Revert "GH-128914: Remove conditional stack effects from bytecodes.c and the code generators (GH-128918)" (GH-129202)

The commit introduced a ~2.5-3% regression in the free threading build.

This reverts commit ab61d3f430.

Tools/cases_generator/stack.py

@@ -24,7 +24,17 @@ def maybe_parenthesize(sym: str) -> str:
 
 
 def var_size(var: StackItem) -> str:
-    if var.size:
+    if var.condition:
+        # Special case simplifications
+        if var.condition == "0":
+            return "0"
+        elif var.condition == "1":
+            return var.get_size()
+        elif var.condition == "oparg & 1" and not var.size:
+            return f"({var.condition})"
+        else:
+            return f"(({var.condition}) ? {var.get_size()} : 0)"
+    elif var.size:
         return var.size
     else:
         return "1"
@@ -80,6 +90,10 @@ class Local:
     def name(self) -> str:
         return self.item.name
 
+    @property
+    def condition(self) -> str | None:
+        return self.item.condition
+
     def is_array(self) -> bool:
         return self.item.is_array()
 
@@ -260,7 +274,16 @@ class Stack:
         self.defined.add(var.name)
         cast = f"({var.type})" if (not indirect and var.type) else ""
         bits = ".bits" if cast and self.extract_bits else ""
-        assign = f"{var.name} = {cast}{indirect}stack_pointer[{self.base_offset.to_c()}]{bits};\n"
+        assign = f"{var.name} = {cast}{indirect}stack_pointer[{self.base_offset.to_c()}]{bits};"
+        if var.condition:
+            if var.condition == "1":
+                assign = f"{assign}\n"
+            elif var.condition == "0":
+                return "", Local.unused(var)
+            else:
+                assign = f"if ({var.condition}) {{ {assign} }}\n"
+        else:
+            assign = f"{assign}\n"
         return assign, Local.from_memory(var)
 
     def push(self, var: Local) -> None:
@@ -280,6 +303,10 @@ class Stack:
     ) -> None:
         cast = f"({cast_type})" if var.type else ""
         bits = ".bits" if cast and extract_bits else ""
+        if var.condition == "0":
+            return
+        if var.condition and var.condition != "1":
+            out.emit(f"if ({var.condition}) ")
         out.emit(f"stack_pointer[{base_offset.to_c()}]{bits} = {cast}{var.name};\n")
 
     def _adjust_stack_pointer(self, out: CWriter, number: str) -> None: