Repository: amaranth-lang/amaranth (python)

Issue:
get_var_name reports wrong name on Python 3.11
```python
class Test(Elaboratable):

    def __init__(self):
        self.i = Signal(4)

    def elaborate(self, platform):
        m = Module()

        a = Signal()
        print(a.name)

        b = Cat(self.i[i] & a for i in range(4))
        c = Signal(4)
        m.d.sync += c.eq(b)

        return m
```
Expected output is "a", but on Python 3.11, it prints "self". This happens when both self and the variable are cell vars (e.g., when they are used in a generator expression).

```python
            name_index = int(code.co_code[index + 1])
            if sys.version_info >= (3, 11):
                name_index -= code.co_nlocals
            return code.co_cellvars[name_index]
```
The line `name_index -= code.co_nlocals` assumes that `code.co_cellvars` does not contains any local variable but this is not the case if self is also a cell var. The code object provides a `_varname_from_oparg` internal method and I wonder if it is a better choice for inferring variable names. 

Pull request:
tracer: fix STORE_DEREF handling, add EXTENDED_ARG support.
This fixes the following issues:

- on Python 3.10 and earlier, storing to free variables is now handled correctly
- on Python 3.11, `_varname_from_oparg` is now used, fixing problems with cell variables that are also arguments
- on all supported versions, EXTENDED_ARG is now parsed, ensuring proper handling for long functions

Fixes #792.

Patch:
diff --git a/amaranth/tracer.py b/amaranth/tracer.py
index 5a21dd0..bc61e2f 100644
--- a/amaranth/tracer.py
+++ b/amaranth/tracer.py
@@ -26,24 +26,40 @@ def get_var_name(depth=2, default=_raise_exception):
             break
     if call_opc not in ("CALL_FUNCTION", "CALL_FUNCTION_KW", "CALL_FUNCTION_EX",
                         "CALL_METHOD", "CALL"):
-        return default
+        if default is _raise_exception:
+            raise NameNotFound
+        else:
+            return default
 
     index = call_index + 2
+    imm = 0
     while True:
         opc = opname[code.co_code[index]]
-        if opc in ("STORE_NAME", "STORE_ATTR"):
-            name_index = int(code.co_code[index + 1])
-            return code.co_names[name_index]
+        if opc == 'EXTENDED_ARG':
+            imm |= int(code.co_code[index + 1])
+            imm <<= 8
+            index += 2
+        elif opc in ("STORE_NAME", "STORE_ATTR"):
+            imm |= int(code.co_code[index + 1])
+            return code.co_names[imm]
         elif opc == "STORE_FAST":
-            name_index = int(code.co_code[index + 1])
-            return code.co_varnames[name_index]
+            imm |= int(code.co_code[index + 1])
+            if sys.version_info >= (3, 11):
+                return code._varname_from_oparg(imm)
+            else:
+                return code.co_varnames[imm]
         elif opc == "STORE_DEREF":
-            name_index = int(code.co_code[index + 1])
+            imm |= int(code.co_code[index + 1])
             if sys.version_info >= (3, 11):
-                name_index -= code.co_nlocals
-            return code.co_cellvars[name_index]
+                return code._varname_from_oparg(imm)
+            else:
+                if imm < len(code.co_cellvars):
+                    return code.co_cellvars[imm]
+                else:
+                    return code.co_freevars[imm - len(code.co_cellvars)]
         elif opc in ("LOAD_GLOBAL", "LOAD_NAME", "LOAD_ATTR", "LOAD_FAST", "LOAD_DEREF",
                      "DUP_TOP", "BUILD_LIST", "CACHE", "COPY"):
+            imm = 0
             index += 2
         else:
             if default is _raise_exception:


Test patch:
diff --git a/tests/test_tracer.py b/tests/test_tracer.py
new file mode 100644
index 0000000..54c6994
--- /dev/null
+++ b/tests/test_tracer.py
@@ -0,0 +1,79 @@
+from amaranth.hdl.ast import *
+from types import SimpleNamespace
+
+from .utils import *
+
+class TracerTestCase(FHDLTestCase):
+    def test_fast(self):
+        s1 = Signal()
+        self.assertEqual(s1.name, "s1")
+        s2 = Signal()
+        self.assertEqual(s2.name, "s2")
+
+    def test_name(self):
+        class Dummy:
+            s1 = Signal()
+            self.assertEqual(s1.name, "s1")
+            s2 = Signal()
+            self.assertEqual(s2.name, "s2")
+
+    def test_attr(self):
+        ns = SimpleNamespace()
+        ns.s1 = Signal()
+        self.assertEqual(ns.s1.name, "s1")
+        ns.s2 = Signal()
+        self.assertEqual(ns.s2.name, "s2")
+
+    def test_index(self):
+        l = [None]
+        l[0] = Signal()
+        self.assertEqual(l[0].name, "$signal")
+
+    def test_deref_cell(self):
+        s1 = Signal()
+        self.assertEqual(s1.name, "s1")
+        s2 = Signal()
+        self.assertEqual(s2.name, "s2")
+
+        def dummy():
+            return s1, s2
+
+    def test_deref_free(self):
+        def inner():
+            nonlocal s3, s4
+            s3 = Signal()
+            s4 = Signal()
+            return s1, s2
+
+        s1 = Signal()
+        s2 = Signal()
+        s3 = None
+        s4 = None
+        inner()
+        self.assertEqual(s1.name, "s1")
+        self.assertEqual(s2.name, "s2")
+        self.assertEqual(s3.name, "s3")
+        self.assertEqual(s4.name, "s4")
+
+    def test_long(self):
+        test = ""
+        for i in range(100000):
+            test += f"dummy{i} = None\n"
+        test += "s1 = Signal()\n"
+        test += "s2 = Signal()\n"
+        ns = {"Signal": Signal}
+        exec(test, ns)
+        self.assertEqual(ns["s1"].name, "s1")
+        self.assertEqual(ns["s2"].name, "s2")
+
+    def test_deref_fast(self):
+        def inner(s2):
+            s1 = Signal()
+            s2 = Signal()
+            self.assertEqual(s1.name, "s1")
+            self.assertEqual(s2.name, "s2")
+
+            def dummy():
+                return s1, s2
+
+        inner(None)