Macro-Op Cache Partial Fallback Abort for Multi-Fetch Blocks
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Solution Overview
Problem
Conventional microprocessor cache systems face inefficiencies in micro-op caching due to fragmentation, power consumption, and misprediction penalties, particularly in high-performance superscalar designs, leading to increased power consumption and decode latency.
Innovation Solution
The implementation of single-fetch and multi-fetch block macro-op cache entries, where single-fetch blocks are optimized by a fusion engine and multi-fetch blocks are built from consistent sequences or loop-unrolled macro-op cache entries, optimizing macro-ops for efficient fetching and reducing power consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If multi-fetch block macro-op cache entries are used to improve cache hit rates, then cache hit rate is improved, but power consumption increases due to full abort processing
Solution Approach 1:
The patent applies partial action by implementing partial fallback abort processing instead of full abort processing. When a multi-fetch block entry is aborted, only the remaining macro-ops after the abort point are re-fetched, while already-fetched macro-ops are retained. This partial re-fetching approach maintains high cache hit rates while significantly reducing the power consumption associated with complete re-fetching operations.
Solution Approach 2:
The patent implements recovery by saving the state of already-fetched macro-ops in the multi-fetch block entry when an abort occurs. Instead of discarding all fetched macro-ops and re-fetching everything, the system recovers and retains the valid macro-ops before the abort point, discarding only the invalid portion after the abort point. This selective recovery mechanism reduces redundant power consumption while maintaining cache effectiveness.
2Reliability
If conventional full abort processing is used for multi-fetch block entries, then correctness is maintained, but decode latency increases
Solution Approach 1:
The patent applies preliminary action by pre-fetching multiple fetch blocks and organizing them into multi-fetch block entries before execution. This allows the processor to have multiple macro-ops ready in the cache entry, so when an abort occurs, only the remaining portion needs re-fetching rather than starting from scratch. This preliminary organization of data reduces the time penalty associated with aborts and maintains processing correctness.
Solution Approach 2:
The patent implements skipping by allowing the processor to skip the re-fetching of macro-ops that are already valid in the multi-fetch block entry. When an abort point is detected, the system rushes through the recovery process by only re-fetching the necessary remaining portion rather than re-processing the entire block, significantly reducing decode latency while maintaining correctness through selective re-fetching.
3Reliability
If larger cache entry sizes are used to reduce fragmentation, then cache hit rate is improved, but cache size and complexity increase
Solution Approach 1:
The patent applies segmentation by dividing the cache entry into multiple fetch block segments, where each segment can be independently managed and processed. This segmentation allows the cache to efficiently utilize space by storing multiple smaller fetch blocks in a single entry, reducing fragmentation without requiring a single large monolithic entry. The segmented structure maintains cache hit rate while avoiding the complexity of managing very large unified entries.
Data Source
AI summary
A microprocessor includes a macro-op (MOP) cache (MOC) that holds MOC entries (MEs), including single-fetch block MEs (SF-MEs) and multi-fetch block MEs (MF-MEs), comprising MOPs decoded from architectural instructions. A prediction circuit generates fetch block (FBlk) start addresses (FBSAs) to make predictions of a sequence of FBlks fetched from an instruction cache and MEs fetched from the MOC. A back end detects that execution of a MOP of an MF-ME needs an abort and generates an abort request. A control circuit flushes all the MF-ME's MOPs and signals the prediction circuit to restart prediction at the MF-ME's FBSA. For each current FBSA of N current FBSAs used to make N predictions starting with the FBSA of the MF-ME, the prediction circuit ignores a hit on any MF-ME and instead, if the current FBSA hits on an SF-ME, predicts the SF-ME, and otherwise predicts a FBlk at the current FBSA.