Comment 0 for bug 1879092

After glibc maintainers changed "math-vector-fortran.h" install directory, gfortran can no longer use it.

This the old behavior Ubuntu 19.10 gcc-9.2.1 glibc 2.30
$ gfortran -O3 -mavx2 vect_test.f90 -o test.x
$ nm ./test.x | grep sin
    U _ZGVdN4v_sin@@GLIBC_2.22
    U _ZGVdN8v_sinf@@GLIBC_2.22

After that change : Ubuntu 20.04 gcc-9.3.0 glibc 2.31
$ gfortran -O3 -mavx2 vect_test.f90 -o test.x
$ nm ./test.x | grep sinn
    U sin@@GLIBC_2.2.5
    U sinf@@GLIBC_2.2.5

$ cat vect_test.f90
program test_vect
  implicit none
  integer, parameter :: n = 64*1024**2
  integer :: i
  integer(8) :: t1, t2, t0
  real(8) :: tic
  !
  real, allocatable :: a32(:), b32(:)
  real(8), allocatable :: a64(:), b64(:)
  !
  allocate( a32(n), a64(n))
  allocate( b32(n), b64(n))
  !
  call random_number(a32 )
  call random_number(a64 )
  !
  call system_clock(t0, tic)
  !
  do i = 1, n
    b32(i) = sin(a32(i))
  end do
  !
  call system_clock(t1)
  print*, (t1-t0)/tic
  !
  do i = 1, n
    b64(i) = sin(a64(i))
  end do
  !
  call system_clock(t2)
  print*, (t2-t1)/tic
  !
end program test_vect

Verbose:

Ubuntu 19.10
$ gfortran -O3 -mavx2 -v vect_test.f90 -o test.x
...
/usr/lib/gcc/x86_64-linux-gnu/9/f951 vect_test.f90 -quiet -dumpbase vect_test.f90 -mavx2 -mtune=generic -march=x86-64 -auxbase vect_test -O3 -version -fintrinsic-modules-path /usr/lib/gcc/x86_64-linux-gnu/9/finclude -fpre-include=/usr/include/finclude/math-vector-fortran.h -o /tmp/cc2E2K8s.s
...

Ubuntu 20.04
$ gfortran -O3 -mavx2 -v vect_test.f90 -o test.x
...
/usr/lib/gcc/x86_64-linux-gnu/9/f951 vect_test.f90 -quiet -dumpbase vect_test.f90 -mavx2 -mtune=generic -march=x86-64 -auxbase vect_test -O3 -version -fintrinsic-modules-path /usr/lib/gcc/x86_64-linux-gnu/9/finclude -o /tmp/cc4swJwz.s
...

As you can see gfortran is not preincluding "math-vector-fortran.h" file on Ubuntu 20.04.
A workaround is to copy "math-vector-fortran.h" to /usr/include/finclude (which is empty!).