Prediction-Built Macro-Op Cache Entries for Consistent Loop Unrolling
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Solution Overview
Problem
Conventional microprocessor caches face inefficiencies due to high power consumption and performance penalties from frequent switching between micro-op and instruction caches, and variable length micro-op cache entries lead to fragmentation and reduced efficiency.
Innovation Solution
Implementing a macro-op cache with single-fetch and multi-fetch block entries, where the prediction unit builds and optimizes multi-fetch block entries based on program instruction stream consistency and loop unrolling, reducing the need for frequent cache switches and optimizing micro-op usage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If the microprocessor frequently switches between micro-op cache and instruction cache, then it can handle diverse instruction types, but power consumption increases and performance decreases
Solution Approach 1:
The patent merges the micro-op cache and instruction cache into a unified cache structure that stores both micro-operations and architectural instructions. This eliminates the need for frequent switching between separate caches, reducing power consumption while maintaining the ability to handle diverse instruction types through a single unified access interface.
Solution Approach 2:
The unified cache is designed to serve multiple functions: storing micro-operations for frequently executed instruction sequences, storing architectural instructions for less frequent access, and providing a single access point that adapts to different instruction types. This multi-functional design reduces the need for cache switching and associated power overhead.
2Adaptability or versatility
If the microprocessor frequently switches between micro-op cache and instruction cache, then it can handle diverse instruction types, but performance decreases due to fetch penalties
Solution Approach 1:
By combining micro-op cache and instruction cache into a unified structure, the patent eliminates cache switching penalties and enables continuous instruction fetch without performance interruptions. The unified cache provides consistent access timing regardless of whether the instruction is a micro-op or architectural instruction.
Solution Approach 2:
The unified cache pre-loads and stores micro-operations for frequently executed instruction sequences in advance, so that when these sequences are encountered during execution, the micro-processor can immediately access the pre-computed micro-ops without performing full instruction decoding, thereby eliminating performance penalties.
3Adaptability or versatility
If variable length micro-op cache entries are used, then different instruction sequences can be optimized, but cache fragmentation increases and efficiency decreases
Solution Approach 1:
The patent implements a unified cache entry length parameter that is consistently applied across all cache entries. Instead of variable length entries that cause fragmentation, the unified cache uses fixed-length entries or dynamically adjustable length that applies uniformly, eliminating fragmentation while maintaining the ability to optimize different instruction sequences through consistent access patterns.
Data Source
AI summary
A microprocessor includes a prediction unit (PRU) that predicts a sequence of fetch blocks (FBlks) in a program instruction stream, a macro-op (MOP) cache (MOC) that comprises MOC entries (MEs), a fusion engine. An ME holds MOPs into which architectural instructions of one or more FBlks are decoded. The PRU detects a loop body ME within the program instruction stream, accumulates loop iteration count information about a series of instances of a loop on the loop body ME in the program instruction stream, updates a consistency counter of the loop body ME while accumulating the loop iteration count information, and in response to detecting that the consistency counter has reached a threshold, instructs the fusion engine to use F copies of the MOPs of the loop body ME to build in the MOC an unrolled loop multi-FBlk ME; F is a loop unroll factor that is at least two.