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Re: Fast trunc() algorithm

From Juha Nieminen <nospam@thanks.invalid>
Newsgroups comp.lang.c++, comp.lang.c, de.comp.lang.c
Subject Re: Fast trunc() algorithm
Date 2022-05-30 06:22 +0000
Organization Aioe.org NNTP Server
Message-ID <t71nqf$6p4$1@gioia.aioe.org> (permalink)
References <t6va0v$eno$1@dont-email.me> <t70p0f$a8$1@dont-email.me>

Cross-posted to 3 groups.

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In comp.lang.c++ Christian Gollwitzer <auriocus@gmx.de> wrote:
> Am 29.05.22 um 10:14 schrieb Bonita Montero:
>> 
>> memcpy() is the only valid way to have writeable aliasing in C++.
>> memcpy() is normally an intrinsic function and in the above code
>> the memcpy()s between integral and floating point types are just
>> moves between general purpose and FPU-registers.
> 
> How about reinterpret_cast<> ?

Doing type punning, ie. interpreting the bit representation of some type
as a value of an incompatible type, by reinterpret-casting a pointer to
that value, or by using a union, is technically speaking UB in C++
(but allowed in C, I think since C11 or the like), even though in
practice it does work (for the same reason it does in C).
The only "official" way of doing it (prior to C++20) was to use
std::memcpy() (and hope that the compiler will optimize it into
equivalent code as using a union).

C++20 added an official way of doing type punning: std::bit_cast.

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Thread

Fast trunc() algorithm Bonita Montero <Bonita.Montero@gmail.com> - 2022-05-29 10:14 +0200
  Re: Fast trunc() algorithm Muttley@dastardlyhq.com - 2022-05-29 08:21 +0000
    Re: Fast trunc() algorithm Bonita Montero <Bonita.Montero@gmail.com> - 2022-05-29 10:29 +0200
      Re: Fast trunc() algorithm Freethinker <freethinker@mymail.com> - 2022-05-29 15:20 +0200
        Re: Fast trunc() algorithm Bonita Montero <Bonita.Montero@gmail.com> - 2022-05-29 15:27 +0200
          Re: Fast trunc() algorithm Freethinker <freethinker@mymail.com> - 2022-05-29 15:37 +0200
            Re: Fast trunc() algorithm Bonita Montero <Bonita.Montero@gmail.com> - 2022-05-29 15:46 +0200
    Re: Fast trunc() algorithm John McCue <jmccue@magnetar.hsd1.ma.comcast.net> - 2022-05-29 21:32 +0000
  Re: Fast trunc() algorithm Christian Gollwitzer <auriocus@gmx.de> - 2022-05-29 23:36 +0200
    Re: Fast trunc() algorithm Juha Nieminen <nospam@thanks.invalid> - 2022-05-30 06:22 +0000
      Re: Fast trunc() algorithm David Brown <david.brown@hesbynett.no> - 2022-05-30 08:59 +0200
      Re: Fast trunc() algorithm muttley@dastardlyhq.com - 2022-05-30 07:52 +0000
      Re: Fast trunc() algorithm Paavo Helde <eesnimi@osa.pri.ee> - 2022-05-30 11:58 +0300
    Re: Fast trunc() algorithm Bonita Montero <Bonita.Montero@gmail.com> - 2022-05-30 11:32 +0200
  Re: Fast trunc() algorithm Bonita Montero <Bonita.Montero@gmail.com> - 2022-05-30 11:34 +0200
  Re: Fast trunc() algorithm Bonita Montero <Bonita.Montero@gmail.com> - 2022-05-31 05:41 +0200
    Re: Fast trunc() algorithm Bonita Montero <Bonita.Montero@gmail.com> - 2022-06-01 18:07 +0200
      Re: Fast trunc() algorithm Bonita Montero <Bonita.Montero@gmail.com> - 2022-06-01 18:09 +0200
      Re: Fast trunc() algorithm Bonita Montero <Bonita.Montero@gmail.com> - 2022-06-02 04:43 +0200
        Re: Fast trunc() algorithm jak <nospam@please.ty> - 2022-06-02 07:44 +0200
          Re: Fast trunc() algorithm Bonita Montero <Bonita.Montero@gmail.com> - 2022-06-02 08:14 +0200
            Re: Fast trunc() algorithm jak <nospam@please.ty> - 2022-06-02 11:14 +0200
              Re: Fast trunc() algorithm Bonita Montero <Bonita.Montero@gmail.com> - 2022-06-02 12:48 +0200

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