# Localizing variables: performance implications?
**URL:** https://discourse.julialang.org/t/localizing-variables-performance-implications/8329
**Category:** Performance
**Created:** [January 12, 2018, 9:30pm UTC](https://discourse.julialang.org/t/localizing-variables-performance-implications/8329 "2018-01-12T21:30:06Z")
**Posts on this page:** 8
**Page:** 1
### Author:  [@SepandMeenu](https://discourse.julialang.org/u/SepandMeenu)
#### Post date: [January 12, 2018, 9:30pm UTC](https://discourse.julialang.org/t/localizing-variables-performance-implications/8329/1 "2018-01-12T21:30:06Z")
Is there any advantage to localizing variables in a Julia function?
As an example, suppose that `MyStruct` is a `mutable struct` with a field `x`:
```julia
mutable struct MyStruct
x0
# ...
end
```
and a function which takes an instance of `MyStruct` as an argument:
```julia
function foo(z::MyStruct)
#... use z.x0 many times ...
end
```
Is it recommended to “localize” `x0` in `foo` as a constant, as
```julia
function foo(z::MyStruct)
const x00 = z.x0
#... use x00 many times ...
end
```
?
---
### Author:  [@yuyichao](https://discourse.julialang.org/u/yuyichao)
#### Post date: [January 12, 2018, 9:45pm UTC](https://discourse.julialang.org/t/localizing-variables-performance-implications/8329/2 "2018-01-12T21:45:56Z")
There’s no advantage and not current allowed to make a variable constant. The compiler doesn’t need that information at all.
There can be an advantage to just do `x00 = z.x0`. It may or may not be possible for the compiler to infer that. The effect of that can range from improving the performance by a order of magnitude (e.g. when that helps alias analysis to enable loop vectorization) to slightly negative (e.g. when the increase in register pressure cause additional spilling)
---
### Author:  [@SepandMeenu](https://discourse.julialang.org/u/SepandMeenu)
#### Post date: [January 16, 2018, 3:16pm UTC](https://discourse.julialang.org/t/localizing-variables-performance-implications/8329/3 "2018-01-16T15:16:27Z")
You mean that the compiler deduces the ‘constantness’ of a variable automatically in any case, or only if one makes an _explicit_ assignment like `x00 = z.x0`?
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### Author:  [@yuyichao](https://discourse.julialang.org/u/yuyichao)
#### Post date: [January 17, 2018, 11:05pm UTC](https://discourse.julialang.org/t/localizing-variables-performance-implications/8329/4 "2018-01-17T23:05:54Z")
I’m not sure how you make a var without an assignment
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### Author:  [@SepandMeenu](https://discourse.julialang.org/u/SepandMeenu)
#### Post date: [January 18, 2018, 8:53am UTC](https://discourse.julialang.org/t/localizing-variables-performance-implications/8329/5 "2018-01-18T08:53:05Z")
Sorry, I don’t get your question 🙄
I just need more clarification on your original answer; it’s rather terse and technical.
---
### Author:  [@mauro3](https://discourse.julialang.org/u/mauro3)
#### Post date: [January 18, 2018, 9:02am UTC](https://discourse.julialang.org/t/localizing-variables-performance-implications/8329/6 "2018-01-18T09:02:40Z")
@yuyichao said that it depends, so best just profile it. 😉
---
### Author:  [@yuyichao](https://discourse.julialang.org/u/yuyichao)
#### Post date: [January 18, 2018, 11:32am UTC](https://discourse.julialang.org/t/localizing-variables-performance-implications/8329/7 "2018-01-18T11:32:59Z")
I did not say “it depends” for constantness. I just don’t know what is the difference between `a variable (automatically) in any case` and `an explicit assignment like x00 = z.x0`
---
### Author:  [@mauro3](https://discourse.julialang.org/u/mauro3)
#### Post date: [January 18, 2018, 12:23pm UTC](https://discourse.julialang.org/t/localizing-variables-performance-implications/8329/8 "2018-01-18T12:23:15Z")
Sorry!