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Episode 3: __builtin_expect(!!(x), 0)
- 2021/04/19
- 再生時間： 39 分
- ポッドキャスト
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サマリー
あらすじ・解説
* 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!