[Bug tree-optimization/58774] tree-switch-conversion doesn't with content in default scase

[ben.maurer at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58774

--- Comment #2 from Ben Maurer <ben.maurer at gmail dot com> ---
I think there might be multiple appropriate transformations. In the
switchwithnoprint example I attached, it is currently compiled as:

<switchwithnoprint>:
sub    $0x1,%edi
xor    %eax,%eax
cmp    $0x5,%edi
ja     RETURN
mov    0x400740(,%rdi,4),%eax
RETURN: repz retq

A good transformation for the switchwithprint function might be to make the
RETURN label do the printf. On the other hand, the switch case with a
non-trivial function might not be as clean as this. Eg, you could have:

switch(val) {
  case 1: return 1;
  case 2: return 2;
  case 3: printf("hello");
  case 4: return 4;
}

In this case you could do something like:

struct {
  bool is_code;
  union {int ret; void* jmp};
} actions[] = ...
if (val >= 1 && val <= 4) {
  auto action = actions[val];
  if (action.is_code) goto action.jmp;
  else return action.ret;
}

In theory, you might even be able to avoid having the is_code value if you knew
that the ret and jmp values never overlapped (eg, if (action >
MAX_POSSIBLE_RETURN_VALUE) goto (void*) action).