Efficient detection of signed overflow?

[Andrew Haley]

Segher Boessenkool wrote:
>>>> The comment is wrong.  The code checks for signed overflow, but the
>>>> following assignment still overflwos when ux is larger than INT_MAX.
>>> No, it doesn't.  This conversion is implementation-defined (6.3.1.3/3),
>>> and GCC does the obvious two's complement thing.  This code is fine.
>> It's fine with GCC 4.4, and likely with GCC 4.5 as well.  But what
>> about GCC 4.6?  And how will a user compiling third-party software
>> notice the discrepancy (if it ever arises)?
> 
> Implementation-defined means the implementation defines
> the behaviour, and GCC defines it like this:

We know, we already discussed this upthread.  The question is not
whether it works, but how to do it portably, not just for gcc.
Depending on 2's complement is fine, but depending on a particular
compiler is less so.

Andrew.