fix[venom]: alloca for default arguments (vyperlang#4155)

this commit fixes an `ir_node_to_venom` translation bug. when there is
a default argument to an external function, it can generate multiple
allocas, because the entry points allocate separate symbol tables, but
actually they should all correspond to the same alloca. for instance,
`external 1 foo(uint256)12345` and `external 1 foo()67890` both feed
into the same `external 1 foo()__common`, but the current translator
mistakenly creates different symbol tables for the two "feeder" entry
points, resulting in separate allocas for the same logical variable.

this commit fixes the bug by fusing the symbol tables for multiple
entry points to the same external function.

vyper/venom/ir_node_to_venom.py

@@ -107,14 +107,16 @@
 NOOP_INSTRUCTIONS = frozenset(["pass", "cleanup_repeat", "var_list", "unique_symbol"])
 
 SymbolTable = dict[str, Optional[IROperand]]
-_global_symbols: SymbolTable = {}
+_global_symbols: SymbolTable = None  # type: ignore
 MAIN_ENTRY_LABEL_NAME = "__main_entry"
+_external_functions: dict[int, SymbolTable] = None  # type: ignore
 
 
 # convert IRnode directly to venom
 def ir_node_to_venom(ir: IRnode) -> IRContext:
-    global _global_symbols
+    global _global_symbols, _external_functions
     _global_symbols = {}
+    _external_functions = {}
 
     ctx = IRContext()
     fn = ctx.create_function(MAIN_ENTRY_LABEL_NAME)
@@ -214,10 +216,6 @@ def _convert_ir_bb_list(fn, ir, symbols):
     return ret
 
 
-current_func = None
-var_list: list[str] = []
-
-
 def pop_source_on_return(func):
     @functools.wraps(func)
     def pop_source(*args, **kwargs):
@@ -232,7 +230,10 @@ def pop_source(*args, **kwargs):
 @pop_source_on_return
 def _convert_ir_bb(fn, ir, symbols):
     assert isinstance(ir, IRnode), ir
-    global _break_target, _continue_target, current_func, var_list, _global_symbols
+    # TODO: refactor these to not be globals
+    global _break_target, _continue_target, _global_symbols, _external_functions
+
+    # keep a map from external functions to all possible entry points
 
     ctx = fn.ctx
     fn.push_source(ir)
@@ -274,7 +275,6 @@ def _convert_ir_bb(fn, ir, symbols):
 
                 return ret
             elif is_external:
-                _global_symbols = {}
                 ret = _convert_ir_bb(fn, ir.args[0], symbols)
                 _append_return_args(fn)
         else:
@@ -382,6 +382,13 @@ def _convert_ir_bb(fn, ir, symbols):
                 data = _convert_ir_bb(fn, c, symbols)
                 ctx.append_data("db", [data])  # type: ignore
     elif ir.value == "label":
+        function_id_pattern = r"external (\d+)"
+        function_name = ir.args[0].value
+        m = re.match(function_id_pattern, function_name)
+        if m is not None:
+            function_id = m.group(1)
+            _global_symbols = _external_functions.setdefault(function_id, {})
+
         label = IRLabel(ir.args[0].value, True)
         bb = fn.get_basic_block()
         if not bb.is_terminated: