compiler: Generate less integer arithmetic

devito/ir/clusters/cluster.py

@@ -297,9 +297,9 @@ def dspace(self):
                 continue
 
             intervals = [Interval(d,
-                                  min([minimum(i) for i in offs]),
-                                  max([maximum(i) for i in offs]))
-                         for d, offs in v.items()]
+                                  min([minimum(i, ispace=self.ispace) for i in o]),
+                                  max([maximum(i, ispace=self.ispace) for i in o]))
+                         for d, o in v.items()]
             intervals = IntervalGroup(intervals)
 
             # Factor in the IterationSpace -- if the min/max points aren't zero,

devito/ir/support/utils.py

@@ -308,30 +308,42 @@ def _relational(expr, callback, udims=None):
     return expr.subs(mapper)
 
 
-def minimum(expr, udims=None):
+def minimum(expr, udims=None, ispace=None):
     """
     Substitute the unbounded Dimensions in `expr` with their minimum point.
 
     Unbounded Dimensions whose possible minimum value is not known are ignored.
     """
-    return _relational(expr, lambda e: e._min, udims)
+    def callback(sd):
+        try:
+            return sd._min + ispace[sd].lower
+        except (TypeError, KeyError):
+            return sd._min
+
+    return _relational(expr, callback, udims)
 
 
-def maximum(expr, udims=None):
+def maximum(expr, udims=None, ispace=None):
     """
     Substitute the unbounded Dimensions in `expr` with their maximum point.
 
     Unbounded Dimensions whose possible maximum value is not known are ignored.
     """
-    return _relational(expr, lambda e: e._max, udims)
+    def callback(sd):
+        try:
+            return sd._max + ispace[sd].upper
+        except (TypeError, KeyError):
+            return sd._max
+
+    return _relational(expr, callback, udims)
 
 
-def extrema(expr):
+def extrema(expr, ispace=None):
     """
     The minimum and maximum extrema assumed by `expr` once the unbounded
     Dimensions are resolved.
     """
-    return Extrema(minimum(expr), maximum(expr))
+    return Extrema(minimum(expr, ispace=ispace), maximum(expr, ispace=ispace))
 
 
 def minmax_index(expr, d):

devito/passes/clusters/derivatives.py

@@ -115,6 +115,12 @@ def _core(expr, c, weights, reusables, mapper, sregistry):
     extra = (c.ispace.itdims + dims,)
     ispace = IterationSpace.union(c.ispace, ispace0, relations=extra)
 
+    # Set the IterationSpace along the StencilDimensions to start from 0
+    # (rather than the default `d._min`) to minimize the amount of integer
+    # arithmetic to calculate the various index access functions
+    for d in dims:
+        ispace = ispace.translate(d, -d._min)
+
     try:
         s = reusables.pop()
         assert s.dtype is w.dtype
@@ -125,12 +131,12 @@ def _core(expr, c, weights, reusables, mapper, sregistry):
     ispace1 = ispace.project(lambda d: d is not dims[-1])
     processed.insert(0, c.rebuild(exprs=expr0, ispace=ispace1))
 
-    # Transform e.g. `w[i0] -> w[i0 + 2]` for alignment with the
-    # StencilDimensions starting points
-    subs = {expr.weights:
-            expr.weights.subs(d, d - d._min)
-            for d in dims}
-    expr1 = Inc(s, uxreplace(expr.expr, subs))
+    # Transform e.g. `r0[x + i0 + 2, y] -> r0[x + i0, y, z]` for alignment
+    # with the shifted `ispace`
+    base = expr.base
+    for d in dims:
+        base = base.subs(d, d + d._min)
+    expr1 = Inc(s, base*expr.weights)
     processed.append(c.rebuild(exprs=expr1, ispace=ispace))
 
     # Track lowered IndexDerivative for subsequent optimization by the caller

tests/test_unexpansion.py

@@ -93,7 +93,7 @@ def test_multiple_cross_derivs(self, coeffs, expected):
 
         # w0, w1, ...
         functions = FindSymbols().visit(op)
-        weights = [f for f in functions if isinstance(f, Weights)]
+        weights = {f for f in functions if isinstance(f, Weights)}
         assert len(weights) == expected