[Bug tree-optimization/91459] Tail-Call Optimization is not performed when return value is assumed.

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91459

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2019-08-16
          Component|middle-end                  |tree-optimization
     Ever confirmed|0                           |1

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
I think this has a duplicate somewhere.  The reason is we end up with

foo1 (int a, int b)
{
  <bb 2> [local count: 1073741824]:
  function_returns_only_1_or_doesnt_return (a_2(D), b_3(D));
  return 1;

and thus no obvious tail-call opportunity.  Here it is EVRP replacing
result_5 with 1 in

  <bb 2> :
  result_5 = function_returns_only_1_or_doesnt_return (a_2(D), b_3(D));
  if (result_5 == 1)
    goto <bb 3>; [INV]
  else
    goto <bb 4>; [INV]

  <bb 3> :
  // predicted unlikely by early return (on trees) predictor.
  return result_5;

  <bb 4> :
  __builtin_unreachable ();

where it would be better to transform this idiom to unconditional
return result_5 early.