• Episode 3: __builtin_expect(!!(x), 0)

  • Apr 19 2021
  • Length: 39 mins
  • Podcast

Episode 3: __builtin_expect(!!(x), 0)

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  • Summary

  • * What is it we know statically?
    * What's effectively discoverable only at runtime?
    * How do we tell "the machine" (compiler and/or hardware):
    * Things we *know* to be true...
    * Things we *expect* to be true...
    * Things we *expect* to be true but *want to do something about* when it's not...
    * Things we have no idea about!
    * How do we collect that information that we have no idea about?
    * What happens if we're wrong? What if the workload is inherently irregular?
    * In the limit, random noise could drive our control decisions!
    * We talked a bit about precise vs imprecise exceptions before and automatic reordering, and we've mentioned vector machines and auto-vectorization.
    * All falls into the broader theme here, but we're always questioning what we can actually cover in an hour...
    * We'll try to give it a go for a subset of these things!
    * Time is often the limiting factor.

    * The episode title is the thing that we often macro define as `#define UNLIKELY`
    * In C/C++ code you might say "this control condition is unlikely", and say `if (UNLIKELY(condition))`
    * In C++20 there was added these `[[likely]]` and `[[unlikely]]` annotations that do the same thing, but with more square brackets, so clearly better!
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