[Bug c++/93008] New: Need a way to make inlining heuristics ignore whether a function is inline

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93008

            Bug ID: 93008
           Summary: Need a way to make inlining heuristics ignore whether
                    a function is inline
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

Currently GCC considers the 'inline' keyword as a hint for inlining, not just a
request for comdat semantics.

This is a problem in C++20 where arbitrarily complex functions are marked
'constexpr' so that they can be used at compile time, and a constexpr function
is implicitly an inline function. But we do not want the function to get
additional weighting for runtime optimizations just because it is eligible for
use in compile-time constant expressions.

It would be useful to have a new attribute that tells the inliner to ignore
whether the function is 'inline' and only consider its size, use in hot/cold
regions etc.

Alternatively, do not interpret 'constexpr' on its own as an inlining hint, and
only treat it as a hint when a constexpr function is explicitly declared with
the 'inline' specifier (or is defined in the class body).


constexpr void f() { }  // not an inline hint
constexpr inline void g() { } // inline hint
inline constexpr void h() { } // inline hint

struct X {
  constexpr void i() { } // inline hint
};