fix[venom]: fix cfg output format (vyperlang#4010)

this commit fixes a regression introduced in ace3789, as we now
have multiple `IRFunction` objects grouped in an `IRContext`. This
commit adds the required `as_graph()` method to `IRContext`. At the same
time the `IRFuntions.as_graph()` is updated to support emitting only a
subgraph without wrapping it into a graphviz `digraph`.

vyper/venom/context.py

@@ -53,6 +53,14 @@ def append_data(self, opcode: str, args: list[IROperand]) -> None:
         """
         self.data_segment.append(IRInstruction(opcode, args))  # type: ignore
 
+    def as_graph(self) -> str:
+        s = ["digraph G {"]
+        for fn in self.functions.values():
+            s.append(fn.as_graph(True))
+            s.append("\n")
+        s.append("}")
+        return "\n".join(s)
+
     def __repr__(self) -> str:
         s = ["IRContext:"]
         for fn in self.functions.values():

vyper/venom/function.py

@@ -214,7 +214,11 @@ def copy(self):
         new.last_variable = self.last_variable
         return new
 
-    def as_graph(self) -> str:
+    def as_graph(self, only_subgraph=False) -> str:
+        """
+        Return the function as a graphviz dot string. If only_subgraph is True, only return the
+        subgraph, not the full digraph -for embedding in a larger graph-
+        """
         import html
 
         def _make_label(bb):
@@ -227,18 +231,25 @@ def _make_label(bb):
             return ret
             # return f"{bb.label.value}:\n" + "\n".join([f"    {inst}" for inst in bb.instructions])
 
-        ret = "digraph G {\n"
+        ret = []
+
+        if not only_subgraph:
+            ret.append("digraph G {{")
+        ret.append(f'subgraph "{self.name}" {{')
 
         for bb in self.get_basic_blocks():
             for out_bb in bb.cfg_out:
-                ret += f'    "{bb.label.value}" -> "{out_bb.label.value}"\n'
+                ret.append(f'    "{bb.label.value}" -> "{out_bb.label.value}"')
 
         for bb in self.get_basic_blocks():
-            ret += f'    "{bb.label.value}" [shape=plaintext, '
-            ret += f'label={_make_label(bb)}, fontname="Courier" fontsize="8"]\n'
+            ret.append(f'    "{bb.label.value}" [shape=plaintext, ')
+            ret.append(f'label={_make_label(bb)}, fontname="Courier" fontsize="8"]')
+
+        ret.append("}\n")
+        if not only_subgraph:
+            ret.append("}\n")
 
-        ret += "}\n"
-        return ret
+        return "\n".join(ret)
 
     def __repr__(self) -> str:
         str = f"IRFunction: {self.name}\n"