perf,memory: fft without precompute #472
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ivokub
approved these changes
Dec 21, 2023
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Conditional accept - I do not have enough knowledge about FFTs and twiddles to understand if done correctly. But making the computation optional seems correct.
If not urgent then I would ask @ThomasPiellard to confirm.
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In short, this PR allows for
fft.NewDomain()to have an optionfft.WithoutPrecompute()which initializes the domain without precomputing (all) twiddles & coset tables.For large domains, this save a huge amount of memory at the cost of 20-30% CPU perf downgrade.
To simplify code, it modifies existing inner DIF and DIT fft methods to take a precomputed twiddle table and a starting stage; so twiddles can still be precomputed but we skip the first stages (each stage we skip we save domain.Cardinality / stage*2 memory).