any expression to tell whether a variable was optimized out?

any expression to tell whether a variable was optimized out?

[Alexandre Oliva]

Hi,

I'm working on a debug info quality testsuite for GCC, and I've been
having some trouble telling, in gdb macros, a variable for which no
location expression is available at a certain point in the program (but
for which we do have locations at other points), from one that evaluates
to zero (lifetime constant, or stored in register or memory).

Consider:

(gdb) p x
$1 = <value optimized out>
(gdb) p y
$2 = 0
(gdb) p x == y
1
(gdb) p &x
Can't take address of "x" which isn't an lvalue.
(gdb) p &y
Address requested for identifier "y" which is in register $r12

(gdb) set $temp1 = x
(gdb) set $temp2 = y
(gdb) set z = $temp1 == $temp2
(gdb) p z
1

(gdb) set $temp1 = x + 1
(gdb) set $temp2 = y + 1
(gdb) set z = $temp1 == $temp2
(gdb) p z
1



Is there really no way to set a gdb variable to something equivalent to
the boolean result of valprint.c:value_optimized_out()?  Any reason for
that, other than that nobody needed it before?

Thanks in advance,

-- 
Alexandre Oliva           http://www.lsd.ic.unicamp.br/~oliva/
You must be the change you wish to see in the world. -- Gandhi
Be Free! -- http://FSFLA.org/   FSF Latin America board member
Free Software Evangelist      Red Hat Brazil Compiler Engineer

Re: any expression to tell whether a variable was optimized out?

[Daniel Jacobowitz]

On Tue, Mar 10, 2009 at 06:48:28AM -0300, Alexandre Oliva wrote:
> Is there really no way to set a gdb variable to something equivalent to
> the boolean result of valprint.c:value_optimized_out()?  Any reason for
> that, other than that nobody needed it before?

Nope, there's no way to do it.  Really, unavailable ought to propogate
through the expression evaluator, and give errors at some points, I
expect...

-- 
Daniel Jacobowitz
CodeSourcery

Re: any expression to tell whether a variable was optimized out?

[Thiago Jung Bauermann]

El mar, 10-03-2009 a las 08:59 -0400, Daniel Jacobowitz escribió:
> On Tue, Mar 10, 2009 at 06:48:28AM -0300, Alexandre Oliva wrote:
> > Is there really no way to set a gdb variable to something equivalent to
> > the boolean result of valprint.c:value_optimized_out()?  Any reason for
> > that, other than that nobody needed it before?
> 
> Nope, there's no way to do it.  Really, unavailable ought to propogate
> through the expression evaluator, and give errors at some points, I
> expect...

It would be easy enough to add a method to the gdb.Value Python class
exposing this boolean. Would it help you?
-- 
[]'s
Thiago Jung Bauermann
IBM Linux Technology Center

Re: any expression to tell whether a variable was optimized out?

[Alexandre Oliva]

On Mar 10, 2009, Thiago Jung Bauermann <bauerman@br.ibm.com> wrote:

> El mar, 10-03-2009 a las 08:59 -0400, Daniel Jacobowitz escribió:
>> On Tue, Mar 10, 2009 at 06:48:28AM -0300, Alexandre Oliva wrote:
>> > Is there really no way to set a gdb variable to something equivalent to
>> > the boolean result of valprint.c:value_optimized_out()?  Any reason for
>> > that, other than that nobody needed it before?
>> 
>> Nope, there's no way to do it.  Really, unavailable ought to propogate
>> through the expression evaluator, and give errors at some points, I
>> expect...

> It would be easy enough to add a method to the gdb.Value Python class
> exposing this boolean. Would it help you?

Maybe.  I was hoping for something already widely deployed, but
failing that, I guess it's ok to rely on future features ;-)

As for whether it would help, I can only assume it would, but I'm not
quite up to speed on how to use such Python interfaces.  If the feature
can be exercised from the GDB textual interactive interface, as part of
normal expression evaluation, it will indeed help.  Otherwise, I may
have to look into it further to tell whether it will help.

Thanks,

-- 
Alexandre Oliva           http://www.lsd.ic.unicamp.br/~oliva/
You must be the change you wish to see in the world. -- Gandhi
Be Free! -- http://FSFLA.org/   FSF Latin America board member
Free Software Evangelist      Red Hat Brazil Compiler Engineer

Re: any expression to tell whether a variable was optimized out?

[Thiago Jung Bauermann]

El mar, 10-03-2009 a las 13:44 -0300, Alexandre Oliva escribió:
> On Mar 10, 2009, Thiago Jung Bauermann <bauerman@br.ibm.com> wrote:
> > It would be easy enough to add a method to the gdb.Value Python class
> > exposing this boolean. Would it help you?
> 
> Maybe.  I was hoping for something already widely deployed, but
> failing that, I guess it's ok to rely on future features ;-)
> 
> As for whether it would help, I can only assume it would, but I'm not
> quite up to speed on how to use such Python interfaces.  If the feature
> can be exercised from the GDB textual interactive interface, as part of
> normal expression evaluation, it will indeed help.  Otherwise, I may
> have to look into it further to tell whether it will help.

Not everything you would need is committed upstream yet, but it's very
close. With the convenience function patch here:

http://sourceware.org/ml/gdb-patches/2009-03/msg00066.html

Plus the frame api patch here:

http://sourceware.org/ml/gdb-patches/2009-03/msg00143.html

Plus the patch at the end of this message, you can create the following
convenience function:

import gdb

class IsOptimizedOut (gdb.Function):

  def __init__ (self):
    super (IsOptimizedOut, self).__init__ ("is_optimized_out")

  def invoke (self, value):
    return value.is_optimized_out ()

IsOptimizedOut ()

Then you can use it in an expression in the GDB command line:

(gdb) print $is_optimized_out ($some_var)
$1 = 1

You can also do the above by checking out the archer-tromey-python
branch, and applying the patch below. I didn't commit this yet because I
don't know if it would be better to have is_optimized_out function as a
method or attribute of gdb.Value...
-- 
[]'s
Thiago Jung Bauermann
IBM Linux Technology Center


diff --git a/gdb/python/python-value.c b/gdb/python/python-value.c
index 2507fcd..debd136 100644
--- a/gdb/python/python-value.c
+++ b/gdb/python/python-value.c
@@ -312,6 +312,18 @@ valpy_str (PyObject *self)
   return result;
 }
 
+/* Implements gdb.Value.is_optimized_out () -> Boolean.  */
+static PyObject *
+valpy_is_optimized_out (PyObject *self, PyObject *args)
+{
+  struct value *value = ((value_object *) self)->value;
+
+  if (value_optimized_out (value))
+    Py_RETURN_TRUE;
+
+  Py_RETURN_FALSE;
+}
+
 enum valpy_opcode
 {
   VALPY_ADD,
@@ -921,6 +933,9 @@ static PyMethodDef value_object_methods[] = {
   { "string", (PyCFunction) valpy_string, METH_VARARGS | METH_KEYWORDS,
     "string ([encoding] [, errors]) -> string\n\
 Return Unicode string representation of the value." },
+  { "is_optimized_out", valpy_is_optimized_out, METH_NOARGS,
+    "is_optimized_out () -> Boolean\n\
+Whether this value is optimized out." },
   {NULL}  /* Sentinel */
 };
 

Re: any expression to tell whether a variable was optimized out?

[Thiago Jung Bauermann]

El mar, 10-03-2009 a las 15:45 -0300, Thiago Jung Bauermann escribió:
> Plus the frame api patch here:
> 
> http://sourceware.org/ml/gdb-patches/2009-03/msg00143.html
> 

Sorry, you don't need this one.
-- 
[]'s
Thiago Jung Bauermann
IBM Linux Technology Center

Re: any expression to tell whether a variable was optimized out?

[Tom Tromey]

>>>>> "Thiago" == Thiago Jung Bauermann <bauerman@br.ibm.com> writes:

Thiago> You can also do the above by checking out the archer-tromey-python
Thiago> branch, and applying the patch below. I didn't commit this yet because I
Thiago> don't know if it would be better to have is_optimized_out function as a
Thiago> method or attribute of gdb.Value...

I think attribute, because (AFAIK) it is immutable.

I'm a little surprised that this is an attribute of values at all.
Doesn't that seem weird?  Is there anything useful that can be done
with a value like this?  Perhaps just fetching its type?

Tom