Re: Fast trunc() algorithm

From	David Brown <david.brown@hesbynett.no>
Newsgroups	comp.lang.c++, comp.lang.c, de.comp.lang.c
Subject	Re: Fast trunc() algorithm
Date	2022-05-30 08:59 +0200
Organization	A noiseless patient Spider
Message-ID	<t71pvp$d5o$1@dont-email.me> (permalink)
References	<t6va0v$eno$1@dont-email.me> <t70p0f$a8$1@dont-email.me> <t71nqf$6p4$1@gioia.aioe.org>

Cross-posted to 3 groups.

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On 30/05/2022 08:22, Juha Nieminen wrote:
> 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).

Using unions for type-punning has, AFAIUI, always been defined behaviour 
in C.  The wording was changed in C99 to make it clear that it is 
allowed, but the way the change was done (as a footnote) indicates that 
this was meant as a clarification, not a change to the language.  In 
C++, however, using unions for type-punning has always been UB.  (This 
can be viewed as an indication that C++ is more strongly typed than C.)

(You are of course correct that reinterpret_cast can't be used for 
accessing one type as an incompatible type.)

> 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).

I believe you could also write your own memcpy version - using char or 
unsigned char to access the data, or std::byte in C++17.  (You might 
want that if you have a poorly optimising compiler that uses a library 
call for memcpy even in simple cases.)

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

Yes.

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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