Address #13: Use sympy.codegen.rewriting.optimize

It's really comfortable to write optimizations in terms of sympy.codegen.rewrite.RewriteOptim:

    # Evaluates all constant terms
    evaluate_constant_terms = ReplaceOptim(
        lambda e: hasattr(e, 'is_constant') and e.is_constant,
        lambda p: p.evalf()
    )

This PR adds a parameter sympy_optimizations to the create_*_kernel functions that applies the list of optimizations to the assignments before creating the AST.

sympy.codegen.rewrite already has some optimizations. Some similar to the optimizations of pystencils. For example create_expand_pow_optimization(limit) is really similar to the logic in CustomSympyPrinter._print_Pow.

See #13 (closed)

Problem: old versions of sympy (e.g. from ubuntu CI) don't have sympy.codegen.rewrite. The optimizations are skipped in that case. test_and_coverage applies all optimizations.

We could also try to implement a fma-optimization (fused-multipy add) with that and sympy.Wild.

pystencils/optimizations.py

@@ -6,10 +6,28 @@ See :func:`sympy.codegen.rewriting.optimize`.
 
 
 try:
-    from sympy.codegen.rewriting import optims_c99
+    from sympy.codegen.rewriting import optims_c99, optimize
+    from sympy.codegen.rewriting import ReplaceOptim
+    HAS_REWRITING = True
+
+    # Evaluates all constant terms
+    evaluate_constant_terms = ReplaceOptim(
+        lambda e: hasattr(e, 'is_constant') and e.is_constant,
+        lambda p: p.evalf()
+    )
+
+    optims_pystencils_cpu = [evaluate_constant_terms] + list(optims_c99)
+    optims_pystencils_gpu = [evaluate_constant_terms] + list(optims_c99)
 except ImportError:
     optims_c99 = []
 
 
-optims_pystencils_cpu = [] + list(optims_c99)
-optims_pystencils_gpu = [] + list(optims_c99)
+# Evaluates all constant terms
+evaluate_constant_terms = ReplaceOptim(
+    lambda e: e.is_constant,
+    lambda p: p.evalf()
+)
+
+
+optims_pystencils_cpu = [evaluate_constant_terms] + list(optims_c99)
+optims_pystencils_gpu = [evaluate_constant_terms] + list(optims_c99)