compiler: Make half arithmetic a symbolic option

Whether an Operator working in half also computes in half decides what
is calculated, not how quickly: the literals and the FD coefficients are
rounded to three decimal digits. That is a mathematical choice, so it
belongs with `interp-mode` in `sym_opt` rather than among the codegen
options, and is validated and defaulted alongside it.

Author: mloubout

devito/core/operator.py

@@ -186,6 +186,14 @@ class BasicOperator(Operator):
     # ------------------------------------------------------------------
 
     INTERP_MODE = 'direct'
+
+    HALF_ARITH = False
+    """
+    Whether an Operator working in half precision carries the arithmetic there
+    too, rounding its literals and its FD weights to half. Off by default: half
+    is a storage format, and giving up the accuracy of the coefficients as well
+    is a mathematical choice rather than a consequence of it.
+    """
     """
     Default for the `sym_opt={'interp-mode': ...}` option. Controls how
     a product of fields living at different staggered locations is mapped
@@ -230,7 +238,8 @@ def _normalize_sym_kwargs(cls, **kwargs):
         the Operator. Returns the normalized `sym_options` dict.
         """
         so = dict(kwargs.get('sym_options', {}))
-        out = {'interp-mode': so.pop('interp-mode', cls.INTERP_MODE)}
+        out = {'interp-mode': so.pop('interp-mode', cls.INTERP_MODE),
+               'half-arith': so.pop('half-arith', cls.HALF_ARITH)}
 
         if so:
             raise InvalidOperator(

devito/operator/operator.py

@@ -813,7 +813,7 @@ def _soname(self):
     def _printer(self):
         # A Target may offer a second printer for Operators that have opted
         # into carrying their precision into the arithmetic
-        if self._options.get('half-arith'):
+        if self._sym_options.get('half-arith'):
             with suppress(AttributeError):
                 return self._Target.HalfArithPrinter
         return self._Target.Printer