Branch Prefetch Buffer Mitigates Frontend Resteers
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Solution Overview
Problem
High single-thread performance remains a challenge in multicore processors, with significant processor cycles lost due to frontend branch resteers, particularly caused by 'cold' CALLs and RETURNs, which exceed current Branch Target Buffer (BTB) sizes, leading to inefficiencies and increased manufacturing costs.
Innovation Solution
Implementing a backward decode logic and Branch Prefetch Buffer (BPB) mechanism that pre-decodes 'cold' branches before instruction cache fill, allowing parallel access to BTB and BPB to mitigate resteers without increasing BTB size, and using a two-phase predecode algorithm to identify shadow branches for insertion into the BPB.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If Branch Target Buffer (BTB) size is increased to accommodate more branch operations, then branch prediction coverage is improved, but hardware manufacturing cost and device complexity increase
Solution Approach 1:
The patent divides the branch target buffer into two segments: a primary BTB for frequently accessed branches and a secondary shadow BTB for cold branches. This segmentation allows the system to handle a larger number of branches without proportionally increasing the size and complexity of a single BTB structure, thereby improving coverage while controlling manufacturing costs.
Solution Approach 2:
The patent implements preliminary decoding of branch instructions during the instruction fetch stage, before execution. By pre-decoding and identifying branch operations in advance, the system can populate the shadow BTB proactively, reducing the need for a larger primary BTB and thereby lowering hardware complexity while maintaining high prediction coverage.
2Productivity
If speculative processing is used to improve execution speed, then processor performance is improved, but branch misprediction penalties increase due to frontend branch resteers
Solution Approach 1:
The patent implements a feedback mechanism where branch prediction outcomes are monitored and used to dynamically adjust the population of the shadow BTB. When mispredictions are detected, the system learns from these errors and adjusts its prediction strategy, thereby reducing the frequency of branch resteers and minimizing the time loss associated with corrective actions.
Solution Approach 2:
By performing preliminary decoding and branch identification during the fetch stage, the system prepares branch information in advance and populates the shadow BTB before execution. This preliminary action enables faster branch resolution during execution, reducing the penalty associated with branch mispredictions and maintaining high processor throughput.
3Measurement precision
If cold branches are decoded after instruction cache fill, then decoding accuracy is improved, but processor cycles are lost due to frontend delays
Solution Approach 1:
The patent performs preliminary decoding of instructions during the cache fill operation itself, rather than waiting until after the cache is filled. By decoding instructions in advance and identifying branch operations during the fill process, the system eliminates the sequential delay between cache filling and decoding, thereby reducing frontend processing time while maintaining decoding accuracy.
Solution Approach 2:
The patent changes the temporal dimension of the decoding operation by performing it in parallel with the cache fill operation rather than sequentially. This dimensional change in the processing timeline allows both operations to occur simultaneously, eliminating the time loss associated with sequential execution while preserving the accuracy benefits of post-fill decoding.
Data Source
AI summary
Methods and apparatus relating to branch prefetch mechanisms for mitigating front-end branch resteers are described. In an embodiment, predecodes an entry in a cache to generate a predecoded branch operation. The entry is associated with a cold branch operation, where the cold branch operation corresponds to an operation that is detected for a first time after storage in an instruction cache and wherein the cold branch operation remains undecoded since it is stored at a location in a cache line prior to a subsequent location of a branch operation in the cache line. The predecoded branch operation is stored in a Branch Prefetch Buffer (BPB) in response to a cache line fill operation of the cold branch operation in an instruction cache. Other embodiments are also disclosed and claimed.