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A number of legitimate warnings get emitted under a 64-bit Windows build (in many places, _pickle uses ints or longs instead of "Py_ssize_t" variable to store various lengths and sizes):
1>..\Modules\_pickle.c(284) : warning C4244: '=' : conversion from 'Py_ssize_t' to 'int', possible loss of data
1>..\Modules\_pickle.c(301) : warning C4244: '=' : conversion from 'Py_ssize_t' to 'int', possible loss of data
1>..\Modules\_pickle.c(461) : warning C4244: '+=' : conversion from 'Py_ssize_t' to 'int', possible loss of data
1>..\Modules\_pickle.c(628) : warning C4244: '=' : conversion from 'Py_ssize_t' to 'long', possible loss of data
1>..\Modules\_pickle.c(647) : warning C4267: '=' : conversion from 'size_t' to 'int', possible loss of data
1>..\Modules\_pickle.c(1320) : warning C4244: '=' : conversion from 'Py_ssize_t' to 'int', possible loss of data
1>..\Modules\_pickle.c(1558) : warning C4244: '=' : conversion from 'Py_ssize_t' to 'int', possible loss of data
1>..\Modules\_pickle.c(1806) : warning C4244: '=' : conversion from 'Py_ssize_t' to 'int', possible loss of data

The warnings at lines 284, 301, 461, 647 are benign. The attached patch fixes them.
The others (lines 628, 1320, 1558, 1806) are real issues: pickle will fail when given a list, a tuple or a dict larger than INT_MAX, or when the memo is too large.
There are other issues in 2.7: pickling a large string fails with a SystemError("Negative size passed to PyString_FromStringAndSize")

See also issue 10640.

> The warnings at lines 284, 301, 461, 647 are benign.
I agree. There is no loss of data because Py_ssize_t variable bounds are checked before these lines are reached.
> The attached patch fixes them.
I don't like these changes:
-Pdata_poptuple(Pdata *self, Py_ssize_t start)
+Pdata_poptuple(Pdata *self, int start)
-Pdata_poplist(Pdata *self, Py_ssize_t start)
+Pdata_poplist(Pdata *self, int start)
These seem to attempt to fix
 Py_SIZE(self) = start;
assignments, but as far as I can tell, Py_SIZE(self) type is Py_ssize_t.
What do I miss here?

The warnings above are a bit old: 027f81579b4a changed Pdata into a PyVarObject, and the "int length" member is now accessed with the Py_SIZE() macro.
Unfortunately, the only win64 buildbot is offline, and I could not find any recent log of compilation warnings.

Attached. Looks like they're still there.

Yes there are still warnings, but in different places; here is an extract of the previous buildlog.html file:
 ..\Modules\_pickle.c(156) : warning C4244:'initializing' : conversion from 'Py_ssize_t' to 'int', possible loss of data
 ..\Modules\_pickle.c(195) : warning C4244: 'initializing' : conversion from 'Py_ssize_t' to 'int', possible loss of data
 ..\Modules\_pickle.c(754) : warning C4244: 'return' : conversion from 'Py_ssize_t' to 'int', possible loss of data
 ..\Modules\_pickle.c(1278) : warning C4244: '=' : conversion from 'Py_ssize_t' to 'long', possible loss of data
 ..\Modules\_pickle.c(1285) : warning C4267: '=' : conversion from 'size_t' to 'int', possible loss of data
 ..\Modules\_pickle.c(1952) : warning C4244: '=' : conversion from 'Py_ssize_t' to 'int', possible loss of data
 ..\Modules\_pickle.c(2233) : warning C4244: '=' : conversion from 'Py_ssize_t' to 'int', possible loss of data
 ..\Modules\_pickle.c(2493) : warning C4244: '=' : conversion from 'Py_ssize_t' to 'int', possible loss of data
 ..\Modules\_pickle.c(4350) : warning C4244: '=' : conversion from 'Py_ssize_t' to 'int', possible loss of data
 ..\Modules\_pickle.c(4615) : warning C4244: 'initializing' : conversion from 'Py_ssize_t' to 'int', possible loss of data
 ..\Modules\_pickle.c(4655) : warning C4244: '=' : conversion from 'Py_ssize_t' to 'int', possible loss of data
 ..\Modules\_pickle.c(4896) : warning C4244: '=' : conversion from 'Py_ssize_t' to 'int', possible loss of data
 ..\Modules\_pickle.c(4943) : warning C4244: 'function' : conversion from 'Py_ssize_t' to 'int', possible loss of data
 ..\Modules\_pickle.c(4960) : warning C4244: '=' : conversion from 'Py_ssize_t' to 'int', possible loss of data
 ..\Modules\_pickle.c(4991) : warning C4244: 'function' : conversion from 'Py_ssize_t' to 'int', possible loss of data
 ..\Modules\_pickle.c(5147) : warning C4244: '=' : conversion from 'Py_ssize_t' to 'int', possible loss of data

On Wed, Apr 27, 2011 at 4:31 AM, Amaury Forgeot d'Arc
<report@bugs.python.org> wrote:
>.. 027f81579b4a changed Pdata into a PyVarObject, and the "int length" member
> is now accessed with the Py_SIZE() macro.
ISTM that with this change Pdata struct is now indistinguishable from
a list. I wonder if the code could be simplified by using a regular
list instead of a custom object.

I believe attached issue9614.diff should fix the warnings, but I don't have a box to test this on.

This issue is obsolete, there are no 64-bit warnings in _pickle.c anymore.
See issue 11564.