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Re: Efficient detection of signed overflow?
>> The test was, if I recall correctly
>>
>> x = a + b;
>> if ((x ^ a) & (x ^ b)) < 0)
>>
>> all you have to do is convert everything to unsigned values, then
>>
>> ux = ua + ub;
>> if ((ux ^ ua) & (ux ^ ub)) & (unsigned)INT_MIN))
>> goto deal_with_overflow;
>> // we now know there is no overflow
>> x = ux;
>>
>> which is exactly the same test as before, but perfectly compliant.
>
> 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.
Segher