[go] Widen the carry type to 64 bits artifically

For performance reasons.  Note that this is unsound, but having the
carry type be 8 bits rather than 1 bit was already unsound, and this is
no more unsound.  The narrowness of the carry type is not used in
practice in the code, though we should still someday include it in the
proofs.

src/Stringification/Go.v

@@ -114,11 +114,13 @@ Module Go.
                 ""]%string)
             ++ (if needs_bits_import then ["import ""math/bits"""; ""]%string else [])
             ++ (let typedefs
-                    := List.flat_map
+                    := let carry_bitwidth := 64 (* c.f. https://github.com/mit-plv/fiat-crypto/pull/1006#issuecomment-892625927 *) in
+                       let carry_typedef_comment := (" // We use uint" ++ Decimal.Z.to_string carry_bitwidth ++ " instead of a more narrow type for performance reasons; see https://github.com/mit-plv/fiat-crypto/pull/1006#issuecomment-892625927")%string in
+                       List.flat_map
                          (fun bw
                           => (if IntSet.mem (int.of_bitwidth false bw) bitwidths_used || IntSet.mem (int.of_bitwidth true bw) bitwidths_used
-                              then [type_prefix ++ int_type_to_string internal_private prefix (int.of_bitwidth false bw) ++ " uint8";
-                                   type_prefix ++ int_type_to_string internal_private prefix (int.of_bitwidth true bw) ++ " int8"]%string (* C: typedef signed challr prefix_int1 *)
+                              then [type_prefix ++ int_type_to_string internal_private prefix (int.of_bitwidth false bw) ++ " uint" ++ Decimal.Z.to_string carry_bitwidth ++ carry_typedef_comment;
+                                   type_prefix ++ int_type_to_string internal_private prefix (int.of_bitwidth true bw) ++ " int" ++ Decimal.Z.to_string carry_bitwidth ++ carry_typedef_comment]%string
                               else []))
                          [1; 2] in
                 let typedefs :=