Report: gmp-6.1.2 on MacOS 10.15 using GCC and two Clang

Hans Åberg haberg-1 at
Sun Oct 20 19:58:29 UTC 2019

> On 20 Oct 2019, at 21:44, Torbjörn Granlund <tg at> wrote:
> Hans Åberg <haberg-1 at> writes:
>  A common programming error is assuming that signed integer types are
>  two’s complement, because even though all current CPUs are that,
>  overflows are undefined in C/C++, and an optimizer can take advantage
>  of that. One example from [1]: checking overflows with x > x + 1 may
>  work as intended with optimization turned off, but when on, the
>  optimizer can assume that it is always false since x + 1 is undefined
>  when overflowed.
> I believe we assume signed integers are in two's complement.
> We don't do a lot of arithmetic on signed integers, though.

Strictly, it is for signed overflows one cannot assume modulo 2^n, n = number of bits. For the unsigned integer types it is required, though. Here is a list for various languages:

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