Illegal parallel code with global symbols in absence of explicit data dependencies

[FabioLuporini]

Reproducer:

import numpy as np

from devito import *
from devito.types import Symbol

configuration['language'] = 'openmp'

grid = Grid(shape=(10,))
x, = grid.dimensions

f = Function(name='f', grid=grid, space_order=0)
s = Symbol(name='s', dtype=np.int32)

eqns = [Eq(s, 0),
        Eq(f[s], x),
        Eq(s, s + 1, implicit_dims=(x,))]

op = Operator(eqns)

op.apply()

assert np.all(f.data == list(range(10)))

Without openmp, this assert passes

[FabioLuporini]

Copy-pasting from slack:

The fix [...] isn’t obvious at all, since here we have sequentialism dictated by an irregular access, not an explicit dependence across two accesses (so it sort of escapes the lamport framework), and so far we’re only looking at explicit dependences (see here https://github.com/devitocodes/devito/blob/master/devito/ir/clusters/analysis.py#L88)

[georgebisbas]

Link to slack: https://devitocodes.slack.com/archives/C7JMLMSG0/p1675846801725969