compiler: Improve IndexDerivative lowering

devito/passes/clusters/derivatives.py

@@ -41,18 +41,36 @@ def dump(exprs, c):
     for c in clusters:
         exprs = []
         for e in c.exprs:
-            expr, v = _core(e, c, weights, mapper, sregistry)
+            # Optimization 1: if the LHS is already a Symbol, then surely it's
+            # usable as a temporary for one of the IndexDerivatives inside `e`
+            if e.lhs.is_Symbol and e.operation is None:
+                reusable = {e.lhs}
+            else:
+                reusable = set()
+
+            expr, v = _core(e, c, weights, reusable, mapper, sregistry)
+
             if v:
                 dump(exprs, c)
                 processed.extend(v)
-            exprs.append(expr)
+
+            if e.lhs is expr.rhs:
+                # Optimization 2: `e` is of the form
+                # `r = IndexDerivative(...)`
+                # Rather than say
+                # `r = foo(IndexDerivative(...))`
+                # Since `r` is reusable (Optimization 1), we now have `r = r`,
+                # which can safely be discarded
+                pass
+            else:
+                exprs.append(expr)
 
         dump(exprs, c)
 
     return processed, weights, mapper
 
 
-def _core(expr, c, weights, mapper, sregistry):
+def _core(expr, c, weights, reusables, mapper, sregistry):
     """
     Recursively carry out the core of `lower_index_derivatives`.
     """
@@ -62,7 +80,7 @@ def _core(expr, c, weights, mapper, sregistry):
     args = []
     processed = []
     for a in expr.args:
-        e, clusters = _core(a, c, weights, mapper, sregistry)
+        e, clusters = _core(a, c, weights, reusables, mapper, sregistry)
         args.append(e)
         processed.extend(clusters)
 
@@ -97,8 +115,12 @@ def _core(expr, c, weights, mapper, sregistry):
     extra = (c.ispace.itdims + dims,)
     ispace = IterationSpace.union(c.ispace, ispace0, relations=extra)
 
-    name = sregistry.make_name(prefix='r')
-    s = Symbol(name=name, dtype=w.dtype)
+    try:
+        s = reusables.pop()
+        assert s.dtype is w.dtype
+    except KeyError:
+        name = sregistry.make_name(prefix='r')
+        s = Symbol(name=name, dtype=w.dtype)
     expr0 = Eq(s, 0.)
     ispace1 = ispace.project(lambda d: d is not dims[-1])
     processed.insert(0, c.rebuild(exprs=expr0, ispace=ispace1))

tests/test_unexpansion.py

@@ -326,7 +326,7 @@ def test_redundant_derivatives(self):
         exprs = FindNodes(Expression).visit(op)
         assert len(exprs) == 6
         temps = [i for i in FindSymbols().visit(exprs) if isinstance(i, Symbol)]
-        assert len(temps) == 3
+        assert len(temps) == 2
 
 
 class Test2Pass(object):