addWithMultiply(FloatType *dest, const FloatType *src1, const FloatType *src2, CountType num)
Multiplies each source1 value by the corresponding source2 value, then adds it to the destination value.
It works as dest[i] = dest[i] + src1[i] * src2[i]. Therefore, in your case out should also be a array.
BTW, in your case, a single loop might be good enough. juce::dsp::FIR is also using a single loop. See:
