# Element-wise vector multiplication and fusing dot

**URL:** <https://discourse.julialang.org/t/element-wise-vector-multiplication-and-fusing-dot/1228>\
**Category:** New to Julia\
**Created:** [December 31, 2016, 1:26pm UTC](https://discourse.julialang.org/t/element-wise-vector-multiplication-and-fusing-dot/1228 "2016-12-31T13:26:13Z")\
**Posts on this page:** 1\
**Showing post:** 2

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**Author:** ![stevengj](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stevengj/32/71_2.png) [@stevengj](https://discourse.julialang.org/u/stevengj)\
**Post date:** [December 31, 2016, 2:03pm UTC](https://discourse.julialang.org/t/element-wise-vector-multiplication-and-fusing-dot/1228/2 "2016-12-31T14:03:36Z")

</div>

Don’t do timing measurements in global scope. I would also strongly recommend the BenchmarkTools package, and using `@benchmark myfunc(...)` rather than `@time myfunc(...)`, since `@benchmark` will run a timing loop for you and gather statistics.

In Julia 0.5 and older versions, the `.*` operator is _defined_, but it is just a function call and is not fusing. In particular, it does not fuse with the assignment in `.=`, so `z .= x .* y` is equivalent to:

```julia
tmp = x .* y # allocate a new temporary array for elementwise x * y
z .= tmp # write the tmp into z, equivalent to copy!(z, tmp)

```

If you want the fusing version of this operation in 0.5, you need `z .= (*).(x,y)`

With `bar!(x, y, z) = z .= (*).(x, y)`, I get:

```julia
julia> @benchmark foo!($x, $y, $z)
BenchmarkTools.Trial: 
  samples: 10000
  evals/sample: 77
  time tolerance: 5.00%
  memory tolerance: 1.00%
  memory estimate: 7.98 kb
  allocs estimate: 3
  minimum time: 795.00 ns (0.00% GC)
  median time: 1.23 μs (0.00% GC)
  mean time: 1.95 μs (25.90% GC)
  maximum time: 36.09 μs (67.13% GC)

julia> @benchmark bar!($x, $y, $z)
BenchmarkTools.Trial: 
  samples: 10000
  evals/sample: 413
  time tolerance: 5.00%
  memory tolerance: 1.00%
  memory estimate: 32.00 bytes
  allocs estimate: 1
  minimum time: 241.00 ns (0.00% GC)
  median time: 248.00 ns (0.00% GC)
  mean time: 289.79 ns (0.58% GC)
  maximum time: 4.35 μs (92.72% GC)

```

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