[Bug c++/83171] New: std::bitset::count not inlining __popcountdi2

[lectem at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83171

            Bug ID: 83171
           Summary: std::bitset::count not inlining __popcountdi2
           Product: gcc
           Version: 7.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: lectem at gmail dot com
  Target Milestone: ---

Even if the size of the std::bitset is smaller than a long long, the compiler
still emits a call to __popcountdi2 to count the number of bits set to 1.

This is unnecessary for example if the size of the bitset is 8. I believe the
issue lies with the fact that __popcountdi2 is not inlined/optimized, even when
using -O3.

Clang (-O2, using libstdc++) and MSVC(/O2, Visual 2017) seem to handle this
correctly. 
Here is a link to the compiler explorer test :

https://godbolt.org/g/QTWgdb

The code used in the test :

//------------------------------------------
#include <bitset>
#include <stdint.h>

size_t foo(uint32_t value)
{
  std::bitset<8> bset = value;
  return bset.count();
}
//------------------------------------------

gcc -v
Target: x86_64-linux-gnu
Configured with: ../gcc-7.2.0/configure --prefix /root/staging
--build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
--disable-multilib --disable-bootstrap --disable-multiarch --with-arch-32=i586
--enable-clocale=gnu --enable-languages=c,c++,go,fortran --enable-ld=yes
--enable-gold=yes --enable-libstdcxx-debug --enable-libstdcxx-time=yes
--enable-linker-build-id --enable-lto --enable-plugins --enable-threads=posix
--with-pkgversion=GCC-Explorer-Build
Thread model: posix
gcc version 7.2.0 (GCC-Explorer-Build)