Generally the semantics of switch statements on architectures that support SIMD+control flow but not indirect divergent branches are such that the condition for every case statement is checked and only those statements with at least one active thread are entered.
So if you had this:
some_function(int condition)
{
switch(condition)
{
case 0: function0(); break;
case 1: function1(); break;
default: break;
}
}
And called some_function(0)
It would check three conditions (0, 1, default), but only execute function0(), not function1(). Note that this is exactly the same as what would happen if you wrote it using a series of if statements. I think that the case instructions are only included in the ISA because it makes it easier to write in assembly.
