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Dear users,
my understanding of memory allocation is that allocatable arrays are put by default on the heap. However the following simple code generate a stack overflow error:
program test implicit none ! Purpose: Generate a random array of dimensions(1000,10,30) ! and limit the maximum value of any array element to be less than or equal to 0.5 ! Write two set of statemente: (1) using nested DO-IF loops, (2) using WHERE !Declaration section integer :: n1,n2,n3 !Dimensions of allocatable arrays integer :: i,j,k,status,status_de real(8), allocatable :: array(:,:,:),array_final(:,:,:),array_final_vec(:,:,:) !Execution section n1 = 1000 n2 = 10 n3 = 30 allocate(array(n1,n2,n3),array_final(n1,n2,n3),array_final_vec(n1,n2,n3),STAT=status) if (status/=0) then write(*,*) 'Allocation failed' stop endif call RANDOM_SEED() call RANDOM_NUMBER(array) !Nested DO-IF loops do k=1,n3 do j=1,n2 do i=1,n1 if (array(i,j,k)<=0.5d0) then array_final(i,j,k) = array(i,j,k) elseif(array(i,j,k)>0.5d0) then array_final(i,j,k) = 0.5d0 else write(*,*) 'Something is wrong!' endif enddo enddo enddo !Vectorized WHERE where (array<=0.5d0) !Here stack overflow error occurs!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! array_final_vec = array elsewhere array_final_vec = 0.5d0 endwhere write(*,'(1X,A)') 'Original vector:' write(*,'(1X,F10.3)') (array(i,1,1), i=1,10) write(*,*) '...' write(*,'(1X,A)') 'Nested do-if loops, results:' write(*,'(1X,F10.3)') (array_final(i,1,1), i=1,10) write(*,*) '...' write(*,'(1X,A)') 'Vectorized WHERE, results:' write(*,'(1X,F10.3)') (array_final_vec(i,1,1), i=1,10) write(*,*) "Ready to deallocate big arrays..." deallocate(array,array_final,array_final_vec,STAT=status_de) if (status_de/=0) then write(*,*) 'De-allocation failed' stop endif end program test
The error occurs when the WHERE statement is reached. The surprising thing is that the allocation, the call to random number and the statements with the nested DO-IF loops are executed ok, but as soon as the code reaches the WHERE statement, it throws a stack overflow error, reported below:
Unhandled exception at 0x00D979D9 in Console1.exe: 0xC00000FD: Stack overflow (parameters: 0x00000000, 0x00A42000)
See attached txt for more info.
I was able to solve the problem by setting Fortran>Optimization>Heap Arrays to 0 (which means that all arrays are put on the heap and not on the stack). Can someone clarify why allocatable arrays are put on the stack?? Thanks
(Note: the code above is just a toy example to understand heap vs stack issues)
Best,
Alessandro
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It's not the allocatable array going on the stack, but a temporary array the compiler created in order to evaluate (array<=0.5d0). By enabling the Heap Arrays option, that temporary array now goes on the heap instead of the stack.
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It's not the allocatable array going on the stack, but a temporary array the compiler created in order to evaluate (array<=0.5d0). By enabling the Heap Arrays option, that temporary array now goes on the heap instead of the stack.
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Thanks a lot, Steve! Now I understand
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Alessandro,
You should be aware that you can conditionally select and not select the heap array option on a file-by-file basis (provided the offending file is not inlined via IPO). IOW select only the source files that require excessively large temporary arrays to have heap arrays enabled. This can result in some performance improvement.
Jim Dempsey
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