[Flang] Pointer assignments for scalar pointers are slower than GFortran

[yus3710-fj]

The execution of the following codes is slower than GFortran.

• test.f90

subroutine pisnnn116(lhs,rhs)
  integer, target :: rhs
  integer, pointer :: lhs
  lhs => rhs
end subroutine

• main.f90

implicit none

integer, parameter :: count = 40000
integer, target :: i,j,k
integer, pointer :: pi
real :: start_t, end_t
interface
  subroutine pisnnn116(lhs,rhs)
    integer, target :: rhs
    integer, pointer :: lhs
  end subroutine
end interface

call cpu_time(start_t)
do i=1,count
  do j=1,count
    call pisnnn116(pi,k)
  end do
end do
call cpu_time(end_t)

print *, end_t - start_t
end

• commands

$ flang -Ofast -mcpu=native test.f90 main.f90 && ./a.out # for A64FX/Grace
$ flang -Ofast -march=native test.f90 main.f90 && ./a.out # for Xeon

	Flang [s]	GFortran [s]
A64FX	4.1902985	2.45159817
Grace	0.9753552	0.975116968
Xeon	1.5322586	1.52703595

[yus3710-fj]

Flang copies not only the address but also other information such as element size. I suspect that, at least for trivial types, only address copies are necessary for pointer assignment.

[llvmbot]

@llvm/issue-subscribers-flang-ir

Author: Yusuke MINATO (yus3710-fj)

The execution of the following codes is slower than GFortran.

• test.f90

subroutine pisnnn116(lhs,rhs)
  integer, target :: rhs
  integer, pointer :: lhs
  lhs => rhs
end subroutine

• main.f90

implicit none

integer, parameter :: count = 40000
integer, target :: i,j,k
integer, pointer :: pi
real :: start_t, end_t
interface
  subroutine pisnnn116(lhs,rhs)
    integer, target :: rhs
    integer, pointer :: lhs
  end subroutine
end interface

call cpu_time(start_t)
do i=1,count
  do j=1,count
    call pisnnn116(pi,k)
  end do
end do
call cpu_time(end_t)

print *, end_t - start_t
end

• commands

$ flang -Ofast -mcpu=native test.f90 main.f90 && ./a.out # for A64FX/Grace
$ flang -Ofast -march=native test.f90 main.f90 && ./a.out # for Xeon

	Flang [s]	GFortran [s]
A64FX	4.1902985	2.45159817
Grace	0.9753552	0.975116968
Xeon	1.5322586	1.52703595

[yus3710-fj]

Closed this issue because it doesn't occur on newer CPUs.