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CPU branch prediction fails when the processor follows a likely execution path but the branch resolves to a different one. The CPU discards work from the wrong path and redirects execution; during recovery, less useful work may move through the pipeline. Replacing the branch with branchless code can help when outcomes are hard to predict, but it can also add instructions or dependencies, so it is not automatically faster.
Why CPUs predict branches
A conditional branch sends execution down one of multiple paths. The condition or destination may not be known when the processor must fetch more instructions. Rather than wait, modern CPUs predict which way execution will go and can begin speculative execution along that path. Intel describes this as speculation based on control-flow prediction; Microsoft explains that prediction lets a CPU start toward a predicted target instead of stalling until the branch target is resolved.
That prediction helps keep instruction fetch moving, but it is a guess. If the branch resolves as predicted, the speculative work can contribute to normal execution. If not, the processor must recover.
What happens after a misprediction
When a prediction is wrong, the processor squashes instructions fetched or executed along the incorrect path and resumes from the correct one. The discarded instructions do not commit as architectural results, but they have already consumed processor resources. Redirecting execution and restoring useful flow can leave fewer useful instructions progressing through the pipeline for a time. This interruption is commonly described as a pipeline bubble.
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A bubble is a useful mental model, not a fixed-size gap that is identical on every CPU. The recovery cost depends on processor design and workload; the cited Intel guidance does not establish one universal cycle penalty. Avoid applying a single misprediction number to all processors or programs.
When branchless code can help
A branchless rewrite replaces control flow with operations such as conditional moves or arithmetic. It can be worth considering when the branch’s outcomes are difficult for the predictor to anticipate. But removing the branch does not make the underlying choice free: the rewrite may do work for both alternatives, add instructions, or create a data dependency that delays later work.
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Intel’s Optimization Reference Manual cautions that conditional moves have overhead and should be weighed against the expected cost of a mispredicted branch. It also says prediction rates are generally high and consistently mispredicted branches are comparatively rare. That is qualitative guidance, not a prediction-accuracy percentage for your code.
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How to judge a branchless rewrite
Compare the alternatives in the context of the real hot path, rather than assuming fewer visible branches means faster execution.
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- Input predictability: Consider how branch outcomes vary in the actual input distribution. A pattern that is predictable in one workload may not be in another.
- Work and dependencies: Count the extra operations the rewrite introduces and check whether it forces computation for both choices or makes later work wait on data.
- Generated machine code: Inspect what your compiler emits for the target CPU. Source-level appearance alone does not show whether a branch, conditional move, or other sequence will be used.
- Representative measurement: Benchmark the real hot workload with representative inputs. A microbenchmark or an unusual input pattern may not reflect the program’s normal behavior.
Intel documents hardware branch-mispredict feedback for profile-guided optimization. Its article says Intel compiler versions 2024.0 and newer can use such profiles to improve code generation, including more aggressive use of conditional moves. This is an Intel compiler capability, not a guarantee that every compiler or codebase will benefit.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why squashed work can still matter for security
Squashed instructions do not change architectural results, but that does not mean every trace of their execution disappears. Speculation can leave microarchitectural effects in structures such as caches. Such residual effects can matter in side-channel vulnerabilities; the Linux kernel’s speculation documentation, for example, discusses bounds-check bypass. Correct-result rollback and absence of all observable side effects are different claims.
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