[Bug tree-optimization/45115] pure functions returning structs are not optimized.

[Bug tree-optimization/45115] pure functions returning structs are not optimized.

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=45115

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jzwinck at gmail dot com

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
*** Bug 108635 has been marked as a duplicate of this bug. ***

[Bug tree-optimization/45115] pure functions returning structs are not optimized.

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=45115

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
This affects C++20 three-way comparisons, which return trivial structs wrapping
an integer.

From PR 108635:

    #include <compare>

    struct S
    {
        std::weak_ordering operator<=>(const S&) const __attribute__((const));
    };

    int compare3way(S& a, S& b)
    {
        return (a < b) ? -1 : (a > b) ? 1 : 0;
    }

I expect operator<=> to be called once, but it is called twice.  This can be a
major missed optimization if operator<=> is expensive.  It happens regardless
of:

 1. Using attribute((const)) or attribute((pure)).
 2. Making operator<=> a free function or a member.
 3. Comparing (a > b) or (a < b) in the second ternary expression.  This is
especially strange, because it's really calling the same pure function twice,
and that's optimized correctly when the function being called is operator<
instead of operator<=>.

Clang optimizes it as expected.

Demo: https://godbolt.org/z/jP51E6xaz

[Bug tree-optimization/45115] pure functions returning structs are not optimized.

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=45115

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |DUPLICATE

--- Comment #5 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Dup of bug 59739; even though that is a newer bug it has some more analysis and
even been assigned (will add the C++20 testcase there too).

*** This bug has been marked as a duplicate of bug 59739 ***

[Bug tree-optimization/45115] pure functions returning structs are not optimized.

[rguenth at gcc dot gnu dot org]

------- Comment #2 from rguenth at gcc dot gnu dot org  2010-07-28 20:05 -------
We can only value-number things which define an SSA name.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |matz at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45115

[Bug tree-optimization/45115] pure functions returning structs are not optimized.

[pinskia at gcc dot gnu dot org]

------- Comment #1 from pinskia at gcc dot gnu dot org  2010-07-28 19:48 -------
Confirmed.


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement
             Status|UNCONFIRMED                 |NEW
          Component|c                           |tree-optimization
     Ever Confirmed|0                           |1
   GCC host triplet|x86_64-linux-gnu            |
           Keywords|                            |missed-optimization
   Last reconfirmed|0000-00-00 00:00:00         |2010-07-28 19:48:03
               date|                            |
            Summary|attribute((pure)) does not  |pure functions returning
                   |work when returning structs |structs are not optimized.


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45115