# Strange Allocation in \`@generated\`
**URL:** https://discourse.julialang.org/t/strange-allocation-in-generated/10521
**Category:** General Usage
**Tags:** metaprogramming
**Created:** [April 24, 2018, 11:49pm UTC](https://discourse.julialang.org/t/strange-allocation-in-generated/10521 "2018-04-24T23:49:13Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@cortner](https://discourse.julialang.org/u/cortner)
#### Post date: [April 24, 2018, 11:49pm UTC](https://discourse.julialang.org/t/strange-allocation-in-generated/10521/1 "2018-04-24T23:49:13Z")
I use a generated function for fast evaluation of monomials and their gradients. The following example could be easily rewritten without a generated function, but I don’t see how to easily implement its gradient without `@generated`. But here the problem I have is easier to see:
```nohighlight
using StaticArrays, BenchmarkTools
@generated function monomial(α, x::SVector{K}) where {K}
# @assert length(α) >= K
ex = "@fastmath "
for i = 1:K
ex *= "x[$i]^α[$i] * "
end
ex = ex[1:end-3]
quote
$(parse(ex))
end
end
α3 = (1,2,3)
x3 = @SVector rand(3)
@btime monomial($α3, $x3) # 15.546 ns (0 allocations: 0 bytes)
α6 = (1,2,3,2,5,2)
x6 = @SVector rand(6)
@btime monomial($α6, $x6) # 2.037 μs (29 allocations: 528 bytes)
```
is the length of x is 1,2,3,4,5 then there is no allocation, if it is 6, then there is an allocation. Does anybody have an idea what is going on here?
EDITS:
- if I turn off the `@fastmath` macro then the allocation goes to zero! But I really need the `@fastmath`, it gives me a factor 5-10 performance gain here.
- If I apply the `@fastmath` to each individual `x[I]^a[I]` then the allocation goes down to zero as well, but when the length of the vector becomes 8, then it start allocating again
---
### Author:  [@cortner](https://discourse.julialang.org/u/cortner)
#### Post date: [April 24, 2018, 11:58pm UTC](https://discourse.julialang.org/t/strange-allocation-in-generated/10521/2 "2018-04-24T23:58:59Z")
I now have a work-around:
```julia
@generated function monomial(α, x::SVector{K, T}) where {K, T}
# @assert length(α) >= K
ex_f = "f = SVector{$K, $T}("
for i = 1:K
ex_f *= "x[$i]^α[$i], "
end
ex_f = ex_f[1:end-2] * ")"
quote
$(parse(ex_f))
return prod(f)
end
end
```
but it is still strange and I’d love to find out what goes wrong?
---
### Author:  [@fredrikekre](https://discourse.julialang.org/u/fredrikekre)
#### Post date: [April 25, 2018, 8:33am UTC](https://discourse.julialang.org/t/strange-allocation-in-generated/10521/3 "2018-04-25T08:33:55Z")
See [https://github.com/JuliaLang/julia/issues/22275](https://github.com/JuliaLang/julia/issues/22275)
FWIW I tested your examples on Julia master and it is definitely better there.
---
### Author:  [@cortner](https://discourse.julialang.org/u/cortner)
#### Post date: [April 25, 2018, 5:41pm UTC](https://discourse.julialang.org/t/strange-allocation-in-generated/10521/4 "2018-04-25T17:41:15Z")
Thank you - that seems to match my problem