Mechanism for reducing cache power consumption using cache way prediction

A mobile device comprising:

• a system memory configured to store instructions and data;
• a processor coupled to the system memory and configured to execute the instructions, wherein the processor includes:
• a cache memory for storing instruction information for one or more instruction fetch groups fetched from a system memory, wherein the cache memory includes a plurality of ways that are each independently controllable;
• a way prediction unit coupled to the cache memory, the way prediction unit comprising a next fetch prediction unit, and a sequential way prediction unit, wherein:
• the next fetch prediction unit is configured to, in response to a branch taken prediction for a next branch instruction, enable, in a next execution cycle, a given way within which instruction information corresponding to a target of the next branch instruction is stored;
• and
• wherein the sequential way prediction unit is configured to, in response to the branch taken prediction for the next branch instruction, enable, one at a time, each corresponding way within which instruction information corresponding to respective sequential instruction fetch groups that follow the next branch instruction are stored;
• wherein the sequential way prediction unit is coupled to receive an indication of a previous hit from the next fetch prediction unit, wherein responsive to an indication of a previous hit in the next fetch prediction unit, the sequential way prediction unit is configured to provide a fixed number of way predictions;
• wherein the next fetch prediction unit is configured to store, in each of a first plurality of entries, information describing a target of a branch instruction and information identifying a given cache way of a cache memory in which the target is stored and the sequential way prediction unit is configured to store, in each of a second plurality of entries corresponding to a unique one of the first plurality of entries, the given cache way of the cache memory in which each of the next instruction fetch groups is stored.

A method comprising:

• storing, in a given entry of a first prediction unit, information describing a target of a branch instruction and information identifying a given cache way of a cache memory in which the target is stored, the information being tagged and indexed and tagged by a fetch address;
• and
• storing, for one or more next instruction fetch groups fetched after the first taken branch instruction, in a respective entry of a second prediction unit that corresponds to the given entry in the first prediction unit, the given cache way of the cache memory in which each of the next instruction fetch groups is stored;
• upon a subsequent fetch of the branch instruction and responsive to a branch taken prediction for the branch instruction, enabling for a next execution cycle, by the first prediction unit, the given cache way of the cache memory that stores the target of the first branch instruction;
• and
• upon taking the branch instruction, enabling, by the second prediction unit, the given cache way that stores each respective next fetch group after the taken branch;
• wherein the method further comprises the first prediction unit providing, the second prediction unit, an indication of a previous hit in the first prediction unit, and the second prediction unit providing a fixed number of way predictions responsive to receiving the indication.