[Flang][OpenMP] - Excessive compile time when `_FortranAAssign` is called in OpenMP target regions

Description

OpenMP target regions that generate calls to _FortranAAssign experience significantly longer compile times. This occurs when using the firstprivate clause, which requires copying array data to the device. The compile time overhead is approximately 30x compared to equivalent code using private (which does not generate _FortranAAssign calls).

Reproducer - Slow Case (with firstprivate)

! test_firstprivate.f90
! This version uses firstprivate, which generates _FortranAAssign calls
program test_firstprivate
  implicit none
  integer, allocatable :: arr(:)
  integer :: source(8)
  integer :: result
  integer :: i

  ! Initialize source array
  do i = 1, 8
    source(i) = i * 10
  end do

  ! Allocate array
  allocate(arr(8))
  arr(:) = 0

  !$omp target firstprivate(source) map(tofrom: arr) map(from: result)
    ! firstprivate(source) causes _FortranAAssign to be called
    arr(:) = source(:)
    result = arr(1) + arr(8)
  !$omp end target

end program test_firstprivate

Compile time: ~29 seconds

Comparison - Fast Case (with private)

! test_firstprivate.f90
! This version uses firstprivate, which generates _FortranAAssign calls
program test_private
  implicit none
  integer, allocatable :: arr(:)
  integer :: source(8)
  integer :: result
  integer :: i

  ! Initialize source array
  do i = 1, 8
    source(i) = i * 10
  end do

  ! Allocate array
  allocate(arr(8))
  arr(:) = 0

  !$omp target private(source) map(tofrom: arr) map(from: result)
    ! firstprivate(source) causes _FortranAAssign to be called
    arr(:) = source(:)
    result = arr(1) + arr(8)
  !$omp end target


  deallocate(arr)
end program test_private

Compile time: ~0.9 seconds

Build Command

flang-new -fopenmp -fopenmp-targets=amdgcn-amd-amdhsa -O2 test_firstprivate.f90 -o test.out

Root Cause

The firstprivate clause requires the runtime to set up array data on the device side based on the array on the host. This generates calls to _FortranAAssign.

The private clause, by contrast, only allocates the array on the device without copying data, avoiding calls to _FortranAAssign entirely.

Impact

• 30x compile-time overhead when using firstprivate compared to private
• This makes development iteration times impractical for codes using firstprivate arrays with OpenMP GPU offloading
• The overhead is present regardless of array size or complexity

Environment

• Compiler: flang-new (upstream LLVM/Flang main branch)
• Target: amdgcn-amd-amdhsa (AMD GPU)
• Optimization level: -O2
• OpenMP flags: -fopenmp -fopenmp-targets=amdgcn-amd-amdhsa

Also related to https://github.com/llvm/llvm-project/issues/200922