doing " += 1" in arrays does not add one in prange loop

[yaoyuki]

Reporting a bug

import numpy as np
import numba as nb

d = 10
k = 2

@nb.njit(parallel = True)
def test_parallel(d, k):

    gg      = np.zeros((d, k), dtype = nb.int32) 
    gg_next = np.zeros((d, k), dtype = nb.int32)   
        
          
    for i_gg in nb.prange(d): #this part can be parallelized.

        # set value for gg[i_gg]
        for n in range(k):
            gg[i_gg, n] = i_gg

        
        gg_next[i_gg, :]  = gg[i_gg, :] # initialize gg_next[i_gg]
        gg_next[i_gg, 0] += 1

    # print errors (without prange to print results nicely)
    for i_gg in range(d):
        print('i_gg = ', i_gg , ', gg = ', gg[i_gg], ', gg_next = ', gg_next[i_gg])

then I got

test_parallel(d, k)

i_gg =  0 , gg =  [0 0] , gg_next =  [1 0]
i_gg =  1 , gg =  [1 1] , gg_next =  [1 1]
i_gg =  2 , gg =  [2 2] , gg_next =  [1 2]
i_gg =  3 , gg =  [3 3] , gg_next =  [1 3]
i_gg =  4 , gg =  [4 4] , gg_next =  [1 4]
i_gg =  5 , gg =  [5 5] , gg_next =  [1 5]
i_gg =  6 , gg =  [6 6] , gg_next =  [1 6]
i_gg =  7 , gg =  [7 7] , gg_next =  [1 7]
i_gg =  8 , gg =  [8 8] , gg_next =  [1 8]
i_gg =  9 , gg =  [9 9] , gg_next =  [1 9]

If we slightly modify code in this way, or simply drop 'parallel = True'

@nb.njit(parallel = True)
def test_parallel_nocolon(d, k):

    gg      = np.zeros((d, k), dtype = nb.int32)
    gg_next = np.zeros((d, k), dtype = nb.int32)   
        
          
    for i_gg in nb.prange(d): #this part can be parallelized.

        # set value for gg[i_gg]
        for n in range(k):
            gg[i_gg, n] = i_gg

        
        gg_next[i_gg]  = gg[i_gg] # <--- only diff
        gg_next[i_gg, 0] += 1

    # print errors (without prange to print results nicely)
    for i_gg in range(d):
        print('i_gg = ', i_gg , ', gg = ', gg[i_gg], ', gg_next = ', gg_next[i_gg])

test_parallel_nocolon(d, k)

then I get

i_gg =  0 , gg =  [0 0] , gg_next =  [1 0]
i_gg =  1 , gg =  [1 1] , gg_next =  [2 1]
i_gg =  2 , gg =  [2 2] , gg_next =  [3 2]
i_gg =  3 , gg =  [3 3] , gg_next =  [4 3]
i_gg =  4 , gg =  [4 4] , gg_next =  [5 4]
i_gg =  5 , gg =  [5 5] , gg_next =  [6 5]
i_gg =  6 , gg =  [6 6] , gg_next =  [7 6]
i_gg =  7 , gg =  [7 7] , gg_next =  [8 7]
i_gg =  8 , gg =  [8 8] , gg_next =  [9 8]
i_gg =  9 , gg =  [9 9] , gg_next =  [10  9]

• I have tried using the latest released version of Numba (most recent is
  visible in the change log (https://github.com/numba/numba/blob/master/CHANGE_LOG).
• I have included below a minimal working reproducer (if you are unsure how
  to write one see http://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports).

[esc]

@yaoyuki thanks for reporting this. I can reproduce this and it looks like a bug in the parfors component of Numba. My guess would be that something is going wrong during array analysis.

[esc]

cc @DrTodd13