[Bug c++/61075] New: parallel std::accumulate reduct type cannot be different than the iterated type

[denes.matetelki at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=61075

Bug ID: 61075
   Summary: parallel std::accumulate reduct type cannot be
different than the iterated type
   Product: gcc
   Version: 4.8.2
Status: UNCONFIRMED
  Severity: normal
  Priority: P3
 Component: c++
  Assignee: unassigned at gcc dot gnu.org
  Reporter: denes.matetelki at gmail dot com

[Bug c++/61075] parallel std::accumulate reduct type cannot be different than the iterated type

[denes.matetelki at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=61075

Denes Matetelki  changed:

   What|Removed |Added

   Host||Linux  3.13.5-gentoo #10
   ||SMP Fri Apr 25 16:12:35
   ||CEST 2014 x86_64 Intel(R)
   ||Xeon(R) CPU W3690 @ 3.47GHz
   ||GenuineIntel GNU/Linux

--- Comment #1 from Denes Matetelki  ---
#include 
#include 
#include 

class Custom
{
public:
  Custom(int i) : m_i(i) {}
  int getI() const { return m_i; }

private:
  int m_i;
};

int main (int /*argc*/, char* /*argv*/[])
{
  std::vector v(10, 1);
  const int v_sum = std::accumulate(v.begin(), v.end(), 0,
[](int sum, int i) { return sum + i; } );


  std::vector v2(10, Custom(1));
  const int v2_sum = std::accumulate(v2.begin(), v2.end(), 0,
[](int sum, const Custom& c) { return sum +
c.getI(); } );

  return 0;
}

Compiling with parallel mode:

parallel_accumulate.cpp:33:93:   required from here
/usr/lib/gcc/x86_64-pc-linux-gnu/4.8.2/include/g++-v4/parallel/par_loop.h:102:15:
error: cannot convert
'std::__iterator_traits<__gnu_cxx::__normal_iterator >, true>::value_type {aka Custom}' to 'int' in
initialization
  __reduct = new _Result(__f(__o, __begin + __start));

[Bug libstdc++/61075] parallel std::accumulate reduct type cannot be different than the iterated type

[denes.matetelki at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=61075

--- Comment #2 from Denes Matetelki  ---
g++ -v

Using built-in specs.
COLLECT_GCC=/usr/x86_64-pc-linux-gnu/gcc-bin/4.8.2/g++
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-pc-linux-gnu/4.8.2/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: /var/tmp/portage/sys-devel/gcc-4.8.2/work/gcc-4.8.2/configure
--host=x86_64-pc-linux-gnu --build=x86_64-pc-linux-gnu --prefix=/usr
--bindir=/usr/x86_64-pc-linux-gnu/gcc-bin/4.8.2
--includedir=/usr/lib/gcc/x86_64-pc-linux-gnu/4.8.2/include
--datadir=/usr/share/gcc-data/x86_64-pc-linux-gnu/4.8.2
--mandir=/usr/share/gcc-data/x86_64-pc-linux-gnu/4.8.2/man
--infodir=/usr/share/gcc-data/x86_64-pc-linux-gnu/4.8.2/info
--with-gxx-include-dir=/usr/lib/gcc/x86_64-pc-linux-gnu/4.8.2/include/g++-v4
--with-python-dir=/share/gcc-data/x86_64-pc-linux-gnu/4.8.2/python
--enable-languages=c,c++,fortran --enable-obsolete --enable-secureplt
--disable-werror --with-system-zlib --enable-nls --without-included-gettext
--enable-checking=release --with-bugurl=https://bugs.gentoo.org/
--with-pkgversion='Gentoo 4.8.2 p1.3r1, pie-0.5.8r1' --enable-libstdcxx-time
--enable-shared --enable-threads=posix --enable-__cxa_atexit
--enable-clocale=gnu --enable-multilib --with-multilib-list=m32,m64
--disable-altivec --disable-fixed-point --enable-targets=all --disable-libgcj
--enable-libgomp --disable-libmudflap --disable-libssp --enable-lto
--without-cloog
Thread model: posix
gcc version 4.8.2 (Gentoo 4.8.2 p1.3r1, pie-0.5.8r1)

[Bug libstdc++/61075] parallel std::accumulate reduct type cannot be different than the iterated type

[denes.matetelki at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61075

--- Comment #4 from Denes Matetelki  ---
Thank you for the reply, Jonathan.

I understand your reasoning and not sure if my desires has much impact in the
future of GCC.

I'm suprised that the same source code cannot be compiled with parallel mode.
It would be ugly to branch with #ifdef _GLIBCXX_PARALLEL. 
Also, I feel it should be allowed for the user to create a custom labda to add
up custom types, just like in the single threaded mode.
The std::experimental::reduce is experimental, how shall I solve the problem
now? The world accumulate shows the intention, shall I use my own
implementation of reduce till that time: a parallel for_each then a single
threaded for to sum up?

[Bug libstdc++/61075] parallel std::accumulate reduct type cannot be different than the iterated type

[denes.matetelki at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61075

Denes Matetelki  changed:

   What|Removed |Added

 Status|NEW |RESOLVED
 Resolution|--- |INVALID

--- Comment #6 from Denes Matetelki  ---
I see. Thank you for your help, 

I'll implement the convert operator.

[Bug libstdc++/61075] parallel std::accumulate reduct type cannot be different than the iterated type

[denes.matetelki at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61075

--- Comment #8 from Denes Matetelki  ---
Just an observation:

The contained type also need to declare and define ctor(int).

Which can be tricky, if it is a template class and:

template 
class Custom
{
public:
  Custom(T t) : m_t(t) {}
  Custom(int i) : m_t() {}
  explicit operator int() const { return convertToInt(m_t); }

private:
  T m_t;
};

Custom c; // ambigous ctor(int)