Microprocessor Phantom Loop Execution for Lower Register File Power
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Solution Overview
Problem
Conventional microprocessors face challenges in efficiently executing loops, leading to high power consumption and complex design due to frequent reading and writing from large register files, especially in vector processors with wide register widths, and dynamic scheduling complicates data dependency resolution.
Innovation Solution
The implementation of phantom loops, where source registers are read once and destination registers are written once, combined with static scheduling using a time counter to manage instruction execution, reduces power consumption and simplifies the design by eliminating dynamic scheduling and resource arbitration.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If dynamic scheduling is used to execute loops in conventional microprocessors, then instruction execution flexibility is improved, but power consumption increases due to frequent reading and writing from large register files
Solution Approach 1:
The patent applies preliminary action by pre-identifying loop structures during compilation and transforming them into phantom loop form before execution. The compiler detects loops and rewrites them to eliminate repeated register file accesses, performing the optimization in advance rather than during runtime execution.
Solution Approach 2:
The patent extracts the loop control mechanism from the execution pipeline by removing branch instructions and iteration counting from the runtime path. The loop iteration count is extracted and used to pre-determine the number of iterations, allowing the loop body to execute without control flow interruptions or repeated register accesses.
2Productivity
If large register files are used in vector processors to handle wide data widths, then processing capacity is improved, but power dissipation increases due to repeated read and write operations
Solution Approach 1:
The compiler performs preliminary analysis to identify phantom loops and transforms them before execution. By detecting the loop structure in advance and rewriting it to execute the loop body a fixed number of times without branch instructions, the transformation eliminates repeated read-write operations to the register file during execution.
Solution Approach 2:
The patent changes the execution parameter from dynamic loop control with repeated register accesses to static phantom loop execution with a predetermined iteration count. This parameter change transforms the loop into a sequence of straightforward instructions that access the register file only once, significantly reducing power dissipation in wide register files.
3Productivity
If out-of-order execution is implemented to improve performance, then instruction throughput is improved, but design complexity increases due to data dependency resolution and resource arbitration
Solution Approach 1:
The patent applies preliminary action by performing loop detection and transformation during compilation before execution. The compiler identifies loop structures and transforms them into phantom loops with predetermined iteration counts, eliminating the need for runtime branch prediction, loop control, and dynamic scheduling of loop iterations, thereby reducing design complexity.
Solution Approach 2:
The patent extracts loop control instructions (branch instructions, iteration counters) from the execution pipeline. By removing these control elements and replacing them with a fixed number of sequential iterations, the complexity of data dependency resolution and resource arbitration within loops is significantly reduced.
Data Source
AI summary
A processor includes a loop detection unit to detect a phantom-loop based on the resources reserved for execution of the phantom loop. The processor executes the phantom loop by reading source operand data on a first iteration of the loop and writing back data on the last iteration of the loop while allowing instructions after the loop to be concurrently executed.