Path: ...!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail
From: Thiago Adams <thiago.adams@gmail.com>
Newsgroups: comp.lang.c
Subject: Re: What is your opinion about unsigned int u = -2 ?
Date: Fri, 2 Aug 2024 22:44:31 -0300
Organization: A noiseless patient Spider
Lines: 32
Message-ID: <v8k21v$33nca$1@dont-email.me>
References: <v8dfo9$1k7cg$1@dont-email.me>
 <pan$d2c8a$8c54ac9f$29a202e0$12c6ce86@invalid.invalid>
 <87bk2cecan.fsf@bsb.me.uk> <v8inds$2qpqh$1@dont-email.me>
 <v8iqnr$7l3c$1@news.xmission.com> <v8irje$2rolg$1@dont-email.me>
 <87r0b6g3qx.fsf@nosuchdomain.example.com> <v8jbj5$2us0r$4@dont-email.me>
 <v8jvln$33atp$1@dont-email.me> <87h6c2fldh.fsf@nosuchdomain.example.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Injection-Date: Sat, 03 Aug 2024 03:44:32 +0200 (CEST)
Injection-Info: dont-email.me; posting-host="6ca7cab00778520cac37785ddd7f8d1f";
	logging-data="3267978"; mail-complaints-to="abuse@eternal-september.org";	posting-account="U2FsdGVkX1+N6JxKloXAzvRRGpl1FIiXJdoducoKRm4="
User-Agent: Mozilla Thunderbird
Cancel-Lock: sha1:o4pfdqAOHfUBhLxag8BtbNLgVJ4=
Content-Language: en-GB
In-Reply-To: <87h6c2fldh.fsf@nosuchdomain.example.com>
Bytes: 2241

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.

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

#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);
}