Float to SInt32

Question

I have series of c++ signal processing classes which use 32 bit floats as their primary sample datatype. For example all the oscillator classes return floats for every sample thats requested. This is the same for all the classes, all calculations of samples are in floating point.

I am porting these classes to iOS.. and for performance issues I want to operate in 8.24 fixed point to get the most out of the processor, word has it there are major performance advantages on iOS to crunching integers instead of floats.. I'm currently doing all the calculations in floats, then converting to SInt32 at the final stage before output which means every sample at the final stage needs to be converted.

Do I simply change the datatype used inside my classes from Float to SInt32. So my oscillators and filters etc calculate in fixed point by passing SInt32's around internally instead of floats ??

is it really this simple ? or do I have to completely rewrite all the different algorithms ?

is there any other voodoo I need to understand before taking on this mission ?

Many Thanks for anyone who finds the time to comment on this.. Its much appreciated..

Answer 1

It's mostly a myth. Floating point performance used to be slow if you compiled for armv6 in Thumb mode; this not an issue in armv7 which supports Thumb 2 (I'll avoid further discussion of armv6 which is no longer supported in Xcode). You also want to avoid using doubles, since floats can use the faster NEON (a.k.a. Advanced SIMD Instructions) unit — this is easy to do accidentally; try enabling -Wshorten.

I also doubt you'll get significantly better performance doing an 8.24 multiply, especially over making use of the NEON unit. Changing float int/int32_t/SInt32 will also not automatically do the necessary shifts for an 8.24 multiply.

If you know that converting floats to ints is the slow bit, consider using some of the functions in Accelerate.framework, namely vDSP_vfix16() or vDSP_vfixr16().