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

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

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

Andrew Pinski  changed:

   What|Removed |Added

   Last reconfirmed|2019-08-16 00:00:00 |2021-12-26

--- Comment #4 from Andrew Pinski  ---
So when DOM3 changes:

  result_5 = function_returns_only_1_or_doesnt_return (a_2(D), b_3(D));
  if (result_5 == 1)
goto ; [100.00%]
  else
goto ; [0.00%]

   [local count: 1073741824]:
  return 1;

   [count: 0]:
  __builtin_unreachable ();

To

  result_5 = function_returns_only_1_or_doesnt_return (a_2(D), b_3(D));
  return 1;

I wonder if it could change 1 back into result_5 because it comes from a
function call.

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

[pinskia at gcc dot gnu.org]

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

--- Comment #3 from Andrew Pinski  ---
Part of that might be because of:
  // predicted unlikely by early return (on trees) predictor.

That seems not true, the other side is more unlikely ...

[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  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  ---
I think this has a duplicate somewhere.  The reason is we end up with

foo1 (int a, int b)
{
   [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

   :
  result_5 = function_returns_only_1_or_doesnt_return (a_2(D), b_3(D));
  if (result_5 == 1)
goto ; [INV]
  else
goto ; [INV]

   :
  // predicted unlikely by early return (on trees) predictor.
  return result_5;

   :
  __builtin_unreachable ();

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