[Bug tree-optimization/77705] New: Optimize away some static constructors

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77705

            Bug ID: 77705
           Summary: Optimize away some static constructors
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jakub at gcc dot gnu.org
  Target Milestone: ---

struct S { int a; };
void baz (S *);
#if __cpp_constexpr >= 200704
constexpr
#endif
inline S foo () { return (S) { 2 }; }

S s = foo ();

for -std=c++98 -O2 we have the runtime initializer.  If we had some pass that
would analyze the ctor functions after they were inlined into and if they only
write constants into file-scope vars defined in the current TU (except for
comdat vars?) turn the ctor function into static initializers for the
file-scope vars.
Not sure if we can do it generally for any (non-comdat?) vars, or if e.g. the
C++ FE wouldn't have to mark such vars some way for us, or if we wouldn't need
to analyze if there aren't some ctors in the same TU that would run earlier
(higher (or is that lower?) priority) and could access those vars, or for vars
visible outside of the TU if some other TU's ctor couldn't access them.
An argument for some guidance from the C++ FE would be that say in C:
int a __attribute__((nocommon));
static __attribute__((constructor)) void
foo (void)
{
  a = 6;
}
some other TU could say in heither priority ctor expect to see a == 0.