Why is a sum statement so bizarrely synthesized?

Question

I have 4 buttons on an FPGA dev board so I wrote

function [HEX0] = Bar(KEY)
  n = uint8(sum(KEY, 'native'));
  ...

Unfortunately, HDL Coder turned it into the following chunk of VHDL:

y := '0';

FOR k IN 0 TO 3 LOOP
  y := y OR KEY(k);
END LOOP;

y_0 := '0' & '0' & '0' & '0' & '0' & '0' & '0' & y;

Which I just don't get. Can you help me figure out what's going on here?

Answer 1

To understand this, you have to understand the matlab sum with logical inputs and native option. The sum of logicals is a logical. Thus sum could be replaced with an or

sum([true,true],'native')

And this is exactly what your Coder puts out. The for-Loop implements the sum (sum(KEY, 'native')), where the coder recognizes that it could be implemented using a OR.

Finally, conversion from logical to uint8 is done padding 7 zero bits.