Extract invert mask calculation to shared helper function

- Add compute_invert_mask() utility function in platform/utils.py
- Replace duplicated inv_mask calculation in silicon.py (2 places) and base.py
- Improves maintainability by centralizing the logic
- Addresses gatecat review comment on silicon.py:109

Author: robtaylor

chipflow_lib/platform/base.py

@@ -10,6 +10,7 @@
 from amaranth import Module
 
 from .io import IOSignature
+from .utils import compute_invert_mask
 
 logger = logging.getLogger(__name__)
 
@@ -84,7 +85,7 @@ def _wire_up_ports(m: Module, top, platform):
                 if hasattr(port, 'wire_up'):
                     port.wire_up(m, wire)
                 else:
-                    inv_mask = sum(inv << bit for bit, inv in enumerate(port.invert))
+                    inv_mask = compute_invert_mask(port.invert)
                     if hasattr(wire, 'i'):
                         m.d.comb += wire.i.eq(port.i ^ inv_mask)
                     if hasattr(wire, 'o'):

chipflow_lib/platform/silicon.py

@@ -23,6 +23,7 @@
 from ..utils import ChipFlowError
 from ..config import Process
 from .io import IOModel, IOTripPoint, Sky130DriveMode
+from .utils import compute_invert_mask
 
 if TYPE_CHECKING:
     from ..config import Config
@@ -105,7 +106,7 @@ def wire_up(self, m, wire):
         if self._oe is not None and len(self._oe) == 1 and len(self._oes) > 1:
             self._oes.eq(self._oe.replicate(len(self._oes)))
 
-        inv_mask = sum(inv << bit for bit, inv in enumerate(self.invert))
+        inv_mask = compute_invert_mask(self.invert)
         if hasattr(wire, 'i') and wire.i is not None:
             assert self._i is not None
             m.d.comb += wire.i.eq(self._i ^ inv_mask)
@@ -349,7 +350,7 @@ def elaborate(self, platform):
             raise TypeError(f"Cannot elaborate SiliconPlatform buffer with port {self.port!r}")
 
         m = Module()
-        invert = sum(bit << idx for idx, bit in enumerate(self.port.invert))
+        invert = compute_invert_mask(self.port.invert)
         if self.direction is not io.Direction.Input:
             if invert != 0:
                 o_inv = Signal.like(self.o)             # type: ignore[reportAttributeAccessIssue]

chipflow_lib/platform/utils.py

@@ -21,6 +21,19 @@
 logger = logging.getLogger(__name__)
 
 
+def compute_invert_mask(invert_list):
+    """
+    Compute a bit mask for signal inversion from a list of boolean invert flags.
+
+    Args:
+        invert_list: List of booleans indicating which bits should be inverted
+
+    Returns:
+        Integer mask where set bits indicate positions to invert
+    """
+    return sum(inv << bit for bit, inv in enumerate(invert_list))
+
+
 def top_components(config: 'Config') -> Dict[str, wiring.Component]:
     """
     Return the top level components for the design, as configured in ``chipflow.toml``