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

[Pedro Alves <palves@redhat.com>]

On 09/20/2016 05:35 PM, Jonathan Wakely wrote:
> 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).

OK.  I remember reading your blog about this mess a while ago.

If easy to do, sounds like we should just do it.  OOC, would calling 
std::floor directly instead of using "using" work just as well?

(This kind of raises the question of which float type / format / representation
to use for arithmetic here -- host's or target's.  gdb currently always
uses host's, but that's a much larger issue that we can just
continue to ignore.)

>> 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

Curious.  Off hand looks like a bug to me.  But since it's orthogonal
to your patch, let's leave it.

>> 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.

Thanks,
Pedro Alves