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


On 20/09/16 17:57 +0100, Pedro Alves wrote:
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?

Yes, and that's exactly what I did locally a few minutes ago. Calling
std::floor explicitly probably makes it more obvious that it relies on
the C++ overloads to preserve the right type.

(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.)

I'm happy to ignore it :-)

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.

Definitely worth testing because my patch gives the wrong result:

(gdb) python print(  gdb.Value(8) // gdb.Value(-3) )
-2

This is because I'm using the same BINOP_DIV operation as GDB's 8/-3
uses, but Python's // should round to -inf. I'll fix that.


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