Branch Target Buffer With Gated Word Lines For Power Reduction
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Solution Overview
Problem
Conventional branch prediction mechanisms in processors are inefficient due to high power consumption and complexity, particularly in accessing the Branch Target Buffer (BTB) memory array, which leads to significant performance penalties and increased power usage, especially in devices with limited power resources like laptops and mobile devices.
Innovation Solution
A Branch Target Buffer (BTB) memory array with gated word lines and a word line gating circuit that conditionally enables access based on branch history data, reducing unnecessary READ operations and conserving power by only accessing data entries predicted to be needed.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by stationary object
If conventional branch prediction mechanisms access the BTB memory array for every branch instruction, then complete branch information is available, but power consumption increases significantly
Solution Approach 1:
The patent applies preliminary action by pre-computing branch history information and storing it in the BTB before actual branch execution. The branch history unit calculates taken/not-taken patterns in advance, and this pre-computed data is stored in the BTB memory array. When a branch instruction is encountered, the pre-stored history information is immediately available, avoiding the need for real-time computation and reducing power consumption during actual branch handling.
Solution Approach 2:
The patent extracts only the essential branch history information (taken/not-taken patterns) from the complete branch data and stores it in the BTB. Instead of accessing and processing all branch instruction details, the system extracts and stores only the critical prediction-relevant data. This extraction reduces the amount of data that needs to be accessed and processed, thereby reducing power consumption while maintaining prediction accuracy.
2Productivity
If the BTB memory array is accessed for every branch instruction, then accurate branch prediction is achieved, but processing speed decreases due to access delays
Solution Approach 1:
The branch history information is pre-computed and stored in the BTB before branch execution occurs. This preliminary preparation eliminates the need for real-time memory access during branch handling, allowing the processor to quickly retrieve prediction information and maintain high processing speed without sacrificing accuracy.
Solution Approach 2:
The BTB acts as an intermediary structure between the branch instruction and the branch prediction logic. Instead of directly accessing complex branch information during execution, the system uses the BTB as a intermediate storage layer that holds pre-processed prediction data. This intermediary approach simplifies the access path and reduces delays in the branch prediction process.
3Reliability
If complex branch prediction mechanisms are implemented, then prediction accuracy improves, but device complexity increases
Solution Approach 1:
The branch prediction mechanism is segmented into distinct functional components: a branch history unit for computing history patterns, a BTB memory array for storing pre-computed information, and a prediction logic unit for using the stored data. This segmentation allows each component to perform its specific function efficiently, reducing overall system complexity while maintaining high prediction accuracy through specialized processing in each segment.
Solution Approach 2:
The patent extracts and isolates the critical branch history computation function into a separate branch history unit, distinct from the main execution pipeline. By extracting this specific function and handling it separately with dedicated hardware, the system achieves accurate branch prediction without requiring complex interwoven logic throughout the entire processor, thereby reducing overall device complexity.
Data Source
AI summary
A branch target buffer (BTB) storing a data entry related to a branch instruction is disclosed. The BTB conditionally enables access to the data entry in response to a word line gating circuit associated with a word line in the BTB. The word line gating circuit stores a word line gating value derived from branch history data related to the instruction. Additionally, a branch prediction unit and a processor incorporating the BTB are disclosed, along with methods for operating the BTB.