Re: [PATCH] Implement floordiv operator for gdb.Value

[Jonathan Wakely <jwakely at redhat dot com>]

On 20/09/16 16:33 +0100, Pedro Alves wrote:
> Hi there,
>
> Thanks!
>
> On 09/20/2016 02:26 PM, Jonathan Wakely wrote:
> > This is my attempt to implement the // operator on gdb.Value objects.
> > There is already BINOP_INTDIV which works fine for integral types, but
> > for floats I use BINOP_DIV and then call floor() on the result. This
> > doesn't support decimal floats though.
> >
> > Is this a reasonable solution? Is the test sufficient?
>
> See below.
>
> > @@ -1142,7 +1160,15 @@ valpy_binop_throw (enum valpy_opcode opcode, PyObject *self, PyObject *other)
> >      }
> >
> >    if (res_val)
> > -    result = value_to_value_object (res_val);
> > +    {
> > +      if (floor_it)
> > +        {
> > +          double d = value_as_double (res_val);
>
> Should be s/double/DOUBLEST, I suppose?

OK - if I do that then floor(d) will convert it back to double,
unless you #include <cmath> and using std::floor, so that the overload
for long double is visible (in C++ <math.h> names like floor are
overloaded so you don't need to use floorf/floor/floorl according to
the type).

> > diff --git a/gdb/testsuite/gdb.python/py-value.exp b/gdb/testsuite/gdb.python/py-value.exp
> > index 57a9ba1..81837e9 100644
> > --- a/gdb/testsuite/gdb.python/py-value.exp
> > +++ b/gdb/testsuite/gdb.python/py-value.exp
> > @@ -87,6 +87,8 @@ proc test_value_numeric_ops {} {
> >    gdb_test "python print ('result = ' + str(f/g))" " = 0.5" "divide two double values"
> >    gdb_test "python print ('result = ' + str(i%j))" " = 1" "take remainder of two integer values"
> >    # Remainder of float is implemented in Python but not in GDB's value system.
> > +  gdb_test "python print ('result = ' + str(i//j))" " = 2" "floor-divide two integer values"
> > +  gdb_test "python print ('result = ' + str(f//g))" " = 0" "floor-divide two double values"
>
> Is the "two double values" test returning an integer somehow?
>
> I ask because IIUC, regardless of Python version, a floor-divide
> involving a float should result in a float, while a floor-divide of
> integers should result in an integer.  And that's what the patch looks
> like should end up with.  So I was expecting to see "0.0" in
> the "two double values" case:
>
> (gdb) python print (5.0//6.0)
> 0.0
> (gdb) python print (5//6)
> 0

This seems to be an existing property of gdb.Value, as even using the
normal division operator (and without my patch) I see floats printed
without a decimal part when they are an integer value:

(gdb) python print (gdb.Value(5.0)/5.0)
1
(gdb) python print (5.0/5.0)
1.0



> I think it'd be good to test with negative numbers too, to make
> sure that we round (and keep rounding) toward the same
> direction Python rounds:
>
> (gdb) python print (8.0//-3)
> -3.0
> (gdb) python print (8//-3)
> -3
> (gdb) print 8/-3
> $1 = -2

Good point, I'll do that.