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From: Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups: comp.lang.c
Subject: Re: What is your opinion about unsigned int u = -2 ?
Date: Fri, 02 Aug 2024 19:04:45 -0700
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Thiago Adams <thiago.adams@gmail.com> writes:
> Em 8/2/2024 10:25 PM, Keith Thompson escreveu:
>> Thiago Adams <thiago.adams@gmail.com> writes:
>> [...]
>>> I am doing experiments with constexpr. What happens with overflow in
>>> compile time? The answer is not so clear. Sometimes it does not
>>> compile and sometimes it works as in runtime.
>> Your first example fails due to signed integer overflow.  Your
>> second
>> example is well defined because there is no such thing as unsigned
>> integer overflow.
>
> I need a new name other than overflow.

Wraparound, or unsigned wraparound if you want to be a little more
precise.

C23 adds this entry to the glossary (3.28):

    wraparound

    the process by which a value is reduced modulo 2^N, where N is the
    width of the resulting type

The standard uses a superscript, which I can't easily reproduce here.
I hope it's sufficiently obvious that I'm not using ^ to mean bitwise xor.

N3220 6.2.5p11 (same or similar wording appears in earlier editions) :

    A computation involving unsigned operands can never produce an
    overflow, because arithmetic for the unsigned type is performed
    modulo 2^N.

> The expression
>
> ULLONG_MAX*ULLONG_MAX/ULLONG_MAX/ULLONG_MAX
>
> has the result 1 if calculated with infinite precision.
>
> But the calculated value here is 0

Right.  The subexpression ULLONG_MAX*ULLONG_MAX in infinite precision
would be 340282366920938463426481119284349108225 in infinite precision,
but assuming 64-bit unsigned long long the actual result is that value
modulo 2^64, or 1.  Dividing 1 by ULLONG_MAX yields 0 (since integer
division truncates); dividing again by ULLONG_MAX yields 0 again.

The reduction modulo 2^64 happens on each arithmitic operation (here one
multiplication and two divisions), not on the expression as a whole.
Since ULLONG_MAX*ULLONG_MAX is 1 (more precisely 1ull), the expression
    ULLONG_MAX*ULLONG_MAX/ULLONG_MAX/ULLONG_MAX
is equivalent to
    1ull/ULLONG_MAX/ULLONG_MAX

> #include <stdio.h>
>
> int main()
> {
>    constexpr unsigned long long ULLONG_MAX = 18446744073709551615;
>    constexpr unsigned long long r =
>    ULLONG_MAX*ULLONG_MAX/ULLONG_MAX/ULLONG_MAX;
>    printf("%llu", r);
> }

-- 
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
void Void(void) { Void(); } /* The recursive call of the void */