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Re: Fast trunc() algorithm

Cross-posted to 3 groups.

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30.05.2022 09:22 Juha Nieminen kirjutas:

> 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).

It might work sometimes, until it doesn't. Here is a counter-example. It 
uses std::launder() (which ought to be kind of a null-op, not quite sure 
why using it makes a difference).

$ cat test1.cpp
#include <iostream>
#include <new>
int main() {
         float x = 3.14;
         int y = *std::launder(reinterpret_cast<int*>(&x));
         std::cout << y << "\n";
}

$ g++ -std=c++20 -O0 -Wall test1.cpp
$ ./a.exe
1078523331

$ g++ -std=c++20 -O3 -Wall test1.cpp
$ ./a.exe
0

You can see with -O3 it has "optimized" the result to be 0. It has all 
the rights to do that because this code contains UB-s.

With memcpy it works as intended also with -O3:

$ cat test1.cpp
#include <iostream>
#include <cstring>
int main() {
	float x = 3.14;
	int y;
	std::memcpy(&y, &x, sizeof(int));
	std::cout << y << "\n";
}

$ g++ -std=c++20 -O0 -Wall test1.cpp
$ ./a.exe
1078523331
$ g++ -std=c++20 -O3 -Wall test1.cpp
$ ./a.exe
1078523331



$ g++ -v
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-pc-cygwin/10/lto-wrapper.exe
Target: x86_64-pc-cygwin
Configured with: 
/mnt/share/cygpkgs/gcc/gcc.x86_64/src/gcc-10.2.0/configure 
--srcdir=/mnt/share/cygpkgs/gcc/gcc.x86_64/src/gcc-10.2.0 --prefix=/usr 
--exec-prefix=/usr --localstatedir=/var --sysconfdir=/etc 
--docdir=/usr/share/doc/gcc --htmldir=/usr/share/doc/gcc/html -C 
--build=x86_64-pc-cygwin --host=x86_64-pc-cygwin 
--target=x86_64-pc-cygwin --without-libiconv-prefix 
--without-libintl-prefix --libexecdir=/usr/lib 
--with-gcc-major-version-only --enable-shared --enable-shared-libgcc 
--enable-static --enable-version-specific-runtime-libs 
--enable-bootstrap --enable-__cxa_atexit --with-dwarf2 
--with-tune=generic --enable-languages=c,c++,fortran,lto,objc,obj-c++ 
--enable-graphite --enable-threads=posix --enable-libatomic 
--enable-libgomp --enable-libquadmath --enable-libquadmath-support 
--disable-libssp --enable-libada --disable-symvers --with-gnu-ld 
--with-gnu-as --with-cloog-include=/usr/include/cloog-isl 
--without-libiconv-prefix --without-libintl-prefix --with-system-zlib 
--enable-linker-build-id --with-default-libstdcxx-abi=gcc4-compatible 
--enable-libstdcxx-filesystem-ts
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 10.2.0 (GCC)

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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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