GPREFETCH Microcode for REP LODS Cache Line Prefetching
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Solution Overview
Problem
Existing microprocessor implementations of repeat string load instructions, such as REP LODS, are inefficient due to the need for a loop that clobbers the AL register and results in slow data fetching, as they execute numerous small load instructions and stall the pipeline on cache misses.
Innovation Solution
Incorporating a guaranteed prefetch (GPREFETCH) instruction in the microcode to proactively fetch entire cache lines implicated by the repeat load string instruction, ensuring that cache lines are loaded into the data cache without stalling the pipeline, even if the memory region is uncacheable or encounters exceptions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If conventional loop-based microcode is used to implement REP LODS, then the instruction can be implemented with simple control logic, but the execution speed is slow due to pipeline stalls on cache misses
Solution Approach 1:
The patent applies preliminary action by implementing guaranteed prefetch instructions that fetch cache lines into the data cache before they are actually needed by the REP LODS instruction. This allows the memory subsystem to proactively load data into the cache, so when the actual load occurs, the data is already available, eliminating pipeline stalls. The microcode generates prefetch addresses in advance and issues prefetch requests, ensuring data is ready before the loop needs it.
Solution Approach 2:
The patent segments the data transfer operation into two distinct phases: a prefetch phase that loads cache lines into the data cache, and a load phase that actually transfers data from cache to the architectural register. This segmentation allows the memory subsystem to operate independently in the background during prefetch, while the main execution pipeline continues without stalls. The complex addressing and prefetch logic is isolated in the memory subsystem rather than complicating the main microcode loop.
2Productivity
If numerous small load instructions are executed in a loop, then the architectural register can be loaded with each value in sequence, but the AL register is clobbered and pipeline performance degrades
Solution Approach 1:
The memory subsystem performs preliminary prefetching of cache lines before the REP LODS instruction needs them. By calculating prefetch addresses in advance and issuing prefetch requests, the system ensures data is loaded into the cache ahead of time. This eliminates the need for the processing pipeline to stall and wait for memory operations, maintaining high productivity without time loss.
Solution Approach 2:
The data cache acts as an intermediary buffer between system memory and the architectural register. Instead of directly executing numerous small load instructions from memory to the register (which causes pipeline stalls), the system uses the cache as an intermediate storage layer. The memory subsystem prefetches data into this intermediary cache, allowing the main pipeline to continue execution without waiting for memory operations to complete.
3Speed
If the memory subsystem waits for cache misses to occur before fetching data, then memory bandwidth is conserved, but the processor pipeline stalls and execution slows down
Solution Approach 1:
The system applies preliminary action by prefetching cache lines into the data cache before they are actually needed by the REP LODS instruction. The memory subsystem calculates prefetch addresses in advance and issues prefetch requests proactively. This ensures data is already in the cache when needed, maintaining high instruction execution speed while allowing the processor to continue execution without stalls.
Data Source
AI summary
A microprocessor invokes microcode in response to encountering a repeat load string instruction. The microcode includes a series of guaranteed prefetch (GPREFETCH) instructions to fetch into a cache memory of the microprocessor a series of cache lines implicated by a string of data bytes specified by the instruction. A memory subsystem of the microprocessor guarantees within architectural limits that the cache line specified by each GPREFETCH instruction will be fetched into the cache. The memory subsystem completes each GPREFETCH instruction once it determines that no conditions exist that would prevent fetching the cache line specified by the GPREFETCH instruction and once it allocates a fill queue buffer to receive the cache line. A retire unit frees a reorder buffer entry allocated to each GPREFETCH instruction in response to completion of the GPREFETCH instruction regardless of whether the cache line specified by the GPREFETCH instruction has been fetched into the cache.