clz

Count leading zero bits

This instruction is defined by:

anyOf:
- B, version >= 0
- Zbb, version >= 0

This instruction is included in the following profiles:

RVA22S64 (Mandatory)
RVA22U64 (Mandatory)

Encoding

Assembly format

clz rd, rs1

Synopsis

This instruction counts the number of 0’s before the first 1, starting at the most-significant bit (i.e., XLEN-1) and progressing to bit 0. Accordingly, if the input is 0, the output is XLEN, and if the most-significant bit of the input is a 1, the output is 0.

Access

M	HS	U	VS	VU
Always	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);
}
X[rd] = (xlen() - 1) - $signed(highest_set_bit(X[rs1]));

{
  let rs1_val = X(rs1);
  result : nat = 0;
  done : bool = false;
  foreach (i from (sizeof(xlen) - 1) downto 0)
    if not(done) then if rs1_val[i] == bitzero
                    then result = result + 1
                    else done = true;
  X(rd) = to_bits(sizeof(xlen), result);
  RETIRE_SUCCESS
}

Exceptions

This instruction may result in the following synchronous exceptions:

IllegalInstruction