[Bug libmudflap/19319] Mudflap produce many violations on simple, correct c++ program

["awl03 at doc dot ic dot ac dot uk" <gcc-bugzilla@gcc.gnu.org>]

------- Comment #27 from awl03 at doc dot ic dot ac dot uk  2007-06-10 19:32 -------
I have been writing my own bounds-checker based on Mudflap.  While doing so I
had to tackle this same problem.  My flatmate and I tracked it down to the fact
that, although function parameters and variables are registered if their
address is ever taken, the return value is not.  This is a problem in
return-by-value where the result is returned directly without an intermediate
variable.  For example:

class bob {
  public:
    int i;
    bob(int n) { i = n; }
};

bob f(int n)
{
  return bob(n);
}

int main()
{
  bob b = f(0);
}

Here bob is constructed directly in the return statement in f().  In GIMPLE
this looks like:

bob f(int) (n)
{
<bb 0>:
  __comp_ctor  (&<retval>, n);
  return <retval>;
}

Notice that <retval> has its address taken.  Inside the constructor
__comp_ctor() the object is created in the location given by <retval>.
<retval> has not been registered by f() as return values are not registered,
nor has it been registered by main() (where the object finally ends up) because
nothing there uses its address.  

This happens a lot in the STL, hence why it shows up whenever template, map
etc., are used:

iterator begin()
{
    return iterator (this->_M_impl._M_start);
}

which is gimplified to into:

iterator begin()
{
    comp_ctor (&<retval>, &this->_M_impl._M_start);
    return <retval>;
} 

If Mudflap is changed to register these return values, the violations go away
:)  I have created a patch that does this but, as I'm a relative newbie, it
could all be complete rubbish in which case I apologise.

This deals with the problem for the initial testcase, the simplified test by
Frank Ch. Eigler and the test by Paul Pluzhnikov.  It does not fix the others
as these are caused by a different problem, namely objects created by external
library calls are not registered by Mudflap and so it thinks there is a
violation if you use one of these foreign pointers.

I hope this helps and I would be very glad of feedback.

Alex Lamaison


-- 

awl03 at doc dot ic dot ac dot uk changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |awl03 at doc dot ic dot ac
                   |                            |dot uk


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19319