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Extend the Non-nullable pointer optimization to handle Option<Bucket<K, V>> #9378
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use std::mem;
struct Bucket<K,V> { x: uint, y: K, z: V }
fn main() {
println!("{} {}",
mem::size_of::<Bucket<~str, ~str>>(),
mem::size_of::<Option<Bucket<~str, ~str>>>())
} prints |
bors
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Dec 29, 2014
This extends the nullable enum opt to traverse beyond just the first level to find possible fields to use as the discriminant. So now, it'll work through structs, tuples, and fixed sized arrays. This also introduces a new lang item, NonZero, that you can use to wrap raw pointers or integral types to indicate to rustc that the underlying value is known to never be 0/NULL. We then use this in Vec, Rc and Arc to have them also benefit from the nullable enum opt. As per rust-lang/rfcs#499 NonZero is not exposed via the `libstd` facade. ``` x86_64 Linux: T Option<T> (Before) Option<T> (After) ---------------------------------------------------------------------------------- Vec<int> 24 32 24 String 24 32 24 Rc<int> 8 16 8 Arc<int> 8 16 8 [Box<int>, ..2] 16 24 16 (String, uint) 32 40 32 ``` Fixes #19419. Fixes #13194. Fixes #9378. Fixes #7576.
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The example is from hashmap, where a bucket is:
struct Bucket<K,V> { hash: uint, key: K, value: V }
and the hashmap keeps a vector of~[Option<Bucket<K,V>>]
If this were a custom enum, the non-nullable pointer optimization could be used to eliminate the enum tag overhead if either
K
orV
were such a non-nullable type.Extend the optimization to handle a case like
Option<Bucket>
when possible.Manual cusom enum was implemented in PR #9212 (not merged).
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