Branch Mispredict

Metric Description

When a branch mispredicts, some instructions from the mispredicted path still move through the pipeline. All work performed on these instructions is wasted since they would not have been executed had the branch been correctly predicted. This metric represents slots fraction the CPU has wasted due to Branch Misprediction. These slots are either wasted by uOps fetched from an incorrectly speculated program path, or stalls when the out-of-order part of the machine needs to recover its state from a speculative path.

Possible Issues

A significant proportion of branches are mispredicted, leading to excessive wasted work or Back-End stalls due to the machine need to recover its state from a speculative path.

Tips

1. Identify heavily mispredicted branches and consider making your algorithm more predictable or reducing the number of branches. You can add more work to 'if' statements and move them higher in the code flow for earlier execution. If using 'switch' or 'case' statements, put the most commonly executed cases first. Avoid using virtual function pointers for heavily executed calls.

2. Use profile-guided optimization in the compiler.

See the Intel 64 and IA-32 Architectures Optimization Reference Manual for general strategies to address branch misprediction issues.