clmul

Carry-less multiply (low-part)

This instruction is defined by:

anyOf:
- B, version >= 0
- Zbc, version >= 0
- Zbkc, version >= 0
- Zk, version >= 0
- Zkn, version >= 0
- Zks, version >= 0

This instruction is included in the following profiles:

Encoding

Assembly format

clmul rd, rs1, rs2

Synopsis

This instruction must have data-independent timing when extension Zkt is enabled.

clmul produces the lower half of the 2*XLEN carry-less product

Access

M	HS	U	VS	VU
Always	Always	Always	Always	Always

Always

Decode Variables

Bits<5> rs2 = $encoding[24:20];
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 rs1_val = X[rs1];
XReg rs2_val = X[rs2];
XReg output = 0;
for (U32 i = 0; i < xlen(); i++) {
  output = (((rs2_val >> i) & 1) == 1) ? output ^ (rs1_val << i) : output;
}
X[rd] = output;

{
  let rs1_val = X(rs1);
  let rs2_val = X(rs2);
  result : xlenbits = zeros();
  foreach (i from 0 to (xlen_val - 1))
    if rs2_val[i] == bitone then result = result ^ (rs1_val << i);
  X(rd) = result;
  RETIRE_SUCCESS
}

Exceptions

This instruction may result in the following synchronous exceptions:

IllegalInstruction