[Bug c++/68316] New: GCC C++ compiler cannot compile a program using RDESED intrinsics

["noloader at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68316

            Bug ID: 68316
           Summary: GCC C++ compiler cannot compile a program using RDESED
                    intrinsics
           Product: gcc
           Version: 5.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: noloader at gmail dot com
  Target Milestone: ---

According the Intel docs, RDSEED intrinsics are available in <immintrin.h>.
Confer,
http://software.intel.com/sites/landingpage/IntrinsicsGuide/#text=rdseed.
However, GCC cannot compile a program using RDSEED and the advertised header.
The same program compiles fine using RDRAND.

It appears the header needed (in this case) is <x86intrin.h>, which does not
appear to be documented anywhere other than this grep:

    $ grep -R rdseed /usr/include
    /usr/include/clang/3.4/include/rdseedintrin.h:#error "Never use
    <rdseedintrin.h> directly; include <x86intrin.h> instead."
                                            ^

This affects GCC 4.8 and 5.2, perhaps more.

**********

$ cat rdseed.cxx
#include <iostream>
#include <immintrin.h>

int main()
{
  unsigned long long val = 0;
  _rdseed64_step(&val);
  _rdseed32_step(&val);
  _rdseed16_step(&val);

  std::cout << val << std::endl;

  return !val;
}

$ g++ -mrdseed rdseed.cxx -o rdseed.exe
rdseed.cxx: In function âint main()â:
rdseed.cxx:7:22: error: â_rdseed64_stepâ was not declared in this scope
   _rdseed64_step(&val);
                      ^
rdseed.cxx:8:22: error: â_rdseed32_stepâ was not declared in this scope
   _rdseed32_step(&val);
                      ^
rdseed.cxx:9:22: error: â_rdseed16_stepâ was not declared in this scope
   _rdseed16_step(&val);
                      ^

**********

$ g++ --version
g++ (Debian 5.2.1-20) 5.2.1 20151002

$ g++ --version
g++ (Ubuntu 4.8.4-2ubuntu1~14.04) 4.8.4

**********

Adding a __has_include to guard <x86intrin.h> appears to open another can of
worms:

    $ g++ -mrdseed rdseed.cxx -o rdseed.exe
    rdseed.cxx:4:45: error: missing binary operator before token "("
     #if defined(__has_include) && (__has_include(<x86intrin.h>))
                                                 ^