add overflow checks for MT_UNSIGNED, MT_NEGATIVE and MT_TAG

[detmat]

[pyropeter]

GCC 4.8.4 was released December 19, 2014.

[pyropeter]

Oh, and __builtin_add_overflow is supported since GCC 5.0.1, released April 22, 2015.

GCC 5 is too new for the oldest LTS releases of Debilian and Ubuntu, which both will reach EOL in the middle of 2018 and 2019 respectively.

[cabo]

So how can we do this in a way that leaves a version that conforms to the C standards?

[jimsch]

__builtin_add_overflow is not supported by my MSVC compiler. I would not be in favor of this.

[pyropeter]

@jimsch It would be interesting to know: how do you do overflow checks? Does MSVC have similar builtins? Do they have library functions doing integer overflow checking?

[jimsch]

I am not aware of any such function that is part of MSVC. The few times that I have cared about it I have always written specific code for that addition. Usually by adding in a longer int and checking the result. The need to make sure that things run on all the different architectures targeted.