Use likely() and unlikely() Hints with Verify Branch Statistics

Likely vs Unlikely Branches

__builtin_expect(expr, c) tells GCC/Clang the probability of a branch, influencing layout so the hot path falls through. Linux kernel likely()/unlikely() macros wrap it. Mis-hinting can slow code by fighting the hardware predictor.

Syntax and Effect

Mark error checks as cold. Keep the common path straight-line when possible.

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• Modern CPUs learn patterns; hints help most on unpredictable branches
• Read perf stat -e branch-misses before sprinkling hints
• PGO often beats manual hints by using real frequencies
• Hints do not change program semantics

Working Through the Exercise

Keep the relevant documentation open while you implement. When your output disagrees with the reference, trace one failing case by hand before changing random lines.

• Trace first: walk one failing input step by step
• Read the manual: skim the official doc for the mechanism named in this task
• Compare baselines: save measurements from the prior step so speedups are honest

Why for this exercise

You will add branch hints to a hot path and compare assembly layout. This exercise requires explaining when hints helped versus when the predictor already learned the branch.

Document one invariant you will assert in tests and how you would detect its violation from observable symptoms.