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Do [[likely]] and [[unlikely]] flags have anything to do with CPU code prediction / speculative execution that has been a burning subject recently (Spectre)?

• secondcoming 6y ago
  Not really. They're hints to the complier about how to order the code for 'if..else' statements.
  • chrisseaton 6y ago
    > They're hints to the complier about how to order the code for 'if..else' statements. It's more than that - it can inform many optimisations, for example keeping values alive in one branch at the expense of another.
• sesuximo 6y ago
  Not really.. in part 1 of a hypothetical spectre attack, you’d have to figure out how to time a branch hit/miss, and the presence of likely may change the times/inputs you’d need to use to do that. These attributes are just hints to move hot/slow code near/far away and to maybe to the right thing if there’s no branch prediction. They cannot prevent speculative execution.
• t-vi 6y ago
  Not directly. It signals that the compiler does well when optimizing the [[likely]] path at the expense of making the other paths less optimized.
  • secondcoming 6y ago
    IIUC, the optimisation is the same, but all it does is tell the compiler what the hot path is likely to be and so the code won't 'jmp' to it; the instructions are likely to be in I-cache.
    • steerablesafe 6y ago
      gcc (maybe other compilers too) can put the cold path to a separate section, so it further improves happy-path code cache locality.
• jcelerier 6y ago
  it's a standard version of e.g. GCC and Clang's __builtin_expect
• mhh__ 6y ago
  No
• nsajko 6y ago
  No, they are intended for helping with performance (letting the compiler know which branch of execution should it make the "default" for the CPU). But see "Don’t use the [[likely]] or [[unlikely]] attributes": https://blog.aaronballman.com/2020/08/dont-use-the-likely-or-unlikely-attributes/ https://blog.aaronballman.com/2020/08/dont-use-the-likely-or...