ast/simplify: improve enum handling.

Before this commit, enum values were serialized as attributes of form
  \enum_<width>_<value>
where <value> was a decimal signed integer.

This has multiple drawbacks:
  * Enums with large values would be hard to process for downstream
    tooling that cannot parse arbitrary precision decimals. (In fact
    Yosys also did not correctly process enums with large values,
    and would overflow `int`.)
  * Enum value attributes were not confined to their own namespace,
    making it harder for downstream tooling to enumerate all such
    attributes, as opposed to looking up any specific value.
  * Enum values could not include x or z, which are explicitly
    permitted in the SystemVerilog standard.

After this commit, enum values are serialized as attributes of form
  \enum_value_<value>
where <value> is a bit sequence of the appropriate width.

2 files changed, 5 insertions, 7 deletions

diff --git a/README.md b/README.md
index ce7b26411..0a81d8bb9 100644
--- a/README.md
+++ b/README.md
@@ -443,8 +443,8 @@ Verilog Attributes and non-standard features
 - The ``wiretype`` attribute is added by the verilog parser for wires of a
   typedef'd type to indicate the type identifier.
 
-- Various ``enum_{width}_{value}`` attributes are added to wires of an
-  enumerated type to give a map of possible enum items to their values.
+- Various ``enum_value_{value}`` attributes are added to wires of an enumerated type
+  to give a map of possible enum items to their values.
 
 - The ``enum_base_type`` attribute is added to enum items to indicate which
   enum they belong to (enums -- anonymous and otherwise -- are
diff --git a/frontends/ast/simplify.cc b/frontends/ast/simplify.cc
index b87af0f8c..5e7518f68 100644
--- a/frontends/ast/simplify.cc
+++ b/frontends/ast/simplify.cc
@@ -907,9 +907,7 @@ bool AstNode::simplify(bool const_fold, bool at_zero, bool in_lvalue, int stage,
 						);
 					}
 					//start building attribute string
-					std::string enum_item_str = "\\enum_";
-					enum_item_str.append(std::to_string(width));
-					enum_item_str.append("_");
+					std::string enum_item_str = "\\enum_value_";
 					//get enum item value
 					if(enum_item->children[0]->type != AST_CONSTANT){
 						log_error("expected const, got %s for %s (%s)\n",
@@ -917,8 +915,8 @@ bool AstNode::simplify(bool const_fold, bool at_zero, bool in_lvalue, int stage,
 								  enum_item->str.c_str(), enum_node->str.c_str()
  								);
 					}
-					int val = enum_item->children[0]->asInt(is_signed);
-					enum_item_str.append(std::to_string(val));
+					RTLIL::Const val = enum_item->children[0]->bitsAsConst(width, is_signed);
+					enum_item_str.append(val.as_string());
 					//set attribute for available val to enum item name mappings
 					attributes[enum_item_str.c_str()] = mkconst_str(enum_item->str);
 				}