Nested Loop Controller With Predicate FIFO for DSP Pipelining
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Solution Overview
Problem
Modern digital signal processors face challenges in efficiently executing nested loops, particularly imperfectly nested loops, due to difficulties in determining predicates for instructions during pipelined execution.
Innovation Solution
A nested loop controller is introduced, which includes an inner loop counter, an outer loop counter, and a predicate FIFO (first-in, first-out buffer) to improve pipelined execution, along with instructions for initializing and controlling the nested loop controller and accessing the predicate FIFO to determine whether predicated instructions are executed.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If nested loops are executed in a pipelined fashion to improve DSP performance, then execution speed is improved, but determining predicates for outer loop instructions becomes difficult
Solution Approach 1:
A predicate FIFO (first-in-first-out buffer) is introduced as an intermediary data structure between the nested loop controller and the instruction execution units. The predicate FIFO stores predicate values generated by the nested loop controller, allowing instruction execution units to efficiently retrieve needed predicates without complex real-time calculations. This mediator resolves the contradiction by enabling fast predicate access during pipelined execution while maintaining correct predicate determination for outer loop instructions.
Solution Approach 2:
The nested loop controller pre-calculates and stores predicate values in the predicate FIFO before instructions need to be executed. By preparing predicate information in advance during loop control operations, the system eliminates the need for complex real-time predicate determination during pipelined instruction execution, thus maintaining both high speed and correct predicate logic.
2Productivity
If a nested loop controller with predicate FIFO is introduced to improve pipelined execution, then execution efficiency is improved, but device complexity increases
Solution Approach 1:
The nested loop controller merges multiple functions into a single integrated unit: it manages both inner and outer loop counters, generates predicate values, and interfaces with the predicate FIFO. By combining these functions in one controller rather than separate control units, the system achieves improved execution efficiency while minimizing the increase in overall device complexity through functional integration.
Data Source
AI summary
A method for compiling and executing a nested loop includes initializing a nested loop controller with an outer loop count value and an inner loop count value. The nested loop controller includes a predicate FIFO. The method also includes coalescing the nested loop and, during execution of the coalesced nested loop, causing the nested loop controller to populate the predicate FIFO and executing a get predicate instruction having an offset value, where the get predicate returns a value from the predicate FIFO specified by the offset value. The method further includes predicating an outer loop instruction on the returned value from the predicate FIFO.