Bitwise Reduction Operators in C

Question

Are there unary bitwise reduction operators in C like there are in Verilog?

Like in Verilog we have:

$display (" &  4'b1001 = %b", (&  4'b1001));

and the output of the function above is:

  &  4'b1001 = 0

I couldn't find a similar operator for C. Is there such an operation in C language?

Answer 1

There are no dedicated operators for this, however in most cases you can achieve the same result using bitwise operators and casting the result to a bool, which effectively is a single bit. For example:

• AND: bitwise invert, convert to bool, negate:

  bool and_reduction_4_bits(int n) {
      return !(~n & 0b1111); // C23 adds binary literals
  }
  

• OR: just convert to bool

  bool or_reduction(int n) {
      return n; // works for any number of bits
  }
  

The tricky one is XOR reduction. This could be done if you have a way to count the number of bits set, and then just check if that number is odd. Some compilers provide a builtin popcount() function to do that. If not, you can create your own function using bit twiddling hacks.

• XOR: count bits, check if odd

  bool xor_reduction(int n) {
      return popcount(n) & 1;
  }
  

Answer 2

In C, assuming unsigned or two’s complement, !~x or ~x == 0 serves as a bitwise AND; it is 1 if and only if each bit of x is 1.

!!x or x != 0 serves as a bitwise OR; it is 1 if any bit of x is 1.

The negations, not properly called NAND or NOR since they do not apply a NAND or NOR in a bitwise fashion but rather apply a NOT to the bitwise AND or OR, are simply !!~x or ~x != 0 and !x or x == 0.

There is no bitwise XOR in the operations specified by the C standard. GCC has __builtin_parity which can provide this.

The above apply to the full width of x. Narrower widths can be implemented by setting the extra bits to identity elements (1 for AND, 0 for OR and XOR).