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


Manage episode 290295747 series 2817205
بواسطة JF Bastien & Chris Leary, JF Bastien, and Chris Leary، اكتشفه Player FM ومجتمعنا ـ حقوق الطبع والنشر مملوكة للناشر وليس لـPlayer FM، والصوت يبث مباشرة من خوادمه. اضغط زر الاشتراك لمتابعة التحديثات في Player FM، أو ألصق رابط التغذية الراجعة في أي تطبيق بودكاست آخر.

* 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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