Does "default" switch case disturb jump table optimization?

Question

In my code I'm used to write fall-back default cases containing asserts like the following, to guard me against forgetting to update the switch in case semantics change

switch(mode) {
case ModeA: ... ;
case ModeB: ... ;
case .. /* many of them ... */
default: {
  assert(0 && "Unknown mode!");
  return ADummyValue();
}
};

Now I wonder whether the artificial fall-back check default case will interfere with jump table generations? Imagine "ModeA" an "ModeB" etc are consecutive so the compiler could optimize into a table. Since the "default" case contains an actual "return" statement (since the assert will disappear in release mode and the compiler will moan about a missing return statement), it seems unlikely the compiler optimizes the default branch away.

What's the best way to handle this? Some friend recommended me to replace "ADummyValue" with a null pointer dereference, so that the compiler, in presence of undefined behavior, could omit to warn about a missing return statement. Are there better ways to solve this?

Answer 1

If your compiler is MSVC, you can use __assume intrinsic : http://msdn.microsoft.com/en-us/library/1b3fsfxw(v=VS.80).aspx

Answer 2

At least with the compilers I've looked at, the answer is generally no. Most of them will compile a switch statement like this to code roughly equivalent to:

if (mode < modeA || mode > modeLast) {
    assert(0 && "Unknown mode!");
    return ADummyValue();
}
switch(mode) { 
    case modeA: ...;
    case modeB: ...;
    case modeC: ...;
    // ...
    case modeLast: ...;
}