[pypy-issue] Issue #2704: Undefined behavior in RPython via the C backend (pypy/pypy)

[Carl Friedrich Bolz-Tereick]

New issue 2704: Undefined behavior in RPython via the C backend
https://bitbucket.org/pypy/pypy/issues/2704/undefined-behavior-in-rpython-via-the-c

Carl Friedrich Bolz-Tereick:

We did not completely decide so far whether int-addition (and other ops) in RPython must behave a) by wrapping around or b) by having undefined behavior on overflow.

Right now, the C backend and parts of the JIT implements b). However, we definitely have code where we use int operations that overflow and we expect a), most notably a lot of hash computations (eg ``W_TupleObject._descr_hash_*``). This has the potential (maybe unlikely though?) of causing miscompilation of PyPy at any point in the future when GCC (or the JIT) gets smarter about reasoning about ints.