[Bug middle-end/48814] [4.4/4.5/4.6/4.7 Regression] Incorrect scalar increment result

["rguenth at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48814

--- Comment #10 from Richard Guenther <rguenth at gcc dot gnu.org> 2012-02-09 13:03:25 UTC ---
For

extern void abort (void);
struct S { int i; };
struct S arr[32];
volatile int count = 0;

struct S __attribute__((noinline))
incr (void)
{
  ++count;
}

int main()
{
  arr[count++] = incr ();
  if (count != 2)
    abort ();
  return 0;
}

we can only avoid reading 'count' too many times (once to get at the
index for the store and once for updating its value) if we can
insert a statement inbetween the call and the store to arr[].  Which
we can do only if we are introducing an aggregate temporary - which
might work in C, but does not work in C++ when we require
return-slot-optimization.  Thus, in the face of volatiles and required RSO
this is unfixable.