Predicting detected branches as taken when cumulative weight values in a weight table selected by history register bits exceed a threshold value

A method comprising:

• storing history data for at least one or more most recently executed branch instructions, in which the history data for a branch represented by a particular branch instruction comprises a multi-bit history value dependent upon a program counter value and a branch target of the branch represented by the particular branch instruction;
• storing a plurality of sets of weight values, one set for each of a group of complementary portions of one or more bits of the history data;
• accessing, for a current branch instruction to be predicted, a weight value for each of the portions of one or more bits of the history data by selecting from the set of weight values in dependence upon a current value of the portions of the history data;
• generating a combined weight value by combining the weight values accessed by the access circuitry;
• comparing the combined weight value with a prediction threshold value to detect whether or not a branch represented by the current branch instruction is predicted to be taken;
• and
• modifying the accessed weight values in dependence upon a resolution of whether the branch represented by the current branch instruction is taken or not.

Program flow prediction circuitry comprising:

• a history register to store history data for at least one or more most recently executed branch instructions;
• a memory to store a plurality of sets of weight values, one set for each of a group of portions of one or more bits of the history data;
• access circuitry to access, for a current branch instruction to be predicted, a weight value for each of the portions of one or more bits of the history data by selecting from the set of weight values in dependence upon a current value of the portions of the history data;
• a combiner to generate a combined weight value by combining the weight values accessed by the access circuitry, in which the combiner comprises an adder;
• a comparator to compare the combined weight value with a prediction threshold value to detect whether or not a branch represented by the current branch instruction is predicted to be taken;
• and
• weight modifier circuitry to modify the accessed weight values in dependence upon a resolution of whether the branch represented by the current branch instruction is taken or not, in which the weight modifier circuitry is configured to increment or decrement the accessed weight values in dependence upon the resolution of whether the branch represented by the current branch instruction is taken or not and to store a pointer indicating the group of portions of the history data used to access weight values for the current branch instruction.