xperm4

Crossbar permutation (nibbles)

The xperm4 instruction operates on nibbles. The rs1 register contains a vector of XLEN/4 4-bit elements. The rs2 register contains a vector of XLEN/4 4-bit indexes. The result is each element in rs2 replaced by the indexed element in rs1, or zero if the index into rs2 is out of bounds.

Assembly format

xperm4 rd, rs1, rs2

Decode Variables

Bits<5> rs2 = $encoding[24:20];
Bits<5> rs1 = $encoding[19:15];
Bits<5> rd = $encoding[11:7];

Execution

IDL
Sail

XReg input1 = X[rs1];
XReg input2 = X[rs2];
XReg output = 0;
for (U32 i = 0; i < (xlen() - 4); i = i + 4) {
  XReg index = input2[i + 3:i];
  if (4 * index < xlen()) {
    output[i + 3:i] = input1[4 * index + 3:4 * index];
  }
}
X[rd] = output;

val xperm4_lookup : (bits(4), xlenbits) -> bits(4)
function xperm4_lookup (idx, lut) = {
  (lut >> (idx @ 0b00))[3..0]
}
function clause execute ( XPERM_4 (rs2,rs1,rd)) = {
  result : xlenbits = EXTZ(0b0);
  foreach(i from 0 to xlen by 4) {
    result[i+3..i] = xperm4_lookup(X(rs2)[i+3..i], X(rs1));
  };
  X(rd) = result;
  RETIRE_SUCCESS
}

Encoding

Defining extension

Zbkx, version >= Zbkx@1.0.0

Access

M	HS	U	VS	VU
Always	Always	Always	Always	Always

Always

Containing profiles

Mandatory:
Optional: RVA22S64, RVA22U64, RVA23M64, RVA23S64, RVB23M64, RVB23S64, RVB23U64