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Comment by anthonj

2 days ago

I think this call for something similar to "__builtin_expect" or linux' likely()/unlikely().

Not very clean, but better than inserting obscure optimisations in the source.

4 comments

anthonj

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gblargg  2 days ago

Assuming compilers are smart enough to insert an unnecessary branch to break the dependency.

  • vitally3643  2 days ago

    That's what the hints are for. Expect/likely/unlikely are the programmer informing the compiler what it should expect and how it should optimize

purplesyringa  2 days ago

That would be great, if only it worked as intended! From the perspective of an optimizing compiler, `a == b ? a : b` is worse than `b` regardless of the probability you assign to `a == b`.

ETA: someone on Lobsters (https://lobste.rs/s/1an425/quadrupling_code_performance_with...) noticed that `[[unlikely]]` actually works on LLVM (not on GCC, and with worse codegen on LLVM, but it's still good to know) -- updated the post.