[Bug other/49666] New: passing nested function to inline function causes a trampoline call

[Bug other/49666] New: passing nested function to inline function causes a trampoline call

[kevin.waugh at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49666

           Summary: passing nested function to inline function causes a
                    trampoline call
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: other
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: kevin.waugh@gmail.com


#include <stdio.h>
static inline int identity(int (*f)()) {
  return f();
}

int n = 0;
int f() {
  return n;
}

int main() {
  int m = 0;
  int g() {
    return m;
  }
  printf("%d\n", identity(f)); /* inlines all the way */
  printf("%d\n", identity(g)); /* calls g through trampoline */
  printf("%d\n", g()); /* inline's g */
}

[Bug tree-optimization/49666] passing nested function to inline function causes a trampoline call

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49666

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |missed-optimization
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2011.07.07 09:22:08
          Component|other                       |tree-optimization
     Ever Confirmed|0                           |1

--- Comment #1 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-07-07 09:22:08 UTC ---
We don't know how to "devirtualize" trampolines.  In fact trampoline setup
isn't very SSA analyzer friendly (look at the hoops IPA points-to analysis
jumps through).  We end up with

<bb 2>:
  __builtin_init_trampoline (&FRAME.0.g, g, &FRAME.0);
  FRAME.0.m = 0;
  D.3251_20 = n;
  printf (&"%d\n"[0], D.3251_20);
  D.3230_3 = __builtin_adjust_trampoline (&FRAME.0.g);
  D.3231_4 = (int (*<T50a>) (void)) D.3230_3;
  D.3245_18 = D.3231_4 ();
  printf (&"%d\n"[0], D.3245_18);

Now we could "devirtualize" the above based on the fact the trampoline
doesn't escape.