# Unnecessary \`where {T}\` causes huge performance drop?

**URL:** <https://discourse.julialang.org/t/unnecessary-where-t-causes-huge-performance-drop/31078>\
**Category:** General Usage\
**Created:** [November 14, 2019, 9:56am UTC](https://discourse.julialang.org/t/unnecessary-where-t-causes-huge-performance-drop/31078 "2019-11-14T09:56:08Z")\
**Posts on this page:** 7\
**Page:** 1

<div class="post-metadata">

**Author:** ![tamasgal](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tamasgal/32/27946_2.png) [@tamasgal](https://discourse.julialang.org/u/tamasgal)\
**Post date:** [November 14, 2019, 9:56am UTC](https://discourse.julialang.org/t/unnecessary-where-t-causes-huge-performance-drop/31078/1 "2019-11-14T09:56:08Z")

</div>

This might be a silly question but I something which I do not understand. I was playing around with different implementations of a function to check the performance changes and sometimes I left an unnecessary `where {T}` in the function line. Then I was wondering about weird results in the `@btime` output and could not reproduce the fastest run, until I figured out that `where {T}` has in an impact, even if no `T` is defined or used anywhere:

```julia
function f1(x) where {T}
    return x*2
end

function f2(x)
    return x*2
end

function f3(::Type{T}, x) where {T}
    return x*2
end

```

I thought all three functions will compile to the same machine code but `f1` does is not playing the game:

```julia
a = rand(1000);

@btime f1.($a);
  14.736 μs (2001 allocations: 39.19 KiB)

@btime f2.($a);
  471.755 ns (1 allocation: 7.94 KiB)

@btime f3.(Bool, $a);
  445.223 ns (1 allocation: 7.94 KiB)

```

What are those allocations and what’s happening here?

---

<div class="post-metadata">

**Author:** ![oheil](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/oheil/32/220745_2.png) [@oheil](https://discourse.julialang.org/u/oheil)\
**Post date:** [November 14, 2019, 10:34am UTC](https://discourse.julialang.org/t/unnecessary-where-t-causes-huge-performance-drop/31078/2 "2019-11-14T10:34:00Z")

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```julia
julia> @code_llvm f1(1.0)

; @ REPL[1]:2 within `f1'
; Function Attrs: uwtable
define nonnull %jl_value_t addrspace(10)* @japi3_f1_16234(%jl_value_t addrspace(10)**, %jl_value_t addrspace(10)*, %jl_value_t addrspace(10)**, i32) #0 {
top:
  %4 = alloca %jl_value_t addrspace(10)**, align 8
  store volatile %jl_value_t addrspace(10)**%2, %jl_value_t addrspace(10)*** %4, align 8
  %5 = call %jl_value_t***inttoptr (i64 1801343456 to %jl_value_t*** ()*)() #3
  %6 = bitcast %jl_value_t addrspace(10)**%2 to double addrspace(10)**
  %7 = load double addrspace(10)*, double addrspace(10)** %6, align 8
; ┌ @ promotion.jl:314 within `*' @ float.jl:399
   %8 = load double, double addrspace(10)* %7, align 8
   %9 = fmul double %8, 2.000000e+00
; └
  %10 = bitcast %jl_value_t*** %5 to i8*
  %11 = call noalias nonnull %jl_value_t addrspace(10)* @jl_gc_pool_alloc(i8* %10, i32 1744, i32 16) #1
  %12 = bitcast %jl_value_t addrspace(10)* %11 to %jl_value_t addrspace(10)* addrspace(10)*
  %13 = getelementptr %jl_value_t addrspace(10)*, %jl_value_t addrspace(10)* addrspace(10)* %12, i64 -1
  store %jl_value_t addrspace(10)* addrspacecast (%jl_value_t* inttoptr (i64 114258656 to %jl_value_t*) to %jl_value_t addrspace(10)*), %jl_value_t addrspace(10)* addrspace(10)* %13
  %14 = bitcast %jl_value_t addrspace(10)* %11 to double addrspace(10)*
  store double %9, double addrspace(10)* %14, align 8
  ret %jl_value_t addrspace(10)* %11
}

julia> @code_llvm f2(1.0)

; @ REPL[2]:2 within `f2'
; Function Attrs: uwtable
define double @julia_f2_16235(double) #0 {
top:
; ┌ @ promotion.jl:314 within `*' @ float.jl:399
   %1 = fmul double %0, 2.000000e+00
; └
  ret double %1
}

```

For me this looks like LLVM is creating a Float64 from whatever is coming, but its more guessing than understanding.

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<div class="post-metadata">

**Author:** ![tamasgal](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tamasgal/32/27946_2.png) [@tamasgal](https://discourse.julialang.org/u/tamasgal)\
**Post date:** [November 14, 2019, 10:43am UTC](https://discourse.julialang.org/t/unnecessary-where-t-causes-huge-performance-drop/31078/3 "2019-11-14T10:43:44Z")

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I don’t understand why the LLVM code differs at all 😕

---

<div class="post-metadata">

**Author:** ![Tamas\_Papp](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tamas_papp/32/25949_2.png) [@Tamas\_Papp](https://discourse.julialang.org/u/Tamas_Papp)\
**Post date:** [November 14, 2019, 11:11am UTC](https://discourse.julialang.org/t/unnecessary-where-t-causes-huge-performance-drop/31078/4 "2019-11-14T11:11:36Z")

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I also find it strange, since

```julia
julia> @code_warntype f1(1.0)
Variables
  #self#::Core.Compiler.Const(f1, false)
  x::Float64

Body::Float64
1 ─ %1 = (x * 2)::Float64
└── return %1

julia> @code_warntype f2(1.0)
Variables
  #self#::Core.Compiler.Const(f2, false)
  x::Float64

Body::Float64
1 ─ %1 = (x * 2)::Float64
└── return %1

```

I can replicate this problem on `v"1.3.0-rc4.1"`. Also note that broadcasting is not needed for an MWE, as

```nohighlight
julia> @btime f1(1.0)
  15.680 ns (1 allocation: 16 bytes)
2.0

julia> @btime f2(1.0)
  0.027 ns (0 allocations: 0 bytes)
2.0

```

Please check if there is an existing issue about this, and if not, open one.

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<div class="post-metadata">

**Author:** ![tamasgal](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tamasgal/32/27946_2.png) [@tamasgal](https://discourse.julialang.org/u/tamasgal)\
**Post date:** [November 14, 2019, 11:25am UTC](https://discourse.julialang.org/t/unnecessary-where-t-causes-huge-performance-drop/31078/5 "2019-11-14T11:25:31Z")

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Thanks for the feedback, I was using 1.3rc2.

Yes the broadcasting was a copy paste leftover 😉

I’ll check older Julia versions too and look through the issues then.

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<div class="post-metadata">

**Author:** ![oxinabox](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/oxinabox/32/206603_2.png) [@oxinabox](https://discourse.julialang.org/u/oxinabox)\
**Post date:** [November 14, 2019, 12:54pm UTC](https://discourse.julialang.org/t/unnecessary-where-t-causes-huge-performance-drop/31078/6 "2019-11-14T12:54:25Z")

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I suspect it is because `@code_warntype` is lying to you  
[https://github.com/JuliaLang/julia/pull/32817](https://github.com/JuliaLang/julia/pull/32817)

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<div class="post-metadata">

**Author:** ![tamasgal](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tamasgal/32/27946_2.png) [@tamasgal](https://discourse.julialang.org/u/tamasgal)\
**Post date:** [November 14, 2019, 2:12pm UTC](https://discourse.julialang.org/t/unnecessary-where-t-causes-huge-performance-drop/31078/7 "2019-11-14T14:12:02Z")

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Alright, I could not find anything related so I quickly opened an issue.

[https://github.com/JuliaLang/julia/issues/33847](https://github.com/JuliaLang/julia/issues/33847)

Update: I did a silly copy&paste mistake, which made me believe that in Julia 0.7 it’s “OK”. It is not… Julia 0.7 shows the same output.