Macro-Op Cache Co-Residence for Unrolled Loop Fetch Efficiency
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Solution Overview
Problem
Conventional microprocessors face inefficiencies in micro-op caches due to fragmentation, conflict misses, and power consumption issues, particularly in high-performance superscalar designs with complex decode logic and variable-length micro-ops, leading to increased latency and power consumption.
Innovation Solution
The implementation of a macro-op cache that includes single-fetch and multi-fetch block entries, where macro-ops are predicted and optimized, allowing for efficient construction of multi-fetch block entries through loop unrolling and sequential fetching, reducing the need for decode operations and improving cache hit rates.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional micro-op cache is used to store decoded micro-operations, then the cache hit rate improves for frequently executed code sequences, but cache fragmentation and conflict misses occur when the same fetch block start address maps to different cache sets
Solution Approach 1:
The patent merges the loop header macro-op cache entry and loop body macro-op cache entry into the same cache set by allowing same-fetch block start address co-residence. This combining approach allows both entries to coexist without causing conflict misses, thereby maintaining high cache hit rates while preventing cache fragmentation.
Solution Approach 2:
The patent creates a multi-functional cache entry structure that can accommodate both loop header and loop body macro-ops using the same fetch block start address. This universal cache structure allows a single cache set to serve multiple purposes: storing loop header information and loop body information simultaneously, eliminating the need for separate cache entries.
2Productivity
If loop unrolling is used to build multi-fetch block macro-op cache entries, then the number of decode operations is reduced and execution efficiency improves, but the complexity of cache entry construction increases
Solution Approach 1:
The patent performs preliminary loop unrolling during the cache entry construction phase. By pre-unrolling the loop body and creating multiple fetch block macro-op cache entries in advance, the system reduces the number of decode operations needed during execution. The loop header entry stores metadata that enables rapid construction of loop body entries without real-time decoding.
Solution Approach 2:
The patent uses copying to construct loop body macro-op cache entries from the loop header entry. Instead of independently decoding the loop body instructions, the system copies the macro-op information from the loop header and increments the fetch block start address to create subsequent loop body entries. This copying mechanism significantly reduces decode complexity while maintaining execution efficiency.
3Device complexity
If the macro-op cache stores both loop header and loop body entries separately, then cache organization is simplified, but cache space is wasted and hit rate decreases due to conflict misses
Solution Approach 1:
The patent merges loop header and loop body macro-op cache entries into the same cache set by allowing same-fetch block start address co-residence. This merging eliminates cache space waste and prevents conflict misses between loop header and loop body entries, thereby improving cache hit rate while maintaining reasonable cache organization through the use of distinct entry types within the same set.
Data Source
AI summary
A microprocessor includes a prediction unit (PRU) that continuously predicts a sequence of fetch block start addresses (FBSAs) that specify a corresponding sequence of fetch blocks (FBlks) in a program instruction stream and a macro-op cache (MOC) having MOC entries (MEs). The PRU installs into the MOC a loop body ME using a first FBSA value that specifies the loop body ME, instructs a fusion engine to build an unrolled loop multi-FBlk ME (ULP-MF-ME) using F copies of the loop body ME, installs the ULP-MF-ME using the same first FBSA value, detects a multiple-hit in the MOC on both the loop body ME and the ULP-MF-ME, and instructs a fetch unit to fetch from the MOC a number of copies of the ULP-MF-ME equal to an unrolled loop iteration count of the ULP-MF-ME and instructs the fetch unit to fetch the loop body ME until the loop exits.