cast

dyn_cast will return NULL or what you want.

isa will return bool, if it can convert.

cast do the convert, you must ensure it can work, or you will get error.

C++

if (ConstantSDNode *N1C = dyn_cast<ConstantSDNode>(N1)) {
  const APInt &C1 = N1C->getAPIntValue();
}

// equal to
if (isa<ConstantSDNode>(N1)) {
  ConstantSDNode *N1C = cast<ConstantSDNode>(N1)
      const APInt &C1 = N1C->getAPIntValue();
}