back.rtlil: fix lowering of Part() on LHS to account for stride.

examples/basic/sel.py

@@ -0,0 +1,30 @@
+from types import SimpleNamespace
+from nmigen import *
+from nmigen.cli import main
+
+
+class FlatGPIO(Elaboratable):
+    def __init__(self, pins, bus):
+        self.pins = pins
+        self.bus  = bus
+
+    def elaborate(self, platform):
+        bus  = self.bus
+
+        m = Module()
+        m.d.comb += bus.r_data.eq(self.pins.word_select(bus.addr, len(bus.r_data)))
+        with m.If(bus.we):
+            m.d.sync += self.pins.word_select(bus.addr, len(bus.w_data)).eq(bus.w_data)
+        return m
+
+
+if __name__ == "__main__":
+    bus = Record([
+        ("addr",   3),
+        ("r_data", 2),
+        ("w_data", 2),
+        ("we",     1),
+    ])
+    pins = Signal(8)
+    gpio = FlatGPIO(pins, bus)
+    main(gpio, ports=[pins, bus.addr, bus.r_data, bus.w_data, bus.we])

nmigen/back/rtlil.py

@@ -622,9 +622,13 @@ class _LHSValueCompiler(_ValueCompiler):
     def on_Part(self, value):
         offset = self.s.expand(value.offset)
         if isinstance(offset, ast.Const):
-            return self(ast.Slice(value.value, offset.value, offset.value + value.width))
+            return self(ast.Slice(value.value,
+                                  offset.value * value.stride,
+                                  offset.value * value.stride + value.width))
         else:
-            raise LegalizeValue(value.offset, range((1 << len(value.offset))), value.src_loc)
+            raise LegalizeValue(value.offset,
+                                range((1 << len(value.offset)) // value.stride),
+                                value.src_loc)
 
     def on_Repl(self, value):
         raise TypeError # :nocov: