4 ms·

The branches still take (instruction) cache. So it is beneficial to reduce the number of branches, even if they could have been predicted accurately.

taeric 4y ago
That said, I have seen people tricked into thinking branches are only if statements. If using polymorphism to reduce if statements, you'll still want to reduce the variations of the types sent through the code. Well, strictly you just want the variations to be predictable, I suppose?
- atq2119 4y ago
  I've been wondering about this recently. Modern branch predictors are really good at predicting correlated conditional branches. That is, if you have two if statements near each other that use the same condition, modern branch predictors have a very good chance of learning to predict the second one perfectly based on what the first one did. Is the same true for indirect calls, i.e. virtual function calls? That could be quite the powerful optimization, but it's probably really hard to do.
  - taeric 4y ago
    My understanding is that this is, essentially, the optimization that you get with many common "entity component system" frameworks. That is, you typically try to keep homogeneous collections of entities so that when you are processing them, you are doing a similar processing in a loop. Amusingly, as I try to google to make sure I am using the right terms, I find https://news.ycombinator.com/item?id=28200030 https://news.ycombinator.com/item?id=28200030 as a good result. Seems to cover the general idea and hints at other terms to look into. In particular, have fun with https://en.wikipedia.org/wiki/AoS_and_SoA https://en.wikipedia.org/wiki/AoS_and_SoA. :D Edit: I should add that I'm pretty sure my memory conjuring up ECS stuff was just mistaken, btw. https://en.wikipedia.org/wiki/Data-oriented_design https://en.wikipedia.org/wiki/Data-oriented_design is probably what I actually had in mind.
- marcosdumay 4y ago
  Things are getting way too high-level here. Nobody will be able to answer your question because it depends on an overwhelming amount of details. Your compiler capabilities will have a much larger influence on your code performance than the lower level properties of your GPU. As always, it's a matter of profiling and discovering what is actually happening. Most times when people use polymorphism, it's free.
  - taeric 4y ago
    Agreed it is tough to answer. Highly polymorphic code is often not that diverse, for one. And many languages that encourage polymorphism are run with a jit. Which can be seen as a higher level branch predictor. And, predictably, benefits code paths that don't have a lot of diversity in the types being sent through code.

taeric 4y ago

That said, I have seen people tricked into thinking branches are only if statements. If using polymorphism to reduce if statements, you'll still want to reduce the variations of the types sent through the code. Well, strictly you just want the variations to be predictable, I suppose?

atq2119 4y ago
I've been wondering about this recently. Modern branch predictors are really good at predicting correlated conditional branches. That is, if you have two if statements near each other that use the same condition, modern branch predictors have a very good chance of learning to predict the second one perfectly based on what the first one did. Is the same true for indirect calls, i.e. virtual function calls? That could be quite the powerful optimization, but it's probably really hard to do.
- taeric 4y ago
  My understanding is that this is, essentially, the optimization that you get with many common "entity component system" frameworks. That is, you typically try to keep homogeneous collections of entities so that when you are processing them, you are doing a similar processing in a loop. Amusingly, as I try to google to make sure I am using the right terms, I find https://news.ycombinator.com/item?id=28200030 https://news.ycombinator.com/item?id=28200030 as a good result. Seems to cover the general idea and hints at other terms to look into. In particular, have fun with https://en.wikipedia.org/wiki/AoS_and_SoA https://en.wikipedia.org/wiki/AoS_and_SoA. :D Edit: I should add that I'm pretty sure my memory conjuring up ECS stuff was just mistaken, btw. https://en.wikipedia.org/wiki/Data-oriented_design https://en.wikipedia.org/wiki/Data-oriented_design is probably what I actually had in mind.
marcosdumay 4y ago
Things are getting way too high-level here. Nobody will be able to answer your question because it depends on an overwhelming amount of details. Your compiler capabilities will have a much larger influence on your code performance than the lower level properties of your GPU. As always, it's a matter of profiling and discovering what is actually happening. Most times when people use polymorphism, it's free.
- taeric 4y ago
  Agreed it is tough to answer. Highly polymorphic code is often not that diverse, for one. And many languages that encourage polymorphism are run with a jit. Which can be seen as a higher level branch predictor. And, predictably, benefits code paths that don't have a lot of diversity in the types being sent through code.