branch prediction

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Branchless Rust: Making a Filter 4x Faster by Removing an If
2 months ago
- Filtering a slice of numbers by a threshold shows performance varying by selectivity, with 50% being slowest due to branch mispredictions.
- The branch predictor guesses which path to take; unpredictable data causes mispredictions, which are costly (15–20 cycles each).
- Sorted input makes the branch predictable (4.5x faster), but sorting is not a practical fix.
- Branchless programming removes the unpredictable branch by always writing elements and using comparison results as numbers to conditionally advance a cursor.
- Branchless code runs in constant time (~1 ms) regardless of selectivity, but is slower for very predictable cases (e.g., 1% kept).
- This technique should only be used on hot paths with unpredictable branches; readability and best-case performance are traded.
Matt Godbolt's blog
2 months ago
- Tail call optimization (TCO) turns recursive functions into loops by replacing calls with jumps, saving stack space and enabling further optimizations.
- Clang's [[clang::musttail]] attribute forces TCO, ensuring tail calls are optimized even in debug mode.
- Arranging function arguments to match the calling convention improves TCO efficiency by reducing register shuffling.
- In CPU emulators, TCO can replace a switch loop with separate instruction handlers, improving branch prediction by giving each handler its own predictor state.
Quadrupling code performance with a "useless" if
2 months ago
- Describes optimizing a domain-specific compressor by improving a critical path loop.
- Highlights the issue of dependency chains preventing instruction-level parallelism.
- Proposes using branch prediction to predict stable values and break dependency chains.
- Suggests a volatile cast trick to prevent compiler optimizations that would remove the branch.
- Mentions potential alternative data structure improvements but notes trade-offs.

branch prediction

Branchless Rust: Making a Filter 4x Faster by Removing an If

Matt Godbolt's blog

Quadrupling code performance with a "useless" if