[Bug tree-optimization/102927] Failure to optimize series of if-else to use array when possible

[marxin at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102927

--- Comment #3 from Martin Liška <marxin at gcc dot gnu.org> ---
> As Richi said, we do the transformation to switch only if we can expand it
> by jump table, bit test or array access.
> 
> You can expand this example with:
> --param case-values-threshold=4

and we end up with array access:

$ gcc -c pr102927.c -O2 --param case-values-threshold=4
-fdump-tree-optimized=/dev/stdout

;; Function foo (foo, funcdef_no=0, decl_uid=1943, cgraph_uid=1,
symbol_order=0)

Removing basic block 5
int foo (int i)
{
  int _1;
  unsigned int _3;
  int _4;

  <bb 2> [local count: 1073741824]:
  _3 = (unsigned int) i_2(D);
  if (_3 <= 3)
    goto <bb 3>; [50.00%]
  else
    goto <bb 4>; [50.00%]

  <bb 3> [local count: 536870913]:
  _4 = CSWTCH.1[_3];

  <bb 4> [local count: 1073741824]:
  # _1 = PHI <42(2), _4(3)>
  return _1;

}