Bug Fix: Sub Kernel issue with input0/1 ordering. (#120)

Bug fix for Sub kernel. Issue: CartesianOp assumed commutative operations (a - b != b - a).

Author: christopherngutierrez

p-isa_tools/kerngen/pisa_generators/basic.py

@@ -68,14 +68,18 @@ def to_pisa(self) -> list[PIsaOp]:
         # Not the same number of parts
         first, second = (self.input0, self.input1) if self.input0.parts < self.input1.parts else (self.input1, self.input0)
 
+        # Preserve the ordering of the inputs (output = input0 op input1) to avoid issues with non-commutative ops (e.g. sub)
+        # For example input0 - input1 != input1 - input0
+        order_preserved = first == self.input0
+
         ls: list[PIsaOp] = []
         for unit, q in it.product(range(self.context.units), range(self.input0.start_rns, self.input0.rns)):
             ls.extend(
                 self.op(
                     self.context.label,
                     self.output(part, q, unit),
-                    first(part, q, unit),
-                    second(0, q, unit),
+                    first(part, q, unit) if order_preserved else second(0, q, unit),
+                    second(0, q, unit) if order_preserved else first(part, q, unit),
                     q,
                 )
                 for part in range(first.parts)