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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 | 2024-08-02 19:04 -0700 |
| Organization | None to speak of |
| Message-ID | <878qxefjk2.fsf@nosuchdomain.example.com> (permalink) |
| References | (6 earlier) <87r0b6g3qx.fsf@nosuchdomain.example.com> <v8jbj5$2us0r$4@dont-email.me> <v8jvln$33atp$1@dont-email.me> <87h6c2fldh.fsf@nosuchdomain.example.com> <v8k21v$33nca$1@dont-email.me> |
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 */
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What is your opinion about unsigned int u = -2 ? Thiago Adams <thiago.adams@gmail.com> - 2024-07-31 10:55 -0300
Re: What is your opinion about unsigned int u = -2 ? Ben Bacarisse <ben@bsb.me.uk> - 2024-07-31 20:29 +0100
Re: What is your opinion about unsigned int u = -2 ? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-08-11 12:33 -0700
Re: What is your opinion about unsigned int u = -2 ? Vir Campestris <vir.campestris@invalid.invalid> - 2024-08-11 21:08 +0100
Re: What is your opinion about unsigned int u = -2 ? Richard Damon <richard@damon-family.org> - 2024-08-11 16:45 -0400
Re: What is your opinion about unsigned int u = -2 ? James Kuyper <jameskuyper@alumni.caltech.edu> - 2024-08-11 16:53 -0400
Re: What is your opinion about unsigned int u = -2 ? Vir Campestris <vir.campestris@invalid.invalid> - 2024-08-12 11:47 +0100
Re: What is your opinion about unsigned int u = -2 ? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-08-11 23:07 -0700
Re: What is your opinion about unsigned int u = -2 ? James Kuyper <jameskuyper@alumni.caltech.edu> - 2024-07-31 17:17 -0400
Re: What is your opinion about unsigned int u = -2 ? Blue-Maned_Hawk <bluemanedhawk@invalid.invalid> - 2024-08-01 06:34 +0000
Re: What is your opinion about unsigned int u = -2 ? Ben Bacarisse <ben@bsb.me.uk> - 2024-08-01 12:02 +0100
Re: What is your opinion about unsigned int u = -2 ? Thiago Adams <thiago.adams@gmail.com> - 2024-08-02 10:36 -0300
Re: What is your opinion about unsigned int u = -2 ? gazelle@shell.xmission.com (Kenny McCormack) - 2024-08-02 14:33 +0000
Re: What is your opinion about unsigned int u = -2 ? Bart <bc@freeuk.com> - 2024-08-02 15:48 +0100
Re: What is your opinion about unsigned int u = -2 ? Ben Bacarisse <ben@bsb.me.uk> - 2024-08-02 16:17 +0100
Re: What is your opinion about unsigned int u = -2 ? Thiago Adams <thiago.adams@gmail.com> - 2024-08-02 15:39 -0300
Re: What is your opinion about unsigned int u = -2 ? Thiago Adams <thiago.adams@gmail.com> - 2024-08-02 15:58 -0300
Re: What is your opinion about unsigned int u = -2 ? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-08-02 11:48 -0700
Re: What is your opinion about unsigned int u = -2 ? James Kuyper <jameskuyper@alumni.caltech.edu> - 2024-08-02 15:21 -0400
Re: What is your opinion about unsigned int u = -2 ? Thiago Adams <thiago.adams@gmail.com> - 2024-08-02 22:03 -0300
Re: What is your opinion about unsigned int u = -2 ? Thiago Adams <thiago.adams@gmail.com> - 2024-08-02 22:12 -0300
Re: What is your opinion about unsigned int u = -2 ? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-08-02 18:31 -0700
Re: What is your opinion about unsigned int u = -2 ? Thiago Adams <thiago.adams@gmail.com> - 2024-08-02 22:50 -0300
Re: What is your opinion about unsigned int u = -2 ? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-08-02 19:29 -0700
Re: What is your opinion about unsigned int u = -2 ? Thiago Adams <thiago.adams@gmail.com> - 2024-08-03 09:36 -0300
Re: What is your opinion about unsigned int u = -2 ? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-08-03 16:10 -0700
Re: What is your opinion about unsigned int u = -2 ? Thiago Adams <thiago.adams@gmail.com> - 2024-08-03 22:46 -0300
Re: What is your opinion about unsigned int u = -2 ? Thiago Adams <thiago.adams@gmail.com> - 2024-08-03 23:04 -0300
Re: What is your opinion about unsigned int u = -2 ? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-08-02 18:25 -0700
Re: What is your opinion about unsigned int u = -2 ? Thiago Adams <thiago.adams@gmail.com> - 2024-08-02 22:44 -0300
Re: What is your opinion about unsigned int u = -2 ? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-08-02 19:04 -0700
Re: What is your opinion about unsigned int u = -2 ? Thiago Adams <thiago.adams@gmail.com> - 2024-08-02 23:27 -0300
Re: What is your opinion about unsigned int u = -2 ? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-08-02 19:40 -0700
Re: What is your opinion about unsigned int u = -2 ? David Brown <david.brown@hesbynett.no> - 2024-08-03 19:43 +0200
Re: What is your opinion about unsigned int u = -2 ? Thiago Adams <thiago.adams@gmail.com> - 2024-08-03 14:46 -0300
Re: What is your opinion about unsigned int u = -2 ? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-08-03 16:52 -0700
Re: What is your opinion about unsigned int u = -2 ? dave_thompson_2@comcast.net - 2024-08-25 16:52 -0400
Re: What is your opinion about unsigned int u = -2 ? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-08-25 14:28 -0700
Re: What is your opinion about unsigned int u = -2 ? David Brown <david.brown@hesbynett.no> - 2024-08-03 19:34 +0200
Re: What is your opinion about unsigned int u = -2 ? Thiago Adams <thiago.adams@gmail.com> - 2024-08-03 14:51 -0300
Re: What is your opinion about unsigned int u = -2 ? Thiago Adams <thiago.adams@gmail.com> - 2024-08-03 15:30 -0300
Re: What is your opinion about unsigned int u = -2 ? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-08-03 16:46 -0700
Re: What is your opinion about unsigned int u = -2 ? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-08-11 13:57 -0700
Re: What is your opinion about unsigned int u = -2 ? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-08-11 14:53 -0700
Re: What is your opinion about unsigned int u = -2 ? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-08-03 16:19 -0700
Re: What is your opinion about unsigned int u = -2 ? Bonita Montero <Bonita.Montero@gmail.com> - 2024-08-09 20:10 +0200
Re: What is your opinion about unsigned int u = -2 ? David Brown <david.brown@hesbynett.no> - 2024-08-03 19:17 +0200
Re: What is your opinion about unsigned int u = -2 ? Bonita Montero <Bonita.Montero@gmail.com> - 2024-08-04 20:29 +0200
Re: What is your opinion about unsigned int u = -2 ? Lawrence D'Oliveiro <ldo@nz.invalid> - 2024-08-07 23:48 +0000
Re: What is your opinion about unsigned int u = -2 ? David Brown <david.brown@hesbynett.no> - 2024-08-08 19:47 +0200
Re: What is your opinion about unsigned int u = -2 ? Bonita Montero <Bonita.Montero@gmail.com> - 2024-08-09 20:08 +0200
Re: What is your opinion about unsigned int u = -2 ? David Brown <david.brown@hesbynett.no> - 2024-08-09 20:19 +0200
Re: What is your opinion about unsigned int u = -2 ? Bonita Montero <Bonita.Montero@gmail.com> - 2024-08-09 21:18 +0200
Re: What is your opinion about unsigned int u = -2 ? David Brown <david.brown@hesbynett.no> - 2024-08-09 21:40 +0200
Re: What is your opinion about unsigned int u = -2 ? Richard Damon <richard@damon-family.org> - 2024-08-09 21:16 -0400
Re: What is your opinion about unsigned int u = -2 ? Bonita Montero <Bonita.Montero@gmail.com> - 2024-08-04 18:16 +0200
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