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Beautiful branchless binary search

(probablydance.com)
386 points ingve | 1 comments | 28 Apr 23 05:46 UTC | HN request time: 0.209s | source
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rowanG077 ◴[28 Apr 23 06:02 UTC] No.35737974[source]▶
Nice algorithm. I don't agree that it is branchless however. Cmov is a branching instruction for sure. And it doesn't really matter whether a branch can be well predicted. The reason you mainly want branchless code is for security reasons where a timing sidechannel could reveal information. Calling this algorithm branchless devalues the term into something meaningless.

Edit: Everybody in the comments is focusing on performance. For performance sensitive code the point is not that it's branchless, the point is that it is fast, that some branchless code is faster than branching code is an implementation detail. During encryption the point is that it certain codepaths MUST be branchless.

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1. frankreyes ◴[28 Apr 23 07:40 UTC] No.35738591[source]▶
Because in the context of security you associate "branchless" with the time it takes is always the same regardless of the data/values, in order to mitigate a timing attack.

In this context, branchless means something different: the performance of the CPU is maximized for the given hardware, by not flushing the speculative execution of the CPU.