Lifted or Undeclared variable scheduled in wrong set of loops

[georgebisbas]

Undeclared var, possibly due to wrong scheduling
(Initial thread from @felipeguedessilva,
here: https://devitocodes.slack.com/archives/C7JMLMSG0/p1736908013546609)

mfe.py

from devito import Grid, TimeFunction, Function, Eq, Operator

# Setup Configuration
grid = Grid(shape=(30, 30))

# Symbolic Dimensions
x, y = grid.dimensions
t = grid.stepping_dim

# Reduced MFE Lame Parameters
VS = Function(name="VS", grid=grid, space_order=2)

# Symbolic Fields Construction
v = TimeFunction(name='v', grid=grid, space_order=2)

# Reduced MFE Higdon Condition - Velocity
eqs = []

beta = (1 + (1/VS))
dz = (1 / beta)

eqv1 = Eq(v.forward, v[t, x, y+1])

eqv2 = Eq(v.forward,
          (v[t+1, x-1, y] +
           v[t+1, x, y-1] +
           (dz)*v[t+1, x, y]))

eqs.append(eqv1)
eqs.append(eqv2)

# Operator Definition
op = Operator(eqs)

op(dt=0.1, time=10)

To reproduce:
DEVITO_LOGGING=DEBUG python mfe.py

Error:

Operator `Kernel` generated in 0.18 s
  * lowering.Clusters: 0.09 s (52.3 %)
     * specializing.Clusters: 0.05 s (29.1 %)
  * lowering.IET: 0.07 s (40.7 %)
     * specializing.IET: 0.04 s (23.3 %)
Flops reduction after symbolic optimization: [6 --> 6]
gcc -march=native -O3 -g -fPIC -Wall -std=c99 -Wno-unused-result -Wno-unused-variable -Wno-unused-but-set-variable -ffast-math -shared -fopenmp /tmp/devito-jitcache-uid1000/5cc44d7ce38ff1f3bbc1699a3fca1f2c9d9896be.c -lm -o /tmp/devito-jitcache-uid1000/5cc44d7ce38ff1f3bbc1699a3fca1f2c9d9896be.so
/tmp/devito-jitcache-uid1000/5cc44d7ce38ff1f3bbc1699a3fca1f2c9d9896be.c: In function ‘Kernel’:
/tmp/devito-jitcache-uid1000/5cc44d7ce38ff1f3bbc1699a3fca1f2c9d9896be.c:53:31: error: ‘r0’ undeclared (first use in this function); did you mean ‘t0’?
   53 |         v[t1][x + 2][y + 2] = r0*v[t1][x + 2][y + 2] + v[t1][x + 1][y + 2] + v[t1][x + 2][y + 1];
      |                               ^~
      |                               t0
/tmp/devito-jitcache-uid1000/5cc44d7ce38ff1f3bbc1699a3fca1f2c9d9896be.c:53:31: note: each undeclared identifier is reported only once for each function it appears in
FAILED compiler invocation: gcc -march=native -O3 -g -fPIC -Wall -std=c99 -Wno-unused-result -Wno-unused-variable -Wno-unused-but-set-variable -ffast-math -shared -fopenmp /tmp/devito-jitcache-uid1000/5cc44d7ce38ff1f3bbc1699a3fca1f2c9d9896be.c -lm -o /tmp/devito-jitcache-uid1000/5cc44d7ce38ff1f3bbc1699a3fca1f2c9d9896be.so
Traceback (most recent call last):
  File "/home/gb4018/workspace/devitocodes/devito/examples/minimal.py", line 35, in <module>
    op(dt=0.1, time=10)
  File "/home/gb4018/workspace/devitocodes/devito/devito/operator/operator.py", line 833, in __call__
    return self.apply(**kwargs)
  File "/home/gb4018/workspace/devitocodes/devito/devito/operator/operator.py", line 899, in apply
    cfunction = self.cfunction
  File "/home/gb4018/workspace/devitocodes/devito/devito/operator/operator.py", line 786, in cfunction
    self._jit_compile()
  File "/home/gb4018/workspace/devitocodes/devito/devito/operator/operator.py", line 772, in _jit_compile
    recompiled, src_file = self._compiler.jit_compile(self._soname, str(self))
  File "/home/gb4018/workspace/devitocodes/devito/devito/arch/compiler.py", line 379, in jit_compile
    _, _, _, recompiled = compile_from_string(self, target, code, src_file,
  File "/home/gb4018/environments/python310-venv/lib/python3.10/site-packages/codepy/jit.py", line 439, in compile_from_string
    toolchain.build_extension(ext_file, source_paths, debug=debug)
  File "/home/gb4018/environments/python310-venv/lib/python3.10/site-packages/codepy/toolchain.py", line 211, in build_extension
    raise CompileError("module compilation failed")
codepy.CompileError: module compilation failed