Zero size array is nnet_common.h

Created by: KOVI89alipes

In C++ it is illegal to declare an array of zero length. https://stackoverflow.com/a/6180200/14582299

But it's allowed in the GNU C extension https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html

Zero size array appears in the reduce template function

https://github.com/fastmachinelearning/hls4ml/blob/c3509cfeb5f80054e01dead612392a9411ebfbe2/hls4ml/templates/vivado/nnet_utils/nnet_common.h#L47

template<class T, int N, class Op>
 T reduce(T* x, Op op){
	static constexpr int leftN = pow2(floorlog2(N - 1)) > 0 ? pow2(floorlog2(N - 1)) : 0;
	static constexpr int rightN = N - leftN > 0 ? N - leftN : 0;
	if(N == 1){
		return x[0];
	}else if(N == 2){
		return op(x[0],x[1]);
	}else{
		T left[leftN];
		T right[rightN]; <<<<<<<<<<<<
		#pragma HLS array_partition variable=left complete
		#pragma HLS array_partition variable=right complete
		ReduceLeft: for(int i = 0; i < leftN; i++){
			left[i] = x[i];
		}
		ReduceRight: for(int i = 0; i < rightN; i++){
			right[i] = x[i+leftN];
		}
		return op(reduce<T,leftN,Op>(left, op), reduce<T,rightN,Op>(right, op));
	}
 }

And it gives an error in MSVC compilation

Possible fix - replace if with if constexpr

 template<class T, int N, class Op>
 T reduce(T* x, Op op){
	static constexpr int leftN = pow2(floorlog2(N - 1)) > 0 ? pow2(floorlog2(N - 1)) : 0;
	static constexpr int rightN = N - leftN > 0 ? N - leftN : 0;
	if constexpr(N == 1){
		return x[0];
	}else if constexpr (N == 2){
		return op(x[0],x[1]);
	}else{
		T left[leftN];
        T right[rightN];
	
    	#pragma HLS array_partition variable=left complete
		#pragma HLS array_partition variable=right complete
		ReduceLeft: for(int i = 0; i < leftN; i++){
			left[i] = x[i];
		}
		ReduceRight: for(int i = 0; i < rightN; i++){
			right[i] = x[i+leftN];
		}
		return op(reduce<T,leftN,Op>(left, op), reduce<T,rightN,Op>(right, op));
	}
 }