clz
Count leading zero bits
This instruction is defined by:
-
Zbb, version >= Zbb@1.0.0
This instruction is included in the following profiles:
-
RVA22S64 (Mandatory)
-
RVA22U64 (Mandatory)
-
RVA23M64 (Mandatory)
-
RVA23S64 (Mandatory)
-
RVA23U64 (Mandatory)
-
RVB23M64 (Mandatory)
-
RVB23S64 (Mandatory)
-
RVB23U64 (Mandatory)
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.
Execution
-
IDL
-
Sail
if (%%LINK%func;implemented?;implemented?%%(ExtensionName::B) && (%%LINK%csr_field;misa.B;CSR[misa].B%% == 1'b0)) {
%%LINK%func;raise;raise%%(ExceptionCode::IllegalInstruction, %%LINK%func;mode;mode%%(), $encoding);
}
X[rd] = (%%LINK%func;xlen;xlen%%() - 1) - $signed(%%LINK%func;highest_set_bit;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
}