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On Windows 64-bit, Visual Studio generates a lot of warnings because Py_ssize_t values are downcasted to int.
Attached patch fixes all warnings and move the final downcast into compiler_addop_i(). This function uses an assertion to check that integer parameter fits into an C int type. I don't know if it's safe to "return 0" on overflow error.
The patch fixes also some indentation issues seen was I wrote the patch.
Nobody complained before, I don't know if the bugs can be seen in practice, so I prefer to not touch Python 2.7 nor 3.2.

You could use the Py_SAFE_DOWNCAST macro everywhere.

> You could use the Py_SAFE_DOWNCAST macro everywhere.
I prefer to store sizes in a size type (Py_ssize_t), and only downcast where it is really needed, in compiler_addop_i(). So in the future, if someone wants to support values larger than INT_MAX, only one function need to be changed.

I meant where you added casts, you could use it.
2013年11月18日 STINNER Victor <report@bugs.python.org>:
>
> STINNER Victor added the comment:
>
>> You could use the Py_SAFE_DOWNCAST macro everywhere.
>
> I prefer to store sizes in a size type (Py_ssize_t), and only downcast where it is really needed, in compiler_addop_i(). So in the future, if someone wants to support values larger than INT_MAX, only one function need to be changed.
>
> ----------
>
> _______________________________________
> Python tracker <report@bugs.python.org>
> <http://bugs.python.org/issue19617>
> _______________________________________

Oops, I forgot to mention this issue number in the commit:
---
changeset: 87277:68fd86a83ece
tag: tip
user: Victor Stinner <victor.stinner@gmail.com>
date: Tue Nov 19 22:23:20 2013 +0100
files: Python/compile.c
description:
Issue #9566: compile.c uses Py_ssize_t instead of int to store sizes to fix
compiler warnings on Windows 64-bit. Use Py_SAFE_DOWNCAST() where the final
downcast is needed.
The bytecode doesn't support integer parameters larger than 32-bit yet.
---
I added some more Py_SAFE_DOWNCAST() in the final commit.

New changeset 8d3e85dfa46f by Victor Stinner in branch 'default':
Issue #9566, #19617: Fix compilation on Windows
http://hg.python.org/cpython/rev/8d3e85dfa46f 

New changeset ee4da7291211 by Victor Stinner in branch 'default':
Issue #9566, #19617: New try to fix compilation on Windows
http://hg.python.org/cpython/rev/ee4da7291211 

New changeset 116bd550e309 by Victor Stinner in branch 'default':
Issue #9566, #19617: Fix more compiler warnings in compile.c on Windows 64-bit
http://hg.python.org/cpython/rev/116bd550e309