[Python-Dev] Re: [snake-farm] test test_slice failed -- [9, 7, 5, 3, 1] == [0]

[Michael Hudson]

Neal Norwitz <neal@metaslash.com> writes:

> On Tue, Nov 05, 2002 at 10:38:18PM +0100, Anders Qvist wrote:
> > Between 1036491804 and 1036522446 (should be GMT 2002-11-05, 10:20 and
> > 2002-11-05, 18:54 respectively), test_slice started failing with the
> > following message:
> > 
> >   test test_slice failed -- [9, 7, 5, 3, 1] == [0]
> > 
> > om moghedien; linux/alpha. There has been some changes containing
> > pointer arithmetic around line 160 of sliceobjects.c, which might be
> > guilty (just a shot in the dark; 64-bit clean?):

Oops!  This is my fault.

> It is a 64-bit problem, but doesn't have to do with the checkin to
> sliceobject.  The new test (below) broke with old code too.
> 
>         vereq(range(10)[::sys.maxint - 1], [0])
> 
> The problem is that sizeof(int) != sizeof(long).  The parameters for
> PySlice_GetIndices() and PySlice_GetIndicesEx() take ints, not longs.
> So the code is doing:
> 
>         range(10)[::-2]
> 
> since the high 32 bits are lost.
> 
> The only solution I can think of is to have 2 interfaces:
>   - the old API which takes int/int*, PySlice_GetIndices()
>     deprecate this API and raise overflow exception
>     if (int)value != (long)value
>   - the new API which takes long/long*, PySlice_GetIndicesEx()
> 
> PySlice_GetIndicesEx() was added in 2.3.

So you suggest changing PySlice_GetIndicesEx(), right?

> Otherwise, we are stuck with slices only being able to support
> 32 bits even on 64 bit architectures.
> 
> Other ideas?

I think the better idea is to call _PyEval_SliceIndex for the step
element of the slice too.  And maybe change the latter from

                else if (x < -INT_MAX)
                        x = 0;

to

                else if (x < -INT_MAX)
                        x = -INT_MAX;

Can you test this on a 64 bit platform or shall I just check it in?

Cheers,
M.

-- 
  After a heavy night I travelled on, my face toward home - the comma
  being by no means guaranteed.           -- paraphrased from cam.misc