Compiler Dependency Analysis for Branch Misprediction Handling
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Solution Overview
Problem
Conventional vector processors waste resources and performance when handling branch mispredictions, as they typically flush the pipeline and fetch new instructions, which is unnecessary and inefficient.
Innovation Solution
The system enables universal parallelization of software applications by expressing dependencies in a compiler's dependency database, allowing for the generation of scalar and vector versions of functions, and decision-making on parallelization based on detected dependencies, thereby optimizing execution and reducing wasteful branch misprediction behavior.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the processor pipeline is flushed and new instructions are fetched from a different address during branch misprediction, then the processor can handle the mispredicted branch, but resources are wasted and performance deteriorates
Solution Approach 1:
The patent performs preliminary analysis during compilation to identify branch instructions that are candidates for misprediction. The compiler generates alternative code paths and prepares vector instructions in advance, so when a misprediction occurs, the processor can switch to pre-prepared alternative paths without flushing the entire pipeline, thus maintaining performance while handling the misprediction correctly
2Reliability
If the processor pipeline is flushed during branch misprediction, then the processor can recover from the misprediction, but power consumption increases due to unnecessary operations
Solution Approach 1:
The compiler prepares alternative execution paths and vector instructions in advance during compilation. When a branch misprediction is detected, the processor can switch to pre-prepared alternative paths without flushing the pipeline, avoiding unnecessary power-consuming operations while maintaining correct branch handling
Solution Approach 2:
The patent applies different handling strategies to different parts of the instruction stream. Instead of flushing the entire pipeline, only the affected instruction segments are handled differently, allowing the rest of the pipeline to continue executing useful instructions, thus reducing power consumption while maintaining correctness
3Device complexity
If scalar code is generated for all functions, then dependency analysis is simpler, but parallelization opportunities are lost and productivity decreases
Solution Approach 1:
The patent segments functions into scalar and vector versions based on dependency analysis. The compiler analyzes dependencies and divides functions into segments that can be safely parallelized (vector versions) and those that cannot (scalar versions), allowing parallelization opportunities to be exploited while keeping compiler complexity manageable through systematic segmentation
4Speed
If vector instructions are used unconditionally, then processing speed increases, but correctness deteriorates when branch misprediction occurs
Solution Approach 1:
The patent generates code that dynamically switches between scalar and vector execution modes based on runtime branch outcomes. The compiler prepares both scalar and vector versions of functions, and the runtime system selects the appropriate version based on actual branch behavior, ensuring correctness while maximizing speed when vectorization is applicable
Data Source
AI summary
System and methods for the parallelization of software applications are described. In some embodiments, a compiler may automatically identify within source code dependencies of a function called by another function. A persistent database may be generated to store identified dependencies. When calls the function are encountered within the source code, the persistent database may be checked, and a parallelized implementation of the function may be employed dependent upon the dependency indicated in the persistent database.