Branch prediction with partially folded global history vector for reduced XOR operation time

A data processing system comprising:

• a memory subsystem;
• and
• a processor having execution units and an instruction cache, coupled to the memory subsystem, said processor including:
• an instruction fetch unit (IFU) having:
• an instruction fetch address register (IFAR);
• a global history vector (GHV) mechanism coupled to the IFAR and which includes:
• an unfolded GHV having a plurality of entries for tracking N groups of fetched instructions, each entry providing a bit indicating whether the group included a branch that was taken, and wherein unfolded means that no logical exclusive OR (XOR) has been performed on each entry;
• a folded GHV having a plurality of entries, which provides M folded entries, each entry providing a bit representing a result of folding two entries of the unfolded GHV, where M is less than N, and wherein folding means logically XORing the two entries of the unfolded GHV;
• first logic for folding a newly received, youngest branch result with an oldest unfolded entry in the unfolded GHV to provide a newest folded result;
• logic for shifting the newest folded result into the folded GHV to provide an updated folded GHV;
• and
• second logic for folding the updated folded GHV with a subset of bits from the IFAR to produce a resulting address that is utilized to address one or more branch history tables.

In a data processing unit having an instruction cache, an instruction fetch unit (IFU) with an instruction fetch address register (IFAR), and branch prediction unit, a method for efficiently tracking branch results, the method comprising:

• providing an unfolded global history vector (GHV) and a folded GHV within a GHV mechanism;
• maintaining both the unfolded GHV and the folded GHV within the GHV mechanism, wherein (a) the unfolded GHV has a plurality of entries for tracking at least N groups of fetched instructions, each entry providing a bit indicating whether the group included a branch that was taken, and wherein unfolded means that no logical exclusive OR (XOR) has been performed on each entry, and (b) the folded GHV has a plurality of entries, which provides M folded entries, each entry providing a bit representing a result of folding two entries of the unfolded GHV, where M is less than N, and wherein folding means logically XORing the two entries of the unfolded GHV;
• and
• responsive to receipt of a youngest branch result;
• folding a newly received, youngest branch result with an oldest unfolded entry in the unfolded GHV to provide a newest folded result;
• shifting the newest folded result into the folded GHV to provide an updated folded GHV;
• and
• folding the updated folded GHV with a subset of bits from the IFAR to produce a resulting address that is utilized to address one or more branch history tables.