// RV64Zbb contains zext.h but not general packw
require(((insn.rs2() == 0) && p->extension_enabled(EXT_ZBB))
  || p->extension_enabled(EXT_ZBKB));
require_rv64;
reg_t lo = zext32(RS1 << 16) >> 16;
reg_t hi = zext32(RS2 << 16);
WRITE_RD(sext32(lo | hi));