Staggering not purged when index contains abs() of a value

[EdCaunt]

If you try to use the absolute value of an index (for example when implementing a free-surface as in the acoustic examples) with a function that is staggered, the staggering is not correctly accounted for during compilation, resulting in a float being used as an index in the generated c code. This causes module compilation to fail.

MFE:

import devito as dv

grid = dv.Grid(shape=(11, 11), extent=(10., 10.))
x, y = grid.dimensions
f = dv.TimeFunction(name='f', grid=grid, space_order=2, staggered=x)

eq = dv.Eq(f.forward, f.dx)

lhs, rhs = eq.evaluate.args

funcs = dv.symbolics.retrieve_functions(rhs)
mapper = {}
for f in funcs:
    # Get the x index
    xind = f.indices[1]
    if (xind - x).as_coeff_Mul()[0] < 0:
        mapper.update({f: f.subs({xind: dv.symbolics.INT(abs(xind))})})

eq2 = dv.Eq(lhs, rhs.subs(mapper))

dv.Operator(eq2)(t_M=1)

Which returns:

/tmp/devito-jitcache-uid1000/1fe24909a093d8c69264ced7adcbb947db766fed.c: In function ‘Kernel’:
/tmp/devito-jitcache-uid1000/1fe24909a093d8c69264ced7adcbb947db766fed.c:47:41: error: array subscript is not an integer
   47 |         f[t1][x + 2][y + 2] = r0*(-f[t0][3.0F/2.0F + (int)(fabs(x - 1.0F/2.0F))][y + 2]) + r0*f[t0][x + 2][y + 2];
      |                                         ^
FAILED compiler invocation:gcc -O3 -g -fPIC -Wall -std=c99 -march=native -Wno-unused-result -Wno-unused-variable -Wno-unused-but-set-variable -ffast-math -shared -fopenmp /tmp/devito-jitcache-uid1000/1fe24909a093d8c69264ced7adcbb947db766fed.c -lm -o /tmp/devito-jitcache-uid1000/1fe24909a093d8c69264ced7adcbb947db766fed.so

to the stderr and:

Traceback (most recent call last):
  File "staggered_absolute_indices_mfe.py", line 23, in <module>
    dv.Operator(eq2)(t_M=1)
  File "/home/ed/devito/devito/operator/operator.py", line 653, in __call__
    return self.apply(**kwargs)
  File "/home/ed/devito/devito/operator/operator.py", line 724, in apply
    cfunction = self.cfunction
  File "/home/ed/devito/devito/operator/operator.py", line 606, in cfunction
    self._jit_compile()
  File "/home/ed/devito/devito/operator/operator.py", line 591, in _jit_compile
    recompiled, src_file = self._compiler.jit_compile(self._soname,
  File "/home/ed/devito/devito/arch/compiler.py", line 319, in jit_compile
    _, _, _, recompiled = compile_from_string(self, target, code, src_file,
  File "/home/ed/anaconda3/envs/devito/lib/python3.8/site-packages/codepy/jit.py", line 433, in compile_from_string
    toolchain.build_extension(ext_file, source_paths, debug=debug)
  File "/home/ed/anaconda3/envs/devito/lib/python3.8/site-packages/codepy/toolchain.py", line 210, in build_extension
    raise CompileError("module compilation failed")
codepy.CompileError: module compilation failed

to the stdout