cpop

Count set bits

This instruction is defined by:

Encoding

Synopsis

This instructions counts the number of 1’s (i.e., set bits) in the source register.

Software Hint

This operations is known as population count, popcount, sideways sum,
bit summation, or Hamming weight.

The GCC builtin function `__builtin_popcount (unsigned int x)` is
implemented by cpop on RV32 and by cpopw on RV64. The GCC builtin
function `__builtin_popcountl (unsigned long x)` for LP64 is
implemented by cpop on RV64.

Access

Always

Decode Variables

Bits<5> rs1 = $encoding[19:15];
Bits<5> rd = $encoding[11:7];

Execution

Pruned, XLEN == 64
Original

XReg bitcount = 0;
XReg rs1_val = X[rs1];
for (U32 i = 0; i < 64; i++) {
  if (rs1_val[i] == 1'b1) {
    bitcount = 1;
  }
}
X[rd] = bitcount;

if (implemented?(ExtensionName::B) && (CSR[misa].B == 1'b0)) {
  raise(ExceptionCode::IllegalInstruction, mode(), $encoding);
}
XReg bitcount = 0;
XReg rs1_val = X[rs1];
for (U32 i = 0; i < xlen(); i++) {
  if (rs1_val[i] == 1'b1) {
    bitcount = bitcount + 1;
  }
}
X[rd] = bitcount;