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Missing Null Pointer Optimization Through Struct Indirection #19419

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mitsuhiko opened this issue Nov 30, 2014 · 1 comment · Fixed by #19765
Closed

Missing Null Pointer Optimization Through Struct Indirection #19419

mitsuhiko opened this issue Nov 30, 2014 · 1 comment · Fixed by #19765

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@mitsuhiko
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Given a small wrapper struct around a box:

struct Wrapper<T> {
    ptr: Box<T>
}

Currently there is no null pointer optimization for a usage like Result<(), Wrapper<X>> whereas there is for Result<(), Box<X>>. The result in the first case will be two words whereas in the latter a single.
@mitsuhiko mitsuhiko mentioned this issue Nov 30, 2014
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@mitsuhiko mitsuhiko changed the title Missing Null Pointer Optimization Through Indirection Missing Null Pointer Optimization Through Struct Indirection Nov 30, 2014
@bluss
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bluss commented Nov 30, 2014

Related issue #9378

@luqmana luqmana mentioned this issue Dec 4, 2014
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@luqmana luqmana mentioned this issue Dec 12, 2014
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bors added a commit that referenced this issue Dec 29, 2014
@bors
auto merge of #19765 : luqmana/rust/nonzero-lang-item, r=nikomatsakis
25fb12b 
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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Enum layout optimizations luqmana/rust
Enum layout optimizations luqmana/rust
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@mitsuhiko @bluss