Throwing and catching exceptions

Exception handling in LLVM IR is closely tied to platform support. Here, we will look at the most common type of exception handling using libunwind. Its full potential is used by C++, so we will look at an example in C++ first, where the bar() function can throw an int or double value:

int bar(int x) {
  if (x == 1) throw 1;
  if (x == 2) throw 42.0;
  return x;
}

The foo() function calls bar(), but only handles a thrown int. It also declares that it only throws int values:

int foo(int x) {
  int y = 0;
  try {
    y = bar(x);
  }
  catch (int e) {
    y = e;
  }
  return y;
}

Throwing an exception requires two calls into the runtime library; this can be seen in the bar() function. First, memory for the exception is allocated with a call to __cxa_allocate_exception(). This function takes the number of bytes to allocate...