cpopw

Count set bits in word

This instruction is defined by:

  • anyOf:

    • B, version >= 0

    • Zbb, version >= 0

This instruction is included in the following profiles:

  • RVA22U64 (Mandatory)

Encoding

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Assembly format

cpopw rd, rs1

Synopsis

This instructions counts the number of 1’s (i.e., set bits) in the least-significant word of the source egister.

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

M HS U VS VU

Always

Always

Always

Always

Always

Decode Variables

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

Execution

  • IDL

  • Sail

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 < 32; i++) {
  if (rs1_val[i] == 1'b1) {
    bitcount = bitcount + 1;
  }
}
X[rd] = bitcount;
{
  let rs1_val = X(rs1);
  result : nat = 0;
  foreach (i from 0 to 31)
    if rs1_val[i] == bitone then result = result + 1;
  X(rd) = to_bits(sizeof(xlen), result);
  RETIRE_SUCCESS
}

Exceptions

This instruction may result in the following synchronous exceptions:

  • IllegalInstruction