Internals: Make node type hashes stable with Ast changes

Instead of hashing node types by type id (which can change when
adding/removing Ast node types), astgen a stable hash based on node
name. This makes output more stable through Ast node
additions/deletions. Largely to help with 'make test-diff', should have
no user noticeable impact.

src/V3Hasher.cpp

@@ -59,7 +59,7 @@ class HasherVisitor final : public VNVisitorConst {
         } else {
             VL_RESTORER(m_hash);
             // Reset accumulator
-            m_hash = V3Hash{nodep->type()};  // Node type
+            m_hash = V3Hash{nodep->type().hash()};  // Node type
             f();  // Node-specific hash
             if (hashDType && nodep != nodep->dtypep())
                 iterateConstNull(nodep->dtypep());  // Node dtype
@@ -77,7 +77,7 @@ class HasherVisitor final : public VNVisitorConst {
     // Each visitor below contributes to the hash any node-specific content
     // that is not dependent on either of the following, as these are
     // included by default by hashNode:
-    // - Node type (as given by AstNode::type())
+    // - Node type (as given by AstNode::type().hash())
     // - Node dtype (unless !hashDType)
     // - child nodes (unless !hashChildren)
     //

src/astgen

@@ -4,6 +4,7 @@
 
 import argparse
 import glob
+import hashlib
 import os
 import re
 import sys
@@ -881,6 +882,15 @@ def write_type_enum(prefix, nodeList):
         fh.write("        return names[m_e];\n")
         fh.write("    }\n")
 
+        fh.write("    uint32_t hash() const VL_MT_SAFE {\n")
+        fh.write("        static const uint32_t hashes[] = {\n")
+        for node in sorted(filter(lambda _: _.isLeaf, nodeList), key=lambda _: _.typeId):
+            digest = hashlib.md5((prefix + node.name).encode("ascii")).hexdigest()
+            fh.write("            0x{h}U,  // {p}{n}\n".format(h=digest[:8], p=prefix, n=node.name))
+        fh.write("        };\n")
+        fh.write("        return hashes[m_e];\n")
+        fh.write("    }\n")
+
 
 def write_type_tests(prefix, nodeList):
     with open_file("V3{p}__gen_type_tests.h".format(p=prefix)) as fh:

test_regress/t/t_debug_emitv.out

@@ -56,8 +56,8 @@ module Vt_debug_emitv_t;
                 $write("");
             end
             $display("%p", $_EXPRSTMT(
-                    __Venum_hf6514fb1__0 = (in);
-                    , __Venum_hf6514fb1__0);
+                    __Venum_h99905942__0 = (in);
+                    , __Venum_h99905942__0);
             );
             if ((| $value$plusargs("TEST=%d", i1))) begin
                 $display("value was %d", i1);
@@ -1096,7 +1096,7 @@ module Vt_debug_emitv_t;
     enum logic [2:0] {
         ZERO = 3'h0,
         ONE = 3'h1
-    } __Venum_hf6514fb1__0;
+    } __Venum_h99905942__0;
 endmodule
 package Vt_debug_emitv___024unit;
     class Vt_debug_emitv_Cls;