# How to make SIMD work?

**URL:** <https://discourse.julialang.org/t/how-to-make-simd-work/10646>\
**Category:** General Usage\
**Created:** [May 2, 2018, 1:26am UTC](https://discourse.julialang.org/t/how-to-make-simd-work/10646 "2018-05-02T01:26:00Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![tk3369](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tk3369/32/2824_2.png) [@tk3369](https://discourse.julialang.org/u/tk3369)\
**Post date:** [May 2, 2018, 1:26am UTC](https://discourse.julialang.org/t/how-to-make-simd-work/10646/1 "2018-05-02T01:26:02Z")

</div>

I’m trying follow this [Demystifying Auto-Vectorization in Julia](https://juliacomputing.com/blog/2017/09/27/auto-vectorization-in-julia.html) blog post. I couldn’t see the nice `vaddpd` instructions, however. Is there something that I need to do to enable SIMD? The [E5-2699 v4 processor](https://ark.intel.com/products/91317/Intel-Xeon-Processor-E5-2699-v4-55M-Cache-2_20-GHz) supports AVX2.

```julia
$ julia -O3

julia> versioninfo()
Julia Version 0.6.2
Commit d386e40c17 (2017-12-13 18:08 UTC)
Platform Info:
  OS: Linux (x86_64-pc-linux-gnu)
  CPU: Intel(R) Xeon(R) CPU E5-2699 v4 @ 2.20GHz
  WORD_SIZE: 64
  BLAS: libopenblas (USE64BITINT DYNAMIC_ARCH NO_AFFINITY Haswell)
  LAPACK: libopenblas64_
  LIBM: libopenlibm
  LLVM: libLLVM-3.9.1 (ORCJIT, broadwell)

julia> function mysum(a::Vector)
           total = zero(eltype(a))
           @simd for x in a
               total += x
           end
           return total
       end
mysum (generic function with 1 method)

julia> @code_native mysum(rand(Float64 , 100000)) ;
        .text
Filename: REPL[8]
Source line: 67
        movq 8(%rdi), %rax
Source line: 64
        movq 24(%rdi), %rdx
        xorl %ecx, %ecx
Source line: 79
        testq %rdx, %rdx
        cmovnsq %rdx, %rcx
Source line: 68
        testq %rax, %rax
        jle L206
Source line: 79
        leaq -1(%rcx), %rsi
        cmpq %rdx, %rsi
        jae L210
Source line: 50
        movq (%rdi), %r9
Source line: 66
        leaq 16(%r9), %r8
        movq %rax, %r10
        andq $-4, %r10
        pxor %xmm0, %xmm0
        xorl %edi, %edi
        jmp L192
Source line: 71
L64:
        testq %rax, %rax
        jle L192
Source line: 50
        cmpq $4, %rax
        jae L79
        xorl %edx, %edx
        jmp L165
L79:
        movq %rax, %rdx
        andq $-4, %rdx
        je L163
        movq %xmm0, %xmm0 # xmm0 = xmm0[0],zero
        xorpd %xmm1, %xmm1
Source line: 74
        movq %r10, %rsi
        movq %r8, %rcx
        nopw %cs:(%rax,%rax)
Source line: 50
L112:
        movupd -16(%rcx), %xmm2
        movupd (%rcx), %xmm3
Source line: 4
        addpd %xmm2, %xmm0
        addpd %xmm3, %xmm1
Source line: 50
        addq $32, %rcx
        addq $-4, %rsi
        jne L112
Source line: 4
        addpd %xmm0, %xmm1
        movapd %xmm1, %xmm0
        shufpd $1, %xmm0, %xmm0 # xmm0 = xmm0[1,0]
        addpd %xmm1, %xmm0
        cmpq %rdx, %rax
        je L192
        jmp L165
L163:
        xorl %edx, %edx
Source line: 50
L165:
        movq %rax, %rcx
        subq %rdx, %rcx
        leaq (%r9,%rdx,8), %rdx
        nop
Source line: 4
L176:
        addsd (%rdx), %xmm0
Source line: 71
        addq $8, %rdx
        decq %rcx
        jne L176
        nopl (%rax)
Source line: 66
L192:
        incq %rdi
        cmpq $2, %rdi
        jne L64
Source line: 6
        retq
L206:
        xorps %xmm0, %xmm0
        retq
L210:
        pushq %rbp
        movq %rsp, %rbp
Source line: 79
        movq %rsp, %rax
        leaq -16(%rax), %rsi
        movq %rsi, %rsp
        movq %rcx, -16(%rax)
        movabsq $jl_bounds_error_ints, %rax
        movl $1, %edx
        callq *%rax
        nopw %cs:(%rax,%rax)

```

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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:** [May 2, 2018, 5:50am UTC](https://discourse.julialang.org/t/how-to-make-simd-work/10646/2 "2018-05-02T05:50:45Z")

</div>

Have you [rebuilt the system image for your machine](https://docs.julialang.org/en/stable/devdocs/sysimg/#Building-the-Julia-system-image-1) (gotcha #7 [here](https://www.juliabloggers.com/7-julia-gotchas-and-how-to-handle-them/))?

---

<div class="post-metadata">

**Author:** ![tk3369](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tk3369/32/2824_2.png) [@tk3369](https://discourse.julialang.org/u/tk3369)\
**Post date:** [May 2, 2018, 6:09am UTC](https://discourse.julialang.org/t/how-to-make-simd-work/10646/4 "2018-05-02T06:09:55Z")

</div>

Gotcha #7 got me 🙂  
Thanks

---

<div class="post-metadata">

**Author:** ![LaurentPlagne](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/laurentplagne/32/10103_2.png) [@LaurentPlagne](https://discourse.julialang.org/u/LaurentPlagne)\
**Post date:** [May 2, 2018, 9:20am UTC](https://discourse.julialang.org/t/how-to-make-simd-work/10646/5 "2018-05-02T09:20:19Z")

</div>

A graphical representation of the @simd + AVX2 effect

```julia
using BenchmarkTools
using Plots

pyplot()

function mysum_orig(a::Vector)
    total = zero(eltype(a))
    @simd for x in a
        total += x
    end
    return total
end
function mysum_nosimd(a::Vector)
    total = zero(eltype(a))
    for x in a
        total += x
    end
    return total
end
function mysum_nosimd_index(a::Vector)
    total = zero(eltype(a))
    n=length(a)
    for i=1:n
        @inbounds total += a[i]
    end
    return total
end
function mysum_native(a::Vector)
    return sum(a)
end

function test_myfunction(myfunction, T::Type,n::Int64)
    a=rand(T , n)
    t=@belapsed s=$myfunction($a)
    gflops=n/(t*1.e9)
    print("GFlops=",gflops," (",string(myfunction),")\n")
    return gflops
end

# for MyFloat in [Float32,Float64]
const MyFloat=Float64
for f in [mysum_orig;mysum_nosimd;mysum_nosimd_index;mysum_native]
    s=10
    gflops=[]
    sizes=[]
    for n=1:21
        push!(gflops,test_myfunction(f,MyFloat,s))
        push!(sizes,s)
        s*=2
    end
    plot!(sizes,gflops,xscale = :log10,linewidth=4,ylabel="GFLOPS",xlabel="arraySize",label=string(string(f)," ",string(MyFloat)))
end

savefig("gflops.pdf")
@code_native(mysum_orig(rand(Float64,1000)))ype or paste code here

```

The index based version looks a bit different than the range version…  
 ![gflops](https://global.discourse-cdn.com/julialang/original/3X/2/1/21048471170a32629e0b478d2ec966a06034ebaa.jpeg)