# FFT planning with flags fails for CUDA

**URL:** <https://discourse.julialang.org/t/fft-planning-with-flags-fails-for-cuda/56870>\
**Category:** GPU\
**Tags:** fftw, cuda\
**Created:** [March 10, 2021, 11:25am UTC](https://discourse.julialang.org/t/fft-planning-with-flags-fails-for-cuda/56870 "2021-03-10T11:25:18Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![roflmaostc](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/roflmaostc/32/30123_2.png) [@roflmaostc](https://discourse.julialang.org/u/roflmaostc)\
**Post date:** [March 10, 2021, 11:25am UTC](https://discourse.julialang.org/t/fft-planning-with-flags-fails-for-cuda/56870/1 "2021-03-10T11:25:19Z")

</div>

Hey,

I was trying to do a FFT plan for a CuArray. The following works:

```julia
julia> using CUDA, CUDA.CUFFT

julia> x = CUDA.rand(2, 2)
2×2 CuArray{Float32, 2}:
 0.318697 0.144699
 0.631969 0.433798

julia> fft(x)
2×2 CuArray{ComplexF32, 2}:
  1.52916+0.0im 0.37217+0.0im
 -0.60237+0.0im -0.0241727+0.0im

julia> p = plan_fft(x);

julia> p * x
2×2 CuArray{ComplexF32, 2}:
  1.52916+0.0im 0.37217+0.0im
 -0.60237+0.0im -0.0241727+0.0im

```

Unfortunately I cannot get planning with a extra flag working:

```julia
help?> plan_fft
search: plan_fft plan_fft! plan_rfft plan_ifft plan_bfft plan_ifft! plan_bfft! plan_irfft plan_brfft

  plan_fft(A [, dims]; flags=FFTW.ESTIMATE, timelimit=Inf)
[...]

julia> p = plan_fft(x, flags=FFTW.ESTIMATE);
ERROR: UndefVarError: FFTW not defined
Stacktrace:
 [1] top-level scope
   @ REPL[10]:1

```

Which makes sense since FFTW is not loaded. But the following is also clear to cause troubles since FFTW is not connected to CUDA (is it?). So the help is quite misleading.

```julia
julia> using FFTW

julia> p = plan_fft(x, flags=FFTW.ESTIMATE);

julia> p * x
ERROR: MethodError: *(::FFTW.cFFTWPlan{ComplexF32, -1, false, 2, UnitRange{Int64}}, ::CuArray{ComplexF32, 2}) is ambiguous. Candidates:
  *(p::AbstractFFTs.Plan{T}, x::CuArray) where T in CUDA.CUFFT at /home/fxw/.julia/packages/CUDA/UC5gt/lib/cufft/fft.jl:11
  *(p::FFTW.cFFTWPlan{T, K, false, N, G} where G where N, x::StridedArray{T, N}) where {T, K, N} in FFTW at /home/fxw/.julia/packages/FFTW/G3lSO/src/fft.jl:714
Possible fix, define
  *(::FFTW.cFFTWPlan{T, K, false, N, G} where G where N, ::CuArray{T, N}) where {T, K, N}
Stacktrace:
 [1] *(p::FFTW.cFFTWPlan{ComplexF32, -1, false, 2, UnitRange{Int64}}, x::CuArray{Float32, 2})
   @ CUDA.CUFFT ~/.julia/packages/CUDA/UC5gt/lib/cufft/fft.jl:11
 [2] top-level scope
   @ REPL[13]:1

```

I tried to find something about the flags for CUFFT but couldn’t find something useful 😕  
So can anybody point me in the right direction?

Thanks,

Felix

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**Author:** ![maleadt](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/maleadt/32/10097_2.png) [@maleadt](https://discourse.julialang.org/u/maleadt)\
**Post date:** [March 10, 2021, 11:51am UTC](https://discourse.julialang.org/t/fft-planning-with-flags-fails-for-cuda/56870/2 "2021-03-10T11:51:41Z")

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That ‘misleading’ docstring comes from AbstractFFTs.jl, and those flags are FFTW.jl specific. AFAIK the CUDA.jl wrappers for CUFFT do not support any flags currently. If that’s a problem, and you want a flag that’s supported by the underlying CUFFT library, you could have a look at exposing that through the wrappers in here: [CUDA.jl/fft.jl at master · JuliaGPU/CUDA.jl · GitHub](https://github.com/JuliaGPU/CUDA.jl/blob/master/lib/cufft/fft.jl)

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**Author:** ![roflmaostc](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/roflmaostc/32/30123_2.png) [@roflmaostc](https://discourse.julialang.org/u/roflmaostc)\
**Post date:** [March 10, 2021, 12:26pm UTC](https://discourse.julialang.org/t/fft-planning-with-flags-fails-for-cuda/56870/3 "2021-03-10T12:26:30Z")

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Thanks a lot!

On Page 82 in the cufft [manual](https://www.google.com/url?q=https://docs.nvidia.com/cuda/pdf/CUFFT_Library.pdf&sa=U&ved=2ahUKEwi-lcqi26XvAhVE2qQKHfvCAmAQFjABegQIBRAB&usg=AOvVaw1SIz1i-M_N8spfk3dO4FOC) it is mentioned:  
“Planner flags are ignored and the same plan is returned regardless”

So I’m not sure whether these flags exist at all? Since it’s not critical for me, I’ll take the default.