# Performance: tuple unpacking

**URL:** <https://discourse.julialang.org/t/performance-tuple-unpacking/6244>\
**Category:** General Usage\
**Created:** [October 4, 2017, 3:53pm UTC](https://discourse.julialang.org/t/performance-tuple-unpacking/6244 "2017-10-04T15:53:27Z")\
**Posts on this page:** 9\
**Page:** 1

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**Author:** ![Marek\_Kukan](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/marek_kukan/32/10430_2.png) [@Marek\_Kukan](https://discourse.julialang.org/u/Marek_Kukan)\
**Post date:** [October 4, 2017, 3:53pm UTC](https://discourse.julialang.org/t/performance-tuple-unpacking/6244/1 "2017-10-04T15:53:27Z")

</div>

Why is the first expression ~2.5x slower?

```julia
julia> x = rand(10^6);

julia> y = rand(10^6);

julia> @btime sum(a * b for (a, b) in zip(x, y));
  4.280 ms (5 allocations: 112 bytes)

julia> @btime sum(t[1] * t[2] for t in zip(x, y));
  1.585 ms (5 allocations: 112 bytes)

```

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<div class="post-metadata">

**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:** [October 4, 2017, 5:15pm UTC](https://discourse.julialang.org/t/performance-tuple-unpacking/6244/2 "2017-10-04T17:15:07Z")

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The generated code looks identical so this is kinda weird.

However, testing this on 0.7, there doesn’t seem to be a difference in timing anymore.

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<div class="post-metadata">

**Author:** ![StefanKarpinski](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stefankarpinski/32/24_2.png) [@StefanKarpinski](https://discourse.julialang.org/u/StefanKarpinski)\
**Post date:** [October 4, 2017, 5:54pm UTC](https://discourse.julialang.org/t/performance-tuple-unpacking/6244/3 "2017-10-04T17:54:00Z")

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What happens if you evaluate them in a fresh session in the opposite order?

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<div class="post-metadata">

**Author:** ![tkoolen](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tkoolen/32/1603_2.png) [@tkoolen](https://discourse.julialang.org/u/tkoolen)\
**Post date:** [October 5, 2017, 4:01am UTC](https://discourse.julialang.org/t/performance-tuple-unpacking/6244/4 "2017-10-05T04:01:43Z")

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Order doesn’t matter on my machine. Using `@time` instead of `@btime` doesn’t result in this discrepancy. Interpolating the variables in the `@btime` version doesn’t change things. The `code_llvm` for the full call including the `btime` macro seems to be a bit different between the two…

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<div class="post-metadata">

**Author:** ![tkoolen](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tkoolen/32/1603_2.png) [@tkoolen](https://discourse.julialang.org/u/tkoolen)\
**Post date:** [October 5, 2017, 4:27am UTC](https://discourse.julialang.org/t/performance-tuple-unpacking/6244/5 "2017-10-05T04:27:33Z")

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Just to be clear, how are you running this code on nightly? With BenchmarkTools 0.0.8 or latest master on Julia 92fa0f3ace (0 days old master), I get

```julia
julia> @btime sum(a * b for (a, b) in zip(x, y));
ERROR: syntax: invalid syntax (escape (call (outerref sum) (call (top Generator) (block (null) (block (= #9 (new ##9#11)) (null) (unnecessary #9))) (call (outerref zip) (outerref x) (outerref y)))))
Stacktrace:
 [1] generate_benchmark_definition(::Module, ::Array{Symbol,1}, ::Array{Any,1}, ::Expr, ::Void, ::Void, ::BenchmarkTools.Parameters) at /Users/twan/code/julia/RigidBodyDynamics/v0.7/BenchmarkTools/src/execution.jl:289

julia> @btime sum(t[1] * t[2] for t in zip(x, y));
ERROR: syntax: invalid syntax (escape (call (outerref sum) (call (top Generator) (block (null) (block (= #12 (new ##12#14)) (null) (unnecessary #12))) (call (outerref zip) (outerref x) (outerref y)))))
Stacktrace:
 [1] generate_benchmark_definition(::Module, ::Array{Symbol,1}, ::Array{Any,1}, ::Expr, ::Void, ::Void, ::BenchmarkTools.Parameters) at /Users/twan/code/julia/RigidBodyDynamics/v0.7/BenchmarkTools/src/execution.jl:289

```

([https://github.com/JuliaCI/BenchmarkTools.jl/issues/69](https://github.com/JuliaCI/BenchmarkTools.jl/issues/69))

I can get things to work using

```julia
b = @benchmarkable sum(a * b for (a, b) in zip(x, y))
run(b)

```

in which case there is no difference between the two (while there still is a difference on 0.6 when you first use `@benchmarkable` and then `run`).

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<div class="post-metadata">

**Author:** ![Ralph\_Smith](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/ralph_smith/32/10344_2.png) [@Ralph\_Smith](https://discourse.julialang.org/u/Ralph_Smith)\
**Post date:** [October 5, 2017, 4:41am UTC](https://discourse.julialang.org/t/performance-tuple-unpacking/6244/6 "2017-10-05T04:41:07Z")

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How do you see the code in the anonymous function built by the `Generator` constructor?

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<div class="post-metadata">

**Author:** ![tkoolen](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tkoolen/32/1603_2.png) [@tkoolen](https://discourse.julialang.org/u/tkoolen)\
**Post date:** [October 5, 2017, 4:52am UTC](https://discourse.julialang.org/t/performance-tuple-unpacking/6244/7 "2017-10-05T04:52:16Z")

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Good call; there is a difference there

```julia
julia> g1 = Base.Generator(a * b for (a, b) in zip(x, y));

julia> @code_llvm g1.f(first(g1.iter))

define double @"julia_#35_61397"([2 x double]* nocapture readonly dereferenceable(16)) #0 !dbg !5 {
top:
  %1 = getelementptr inbounds [2 x double], [2 x double]* %0, i64 0, i64 0
  %2 = load double, double* %1, align 8
  %3 = getelementptr inbounds [2 x double], [2 x double]* %0, i64 0, i64 1
  %4 = load double, double* %3, align 8
  %5 = fmul double %2, %4
  ret double %5
}

julia> g2 = Base.Generator(t[1] * t[2] for t in zip(x, y));

julia> @code_llvm g2.f(first(g2.iter))

define double @"julia_#37_61406"([2 x double]* nocapture readonly dereferenceable(16)) #0 !dbg !5 {
top:
  %1 = getelementptr inbounds [2 x double], [2 x double]* %0, i64 0, i64 0
  %2 = getelementptr inbounds [2 x double], [2 x double]* %0, i64 0, i64 1
  %3 = load double, double* %1, align 8
  %4 = load double, double* %2, align 8
  %5 = fmul double %3, %4
  ret double %5
}

```

edit: but not in the `code_native`; I’m definitely not an expert on LLVM IR (does it just not matter that things are shuffled around in `code_llvm`?)

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<div class="post-metadata">

**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:** [October 5, 2017, 6:03am UTC](https://discourse.julialang.org/t/performance-tuple-unpacking/6244/8 "2017-10-05T06:03:24Z")

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You put it in a function.

Edit: Perhaps this doesn’t work and I might have just seen the setup to the call. In that case, woops.

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<div class="post-metadata">

**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:** [October 5, 2017, 6:11am UTC](https://discourse.julialang.org/t/performance-tuple-unpacking/6244/9 "2017-10-05T06:11:14Z")

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I saw the difference with `@time` as well.