GCC precompiled header bug on bdver2 with -march=native

a.cpp

#include "stdafx.h"
int main(int argc, char**argv)
{
  std::string s = "Hi";
  std::cout << s << std::endl;
  return 0;
}

build.sh

#!/bin/bash

set -euo pipefail


CXXFLAGS="-O3 -march=native -pipe -Winvalid-pch"
# on an AMD FX-8320E this appears, about 40% of the time, with both gcc-7.3.0 and gcc-8.1.0, on Gentoo ~amd64:
#a.cpp:1:20: warning: stdafx.h.gch: created and used with differing settings of '-mlwp'
# #include "stdafx.h"
#                    ^

#CXXFLAGS="-O3 -march=bdver2 -pipe -Winvalid-pch"
# no warning with an explicit -march

rm -f stdafx.h.gch a
g++ $CXXFLAGS -c stdafx.h -o stdafx.h.gch
g++ $CXXFLAGS -o a a.cpp

stdafx.h

#include <string>
#include <iostream>