# @turbo macro gives incorrect results

**URL:** <https://discourse.julialang.org/t/turbo-macro-gives-incorrect-results/89280>\
**Category:** Performance\
**Tags:** loopvectorization\
**Created:** [October 26, 2022, 8:00am UTC](https://discourse.julialang.org/t/turbo-macro-gives-incorrect-results/89280 "2022-10-26T08:00:25Z")\
**Posts on this page:** 5\
**Page:** 1

<div class="post-metadata">

**Author:** ![sidelkin](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/sidelkin/32/43911_2.png) [@sidelkin](https://discourse.julialang.org/u/sidelkin)\
**Post date:** [October 26, 2022, 8:00am UTC](https://discourse.julialang.org/t/turbo-macro-gives-incorrect-results/89280/1 "2022-10-26T08:00:25Z")

</div>

I often use the `@turbo` macro for calculations. But I recently discovered that using it sometimes gives incorrect results. Below are minimal working examples where different options are compared.

```julia
using LoopVectorization

tbuf = [0.0, 0.0]
tbuf2 = [0.0, 0.0]
tbuf3 = [0.0, 0.0]

@turbo for i = 1:length(tbuf)    
    tbuf[i] = 1.0
    tbuf[i] += 1.0
    tbuf[i] *= -1.0
end

@inbounds @simd for i = 1:length(tbuf2)    
    tbuf2[i] = 1.0
    tbuf2[i] += 1.0
    tbuf2[i] *= -1.0
end

@inbounds for i = 1:length(tbuf3)    
    tbuf3[i] = 1.0
    tbuf3[i] += 1.0
    tbuf3[i] *= -1.0
end

```

As a result, we have

```julia
julia> tbuf
2-element Vector{Float64}:
 -1.0
 -1.0

julia> tbuf2
2-element Vector{Float64}:
 -2.0
 -2.0

julia> tbuf3
2-element Vector{Float64}:
 -2.0
 -2.0

```

Can someone explain why that is?

My software versions are

```julia
julia> versioninfo()
Julia Version 1.6.7
Commit 3b76b25b64 (2022-07-19 15:11 UTC)
Platform Info:
  OS: Windows (x86_64-w64-mingw32)
  CPU: Intel(R) Xeon(R) CPU E5-1620 v2 @ 3.70GHz
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-11.0.1 (ORCJIT, ivybridge)
(@v1.6) pkg> st
  [bdcacae8] LoopVectorization v0.12.136

```

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<div class="post-metadata">

**Author:** ![nilshg](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/nilshg/32/2283_2.png) [@nilshg](https://discourse.julialang.org/u/nilshg)\
**Post date:** [October 26, 2022, 8:08am UTC](https://discourse.julialang.org/t/turbo-macro-gives-incorrect-results/89280/2 "2022-10-26T08:08:32Z")

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I’m not a LoopVectorization user so others will chime in but did you look at the package’s Readme, in particular this part:

> We expect that any time you use the `@turbo` macro with a given block of code that you:  
> …  
> 3. Are not relying on a specific execution order. `@turbo` can and will re-order operations and loops inside its scope, so the correctness cannot depend on a particular order. You cannot implement `cumsum` with `@turbo`.

---

<div class="post-metadata">

**Author:** ![sidelkin](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/sidelkin/32/43911_2.png) [@sidelkin](https://discourse.julialang.org/u/sidelkin)\
**Post date:** [October 26, 2022, 8:17am UTC](https://discourse.julialang.org/t/turbo-macro-gives-incorrect-results/89280/3 "2022-10-26T08:17:34Z")

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Thank you! Apparently this is the reason.

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<div class="post-metadata">

**Author:** ![mikmoore](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mikmoore/32/31109_2.png) [@mikmoore](https://discourse.julialang.org/u/mikmoore)\
**Post date:** [October 26, 2022, 3:10pm UTC](https://discourse.julialang.org/t/turbo-macro-gives-incorrect-results/89280/4 "2022-10-26T15:10:53Z")

</div>

> [@nilshg](#):
>
> 1. Are not relying on a specific execution order. `@turbo` can and will re-order operations and loops inside its scope, so the correctness cannot depend on a particular order. You cannot implement `cumsum` with `@turbo`.

I admit I’m still a little confused here. Clearly, this example does not rely on the loop execution order – each loop operates on only a single element. But they do rely on the instruction order within a single loop iteration. Almost every program does, for that matter.

I haven’t used it before, but I had assumed LoopVectorization was not usually in the business of transforming dependent instruction orderings. It’s hard to write a program when it will occasionally transform `(a + b) * c` to `a * c`, as appears to have happened here.

Why did this fail? ~~Would it have worked if it were instead written as this?~~ This appears to work correctly

```julia
@turbo for i = 1:length(tbuf)    
    z = 1.0
    z += 1.0
    tbuf[i] = z * -1.0
end

```

So the issue appears related to the repeated accesses to `tbuf[i]` in the original. Can someone clarify what patterns are and aren’t safe?

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<div class="post-metadata">

**Author:** ![dlakelan](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/dlakelan/32/8491_2.png) [@dlakelan](https://discourse.julialang.org/u/dlakelan)\
**Post date:** [October 26, 2022, 3:53pm UTC](https://discourse.julialang.org/t/turbo-macro-gives-incorrect-results/89280/5 "2022-10-26T15:53:47Z")

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> [@mikmoore](#):
>
> Can someone clarify what patterns are and aren’t safe?

I’m not sure but if I were to guess, it won’t reorder operations within a single expression, but it may reorder statements, so you can’t rely on assignment occurring in the order you specify.