add default_cost_fn

diff --git a/src/soc/fu/div/experiment/goldschmidt_div_sqrt.py b/src/soc/fu/div/experiment/goldschmidt_div_sqrt.py
index 03378048810b08eb2e87e10d77d4576625338279..e319b9f630ab17744d6c7f1f6952495d9b159c18 100644 (file)
--- a/src/soc/fu/div/experiment/goldschmidt_div_sqrt.py
+++ b/src/soc/fu/div/experiment/goldschmidt_div_sqrt.py
@@ -699,6 +699,26 @@ class GoldschmidtDivParams:
 
         yield GoldschmidtDivOp.CalcResult
 
+    def default_cost_fn(self):
+        """ calculate the estimated cost on an arbitrary scale of implementing
+        goldschmidt division with the specified parameters. larger cost
+        values mean worse parameters.
+
+        This is the default cost function for `GoldschmidtDivParams.get`.
+
+        returns: float
+        """
+        rom_cells = self.table_data_bits << self.table_addr_bits
+        cost = float(rom_cells)
+        for op in self.ops:
+            if op == GoldschmidtDivOp.MulNByF \
+                    or op == GoldschmidtDivOp.MulDByF:
+                mul_cost = self.expanded_width ** 2
+                mul_cost *= self.expanded_width.bit_length()
+                cost += mul_cost
+        cost += 1e6 * self.iter_count
+        return cost
+
     @staticmethod
     def get(io_width):
         """ find efficient parameters for a goldschmidt division algorithm