adder: Remove unnecessary xor of lowest bit

Mikey noticed the lowest bit of the adder has an xor with 0.

To fix this, we have to move the flattening of the array of signals
later.

Signed-off-by: Anton Blanchard <[email protected]>

vlsi_arithmetic/adder.py

@@ -1,6 +1,6 @@
 import math
 
-from amaranth import Elaboratable, Module, Signal, Const
+from amaranth import Elaboratable, Module, Signal, Cat
 
 
 class AdderFramework(Elaboratable):
@@ -68,32 +68,30 @@ def elaborate(self, platform):
             m.d.comb += p_tmp[i].eq(self._p[i])
 
         self._calculate_pg()
+        o = [Signal() for i in range(self._bits)]
 
         # g is the carry out signal. We need to shift it left one bit then
-        # xor it with the sum (ie p_tmp). Since we have a list of 1 bit
-        # signals, just insert a constant zero signal at the head of of the
-        # list to shift g.
-        self._g.insert(0, Const(0))
+        # xor it with the sum (ie p_tmp). This means bit 0 is just p.
+        m.d.comb += o[0].eq(p_tmp[0])
 
-        o = Signal(self._bits)
-        for i in range(self._bits):
-            # This also flattens the list of bits when writing to o
-            self._generate_xor(p_tmp[i], self._g[i], o[i])
+        for i in range(1, self._bits):
+            self._generate_xor(p_tmp[i], self._g[i - 1], o[i])
 
         if self._carry_out:
             carry_out = Signal()
-            m.d.comb += carry_out.eq(self._g[self._bits])
+            m.d.comb += carry_out.eq(self._g[self._bits - 1])
 
             if self._register_output:
                 m.d.sync += self.carry_out.eq(carry_out)
             else:
                 m.d.comb += self.carry_out.eq(carry_out)
 
         o2 = Signal(self._bits, reset_less=True)
+        # This also flattens the list of bits when writing to o2
         if self._register_output:
-            m.d.sync += o2.eq(o)
+            m.d.sync += o2.eq(Cat(o[n] for n in range(len(o))))
         else:
-            m.d.comb += o2.eq(o)
+            m.d.comb += o2.eq(Cat(o[n] for n in range(len(o))))
 
         m.d.comb += self.o.eq(o2)
         return m