repl const Arrays with Consts in optimize (#854)

To speed-up repeated evaluation of the same `Evaluable`, this PR introduces
`Evaluable._deep_flatten_constants`, which replaces constant `Array`s with
`Constant`s, excluding the no-op arrays `Transpose` and `InsertAxis`, and
includes this in `optimized_for_numpy`.

nutils/__init__.py

@@ -1,4 +1,4 @@
 'Numerical Utilities for Finite Element Analysis'
 
-__version__ = version = '9a15'
+__version__ = version = '9a16'
 version_name = 'jook-sing'

nutils/evaluable.py

@@ -449,6 +449,7 @@ def _simplified(self):
     @cached_property
     def optimized_for_numpy(self):
         retval = self.simplified._optimized_for_numpy1() or self
+        retval = retval._deep_flatten_constants() or retval
         return retval._combine_loop_concatenates(frozenset())
 
     @replace(depthfirst=True, recursive=True)
@@ -462,6 +463,11 @@ def _optimized_for_numpy1(obj):
     def _optimized_for_numpy(self):
         return
 
+    @replace(depthfirst=False, recursive=False)
+    def _deep_flatten_constants(self):
+        if isinstance(self, Array):
+            return self._flatten_constant()
+
     @cached_property
     def _loop_concatenate_deps(self):
         deps = []
@@ -997,6 +1003,10 @@ def _const_uniform(self):
             lower, upper = self._intbounds
             return lower if lower == upper else None
 
+    def _flatten_constant(self):
+        if self.isconstant:
+            return constant(self.eval())
+
 
 class Orthonormal(Array):
     'make a vector orthonormal to a subspace'
@@ -1177,6 +1187,9 @@ def _const_uniform(self):
         if self.ndim == 0:
             return self.dtype(self.value[()])
 
+    def _flatten_constant(self):
+        pass
+
 
 class InsertAxis(Array):
 
@@ -1296,6 +1309,9 @@ def _intbounds_impl(self):
     def _const_uniform(self):
         return self.func._const_uniform
 
+    def _flatten_constant(self):
+        pass
+
 
 class Transpose(Array):
 
@@ -1491,6 +1507,9 @@ def _intbounds_impl(self):
     def _const_uniform(self):
         return self.func._const_uniform
 
+    def _flatten_constant(self):
+        pass
+
 
 class Product(Array):
 
@@ -2178,11 +2197,6 @@ def _simplified(self):
         if len(where) != self.ndim:
             return align(self._newargs(*uninserted), where, self.shape)
 
-    def _optimized_for_numpy(self):
-        if self.isconstant:
-            retval = self.eval()
-            return constant(retval)
-
     def _derivative(self, var, seen):
         if self.dtype == complex or var.dtype == complex:
             raise NotImplementedError('The complex derivative is not implemented.')