Adjustment to DW_TAG_GNU_call_site patch for ICF debug

Hi!

This patch is an attempt to provide all info needed for ICF disambiguation
in DW_TAG_GNU_call_site DIEs, so far just for -O1 -g and above, not for -O0
-g (but that is possible to handle, by setting some flag that dwarf2out
shouldn't be expecting call arg notes and instead look at CALL_INSNs alone).
For the direct calls, I believe the
http://gcc.gnu.org/ml/gcc-patches/2010-12/msg01793.html
http://gcc.gnu.org/ml/gcc-patches/2010-12/msg01794.html
provide everything the debugger needs to disambiguate, for most virtual
calls they don't, because most often there isn't any
DW_AT_GNU_call_site_target that can be provided.

Roland wanted call site's DW_AT_abstract_origin in these cases to contain
a reference to the fn pointer that is called, but in the virtual calls
we don't have anything like that, all we have is DW_TAG_subprogram
in the class type with DW_AT_virtuality saying it is a virtual call.
If we referenced that, it would be ambiguous whether it is a direct call
to the virtual method (e.g. discovered through devirtualization), or
just indirect call to the virtual method that could be at runtime a call to
a different method.

The patch below arranges in that case (as can be seen on the testcase below)
to have DW_AT_GNU_call_site_target_clobbered (e.g. on x86_64 if this is
passed in %rdi and OBJ_TYPE_REF token is 0 it is
DW_OP_breg5 0 DW_OP_deref DW_OP_deref), i.e. where at the call time can be
the address found.  The debugger could check that %rdi in this case in the
called function is the artificial this parameter, assume in C++ it doesn't
change in the function and use that to discover the called function's
vtable.

And/or we could invent a new attribute which would contain what the
.debug_vcall function contained, i.e. the OBJ_TYPE_REF_TOKEN value.

I haven't seen .debug_dcall/.debug_vcall support patches for gdb anywhere,
and the patch doesn't disable generation of those sections (though, that
could be simplified now that the OBJ_TYPE_REF should be available from the
CALL_INSN).

Also note that at least ia64 would need some further changes, because its
call splitters recreate the call MEMs and thus throw their MEM_EXPR down on
the floor and thus don't preserve it till var-tracking/dwarf2out.

Comments/suggestions?

2010-12-23  Jakub Jelinek  <jakub@redhat.com>

	* calls.c (emit_call_1): Set MEM_EXPR on call's MEM.
	* var-tracking.c (prepare_call_arguments): Use MEM_EXPR on
	call's MEM.  Handle functions returning aggregate through a hidden
	first pointer.  For virtual calls add clobbered pc to call arguments
	chain.
	* dwarf2out.c (gen_subprogram_die): Emit
	DW_AT_GNU_call_site_target_clobbered if DW_AT_GNU_call_site_target
	can't be emitted.

	Jakub
__attribute__((noinline, noclone)) void
f1 (int i)
{
  asm volatile ("" : : "r" (i) : "memory");
}

__attribute__((noinline, noclone)) void
f2 (int i)
{
  asm volatile ("" : : "r" (i) : "memory");
}

__attribute__((noinline, noclone)) void
f3 (int i)
{
  asm volatile ("" : : "r" (i) : "memory");
}

__attribute__((noinline, noclone)) int
bar (int i)
{
  f1 (i * 4 + 2);
  f2 (i * 4 + 2);
  f3 (i * 4 + 2);
  return 4;
}

#ifdef RETURN_STRUCT
struct S
{
  S ();
  char c[1024];
};
#else
#define S void
#endif

struct A
{
  A () { int i = 4; baz (i); foo (); }
  void foo () __attribute__((noinline, noclone));
  virtual S baz (int &) __attribute__((noinline, noclone));
  virtual ~A () {}
};

struct B : A
{
  B () { int i = 5; baz (i); foo (); }
  virtual S baz (int &) __attribute__((noinline, noclone));
  virtual ~B () {}
};

struct C : A
{
  C () { int i = 6; baz (i); foo (); }
  virtual S baz (int &) __attribute__((noinline, noclone));
  virtual S baz2 (int &) __attribute__((noinline, noclone));
  virtual ~C () {}
};

struct D : B, C
{
  D () { int i = 7; baz (i); foo (); }
  void foo () __attribute__((noinline, noclone));
  virtual S baz (int &) __attribute__((noinline, noclone));
  virtual ~D () {}
};

void
A::foo ()
{
  int i = 8;
  baz (i);
}

#ifdef RETURN_STRUCT
S s;
#else
#define s
#endif

S
A::baz (int &x)
{
  asm volatile ("" : : "r" (x) : "memory");
  return s;
}

S
B::baz (int &x)
{
  asm volatile ("" : : "r" (x) : "memory");
  return s;
}

S
C::baz (int &x)
{
  asm volatile ("" : : "r" (x) : "memory");
  return s;
}

void
D::foo ()
{
  int i = 9;
  baz (i);
  baz2 (i);
}

S
D::baz (int &x)
{
  asm volatile ("" : : "r" (x) : "memory");
  return s;
}

D d;

int
main ()
{
  bar (5);
}

Adjustment to DW_TAG_GNU_call_site patch for ICF debug

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