[Bug middle-end/114185] New: Missed tail-call optimization due to an argument whose address is taken

[Bug middle-end/114185] New: Missed tail-call optimization due to an argument whose address is taken

[dizhao at os dot amperecomputing.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114185

            Bug ID: 114185
           Summary: Missed tail-call optimization due to an argument whose
                    address is taken
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dizhao at os dot amperecomputing.com
  Target Milestone: ---

GCC (with "-Ofast") doesn't perform tail call optimization on a function like:

    long test_func (long n, unsigned long u64arg,
                    unsigned int u32arg, unsigned long *p)
    {
      if (n > 0)
        return test_func (n - 1, u64arg + (unsigned long) u32arg, u32arg, p);
      else
        return &u64arg - p;
    }

llvm can optimize the tail call.

GCC gave up tail call optimization, because the following check in
find_tail_calls() failed on argument "u64arg":

              /* The parameter should be a real operand, so that phi node
                 created for it at the start of the function has the meaning
                 of copying the value.  This test implies is_gimple_reg_type
                 from the previous condition, however this one could be
                 relaxed by being more careful with copying the new value
                 of the parameter (emitting appropriate GIMPLE_ASSIGN and
                 updating the virtual operands).  */
              if (!is_gimple_reg (param))
                break;

The check was to fix this ICE:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93487 . But in this case,
"&u64arg" won't be used in PHI argument, so it seems ok for tail call
optimization. (BTW, I tried current GCC trunk on the example code in PR 93487,
but haven't encounter the ICE with that check removed, because the PHI
arguments are "&y" and "&x" now.)

[Bug middle-end/114185] Missed tail-call optimization due to an argument whose address is taken

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114185

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
This code is undefined if the if is not taken.

[Bug middle-end/114185] Missed tail-call optimization due to an argument whose address is taken

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114185

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Also the PR # you gave is wrong.

[Bug middle-end/114185] Missed tail-call optimization due to an argument whose address is taken

[dizhao at os dot amperecomputing.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114185

--- Comment #3 from Di Zhao <dizhao at os dot amperecomputing.com> ---
Sorry, the old tracker for the code is PR 17749 .

[Bug middle-end/114185] Missed tail-call optimization due to an argument whose address is taken

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114185

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |missed-optimization

--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
The tail recursion optimization doesn't support non-register argument passing.

Of course the example is also broken as Andrew says.

A "better" example might be taking the address with like

  asm ("":: "g" (&u64arg));

Better make it meaningfully do sth with no invisible side-effects or escaping
of the address.

We should be pretty good these days with not setting TREE_ADDRESSABLE
unnecessarily (but I guess DECL_NOT_GIMPLE_REG should also inhibit
tail-recursion).