On Monday, October 04, 2010 3:01:22 pm Ulrich Weigand wrote:
> Ken Werner wrote:
> > -	  return value_assign (arg1, arg2);
> > +	  /* Prevent to return a lvalue.  */
> > +	  arg3 = value_assign (arg1, arg2);
> > +	  VALUE_LVAL (arg3) = not_lval;
> > +	  return arg3;
> 
> We want to get away from changing core properties like lval
> in values after the fact ...   In any case, hard-coding the
> lval to non_lval without any further change can cause problems,
> e.g. if the value is lazy.

Thank you for looking into the patch and clarifying things.

> I think there is a more general issue underlying this particular
> change.  You're right that the result of a preincrement should
> not be an lvalue.  But the same is true for results of assignment
> operators in general.

Interesting - fixed. : )

> Note that value_assign is used only to implement such operators
> (simple assignment, compound assignment, pre/postfix operators).
> Since *all* of them return non-lvalues, it might make sense to
> simply change value_assign to return a non-lvalue in the first
> place ...

Sounds good because value_assign would match the C semantic then. 
Unfortunately the varobj.c:varobj_set_value function (through the 
gdb_value_assign wrapper) seems to rely on the return value of value_assing 
beeing a lvalue. Therefore I decided to adjust the places where value_assign 
is called.

> > +	  type = value_type (arg1);
> > +	  arg3 = allocate_value (type);
> > +
> > +	  /* Copy the value to prevent to return a lvalue.  */
> > +	  memcpy (value_contents_raw (arg3), value_contents (arg1),
> > +		  TYPE_LENGTH (type));
> 
> I'd prefer to encapsulate this in a function, e.g. value_non_lval (...)
> or a similar name, which returns a version of the value that is non_lval.
> This could then address a couple of additional issues:
> - if the value is already non_lval, no need to create an extra copy
> - it might be better to copy the full contents / enclosing type
>   for C++ objects

The attached patch introduces a new function called value_non_lval that 
returns a non-lval version of the given value. This function is called prior 
the simple assignment, compound assignment and pre/postfix routines return. Any 
comments are appreciated.

Regards
Ken Werner