Flang-new: How to force arrays to be allocated on the heap? (a possible bug)

According to information on GitHub, flang -mmlir -fdynamic-heap-array should force dynamic arrays to be allocated on the heap and hence avoid stackoverflows when large dynamic arrays are present. The example mentioned on GitHub works but the following does not.

! array.f90

module outprod_mod
function outprod(y, z) result(x)
implicit none
real, intent(in) :: y(:), z(:)
real :: x(size(y), size(z))
integer :: i, j, n, m

m = size(y)
n = size(z)
do i = 1, m
    do j = 1, n
        x(i, j) = y(i) * z(j)
    end do
end do
end function
end module outprod_mod

program prog
use outprod_mod, only : outprod
implicit none

integer, parameter :: k = 10000
integer :: i
real :: y(k)
y = 1.0
do i = 1, k
    write (*, *) i, size(outprod(y(1:i), y(1:i)))
end do

With flang-new, it crashes with a segfault as follows.

$ uname -a && flang --version && flang -fno-stack-arrays -mmlir -fdynamic-heap-array -o array array.f90 && ./array

Linux 6.5.0-26-generic #26~22.04.1-Ubuntu SMP PREEMPT_DYNAMIC Tue Mar 12 10:22:43 UTC 2 x86_64 x86_64 x86_64 GNU/Linux
flang-new version 19.0.0git (git@github.com:llvm/llvm-project.git 37c175af955f0aeab67e8c553a0a47b2ed0fdba2)
Target: x86_64-unknown-linux-gnu
Thread model: posix
 ... ...
 1445 2088025
 1446 2090916
 Segmentation fault (core dumped)


  • What are the correct flags to effectively force dynamic arrays to be allocated on the heap?
  • -fdynamic-heap-array alone cannot be recognized by the compiler. It must be -mmlir -fdynamic-heap-array. Is this intended?


  • ulimit -s unlimited seems to help, but we need a solution on flang’s side.
  • Compiled with Classic Flang 15.0.3 without any flags, the code runs fine.

See also flang-new: forcing arrays to be allocated on the heap and a possible bug · Issue #88344 · llvm/llvm-project · GitHub .