wherein the front-end logic section comprises:
• a main-branch target buffer (BTB);
• a micro-BTB separate from and smaller than the main-BTB, and configured to produce prediction information associated with a branching instruction;
• and
• a heuristic logic circuit comprising a plurality of counters configured to measure the effectiveness of the main-BTB and the micro-BTB, and based, at least in part, upon the plurality of counters, determine whether to enable a usage of the micro-B TB's prediction information;
• wherein the plurality of counters are updated, at least in part, in response to a commitment of a branch instruction.