[Bug c++/127825] New: (pure|const) attributed functions are not CSE if inlined

[marco.rubini08 at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=127825

            Bug ID: 127825
           Summary: (pure|const) attributed functions are not CSE if
                    inlined
           Product: gcc
           Version: 17.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: marco.rubini08 at gmail dot com
  Target Milestone: ---

GCC can simplify common subexpressions involving pure and const attributed
functions, but some optimizations are missed if the function code is inlined in
the caller.

Example code on Godbolt: https://godbolt.org/z/v68scEbKb

Example copied verbatim, add/remove the gnu::noinline attribute and see the
difference in code generation with optimizations enabled.

If the function is inlined, GCC reloads the x variable unnecessarily.
It is an optimization bug because inlining shouldn't reduce optimization
opportunities, at least in simplified cases.

=====
extern int x;

[[gnu::noinline, gnu::const]]
inline int get_x() 
{ return x; }

void opaque();

int foo()
{
    int p = get_x();
    opaque();
    int q = get_x();
    return (p ^ q);
}
=====