Memory leak in gate bootstrapping API

[j2kun]

The gate bootstrapping API appears to have a memory leak. I've observed this when running long programs, that for each call to bootsAND (or any other function), a new instance of FFT_Processor_Spqlios is created, which in turn allocates new fft/ifft lookup tables, and they are never freed (see the commented-out delete calls in https://github.com/tfhe/tfhe/blob/master/src/libtfhe/fft_processors/spqlios/fft_processor_spqlios.cpp#L118-L119) and the comment in

tfhe/src/libtfhe/tgsw-fft-operations.cpp

Line 95 in be526fb

//TODO attention, improve these new/delete...

that mentions the new/delete need to be improved.

As I understand it, no more development work will go into this repository, so this issue is more of a note to any other users that this code contains a memory leak, and that the fftw backend should probably be used instead. I may try to patch the memory leak myself, but given that there have been no feature/bug fix commits on this repository for almost two years, I will probably not try to get a fix upstreamed.

That said, allocating these lookup tables once instead of on every single gate bootstrapping call would probably improve the performance of the Spqlios part of this library.