# Avoiding branching in loops with macros

**URL:** <https://discourse.julialang.org/t/avoiding-branching-in-loops-with-macros/10453>\
**Category:** General Usage\
**Tags:** metaprogramming\
**Created:** [April 20, 2018, 12:07pm UTC](https://discourse.julialang.org/t/avoiding-branching-in-loops-with-macros/10453 "2018-04-20T12:07:14Z")\
**Posts on this page:** 7\
**Page:** 1

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**Author:** ![vilim](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/vilim/32/12164_2.png) [@vilim](https://discourse.julialang.org/u/vilim)\
**Post date:** [April 20, 2018, 12:07pm UTC](https://discourse.julialang.org/t/avoiding-branching-in-loops-with-macros/10453/1 "2018-04-20T12:07:14Z")

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Is there a common pattern in Julia (perhaps with some kind of a macro) for the following case?  
I have a function that runs a model through several thousands of time steps, and I want to have two versions:

- one where each time-step is saved,
- just a sum over all time steps is computed  
As the parameters of this model will be optimized using Optim.jl on many sets, I would like to keep the functions as performant as possible and avoid conditions inside the loop, yet without copy-pasting a large part of code (only two lines are different).

---

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**Author:** ![kristoffer.carlsson](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/kristoffer.carlsson/32/22_2.png) [@kristoffer.carlsson](https://discourse.julialang.org/u/kristoffer.carlsson)\
**Post date:** [April 20, 2018, 12:18pm UTC](https://discourse.julialang.org/t/avoiding-branching-in-loops-with-macros/10453/2 "2018-04-20T12:18:07Z")

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You can use `@static if` e.g.

```julia
const SAVE_TIMESTEP = false

function f()
    @static if SAVE_TIMESTEP
        ....
    end
    ....
end

```

but if the conditional is statically known, it is likely to get optimized away without need of `@static`.

Have you benchmarked to see that the conditionals are actually expensive. CPUs are very good at predicting branches.

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**Author:** ![foobar\_lv2](https://avatars.discourse-cdn.com/v4/letter/f/ee59a6/32.png) [@foobar\_lv2](https://discourse.julialang.org/u/foobar_lv2)\
**Post date:** [April 20, 2018, 2:33pm UTC](https://discourse.julialang.org/t/avoiding-branching-in-loops-with-macros/10453/3 "2018-04-20T14:33:19Z")

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```julia
function foo(x,v::Val{T}) where T
       if T 
          return x
       else 
         return -x
       end
end

julia> @code_llvm foo(12, Val(true));

define i64 @julia_foo_66629(i64) #0 !dbg !5 {
top:
  ret i64 %0
}

julia> @code_llvm foo(12, Val(false));

define i64 @julia_foo_66663(i64) #0 !dbg !5 {
top:
  %1 = sub i64 0, %0
  ret i64 %1
}

```

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<div class="post-metadata">

**Author:** ![vilim](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/vilim/32/12164_2.png) [@vilim](https://discourse.julialang.org/u/vilim)\
**Post date:** [April 20, 2018, 4:21pm UTC](https://discourse.julialang.org/t/avoiding-branching-in-loops-with-macros/10453/4 "2018-04-20T16:21:21Z")

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@kristoffer.carlsson @foobar_lv2 thank you for the fast responses!

I was hanging on too much on my lessons from my early computer science courses which assumed very little cleverness on the part of the compiler.

Is the following then a good pattern to handle multiple dispatch of optional parameters within the function body?

```julia
function foo(x, y::T=nothing) where T
    if T == Nothing
        sum = zero(eltype(x))
    end
    for i in eachindex(x)
        if T == Nothing
            sum += x[i]
        else
            y[i] = x[i]+1
        end
    end
    if T == Nothing
        return sum
    else
        return y
    end
end

```

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<div class="post-metadata">

**Author:** ![mbauman](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mbauman/32/31082_2.png) [@mbauman](https://discourse.julialang.org/u/mbauman)\
**Post date:** [April 20, 2018, 4:23pm UTC](https://discourse.julialang.org/t/avoiding-branching-in-loops-with-macros/10453/5 "2018-04-20T16:23:00Z")

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You don’t even need the type parameter — just doing `if y isa Nothing` has enough static information that those branches will be compiled away.

---

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**Author:** ![foobar\_lv2](https://avatars.discourse-cdn.com/v4/letter/f/ee59a6/32.png) [@foobar\_lv2](https://discourse.julialang.org/u/foobar_lv2)\
**Post date:** [April 20, 2018, 6:39pm UTC](https://discourse.julialang.org/t/avoiding-branching-in-loops-with-macros/10453/6 "2018-04-20T18:39:18Z")

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I think the official way is something like

```julia
struct Verbose_T end
const verbose = Verbose_T()
struct Debug_T end
const debug = Debug_T()

function foo(x, loglevel=nothing)
    if loglevel==debug
       @show x
    elseif loglevel == verbose
       println("entering foo")
    else
       #we need no default branch today
    end
    return x   
end

```

Feel free to use more reasonable / julian type names. I like to use intentionally lenghty/nonclashing names for stuff that really is just internals.

edit: You call by

```julia
foo(12)
foo(12, debug)
foo(12, verbose)

```

Also, my suggestions are slightly outdated on current master. I think nowadays keyword parameters are the way to go.

---

<div class="post-metadata">

**Author:** ![vilim](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/vilim/32/12164_2.png) [@vilim](https://discourse.julialang.org/u/vilim)\
**Post date:** [April 23, 2018, 11:21am UTC](https://discourse.julialang.org/t/avoiding-branching-in-loops-with-macros/10453/7 "2018-04-23T11:21:50Z")

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Thanks for the replies! I suppose I was aiming at making types explicit, since the function is going to be run many times inside the optimization function and I wanted to avoid type instability, but I see now that it is not necessary.