[patch] improve entry/exit instrumentation wrt trampolines

[Olivier Hainque]

Hello,

Execution of the example program below shows that when
a nested function is called by way of a stack trampoline,
the -finstrument-functions hooks receive the address of
the trampoline as the "this function" parameter.

Receiving the actual function address would seem more
useful for function-instrumentation purposes, and this
patch adjusts the compiler to do that instead.

On x86_64-linux, without the patch, the code below compiled
with -finstrument-functions outputs something like

  entering function at 0x4005a1
  entering function at 0x7fffae4acc34

And with the patch, we get instead:

  entering function at 0x400583
  entering function at 0x400557

where the second address corresponds to the bump_by
symbol address.

Bootstrapped and regression tested on x86_64-linux.

Ok to commit ?

Thanks in advance,

With Kind Regards,

2018-06-22  Olivier Hainque  <hain...@adacore.com>

        * gimplify.c (gimplify_function_tree): Prevent creation
        of a trampoline for the address of the current function
        passed to entry/exit instrumentation hooks.

---

int main ()
{
  volatile int count = 0;

  void bump_by (int x)
  {
    count += x;
  }

  { void (* bumper)(int) = &bump_by;
    (*bumper)(5);
  }
   
  return 0;
}

#include <stdio.h>

__attribute__((no_instrument_function))
void __cyg_profile_func_exit (void * this_fn, void * call_site)
{
}

__attribute__((no_instrument_function))
void __cyg_profile_func_enter (void * this_fn, void * call_site)
{
  printf ("entering function at %p\n", this_fn);
}