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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug ipa/113478] -Os does not inline single instruction function
Date: Thu, 18 Jan 2024 13:12:53 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D113478

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2024-01-18
                 CC|                            |hubicka at gcc dot gnu.org
          Component|c++                         |ipa

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
t.C:35:19: note: Considering inline candidate T D<T>::test() [with T =3D
char]/231.
   Estimating body: T D<T>::test() [with T =3D char]/231
   Known to be false: not inlined, op0 changed
   size:4
   Estimating body: T D<T>::test() [with T =3D char]/231
   Known to be false: not inlined, op0 changed
   size:4
t.C:35:19: missed:   will not early inline: int main()/185->T D<T>::test()
[with T =3D char]/231, call is cold and code would grow by 1

which is because

Analyzing function body size: T D<T>::test() [with T =3D char]/231
                Accounting size:2.00, time:0.00 on new predicate exec:(not
inlined)

 BB 2 predicate:(true)
  _3 =3D &MEM[(const struct __atomic_base *)this_1(D)]._M_i;
                freq:1.00 size:  0 time:  0
  _4 =3D __atomic_load_1 (_3, 0);
                freq:1.00 size:  4 time: 13
  _5 =3D (char) _4;
                freq:1.00 size:  0 time:  0
  return _5;
                freq:1.00 size:  1 time:  2
                Will be eliminated by inlining
                Accounting size:1.00, time:2.00 on predicate exec:(not inli=
ned)

so the __atomic_load_1 "call"s are not special-cased during size estimation
and we assume you'd get parameter setup and call.  Note some targets
might expand this to a call to libatomic (there's also -finline-atomics).

We might want to consider improving heuristics here.=