Don't write to locals if they are never read

library/_compile_test.py

@@ -1486,5 +1486,89 @@ async def foo():
     # TODO(emacs): Test with (multiple context managers)
 
 
+@pyro_only
+class OptStoreFastTests(unittest.TestCase):
+    def test_store_with_load_not_replaced(self):
+        source = """
+def foo():
+    _ = 123
+    return _
+"""
+        func = compile_function(source, "foo")
+        self.assertEqual(
+            dis(func.__code__),
+            """\
+LOAD_CONST 123
+STORE_FAST_REVERSE _
+LOAD_FAST_REVERSE_UNCHECKED _
+RETURN_VALUE
+""",
+        )
+        self.assertEqual(func(), 123)
+
+    def test_store_with_no_load_replaced_with_pop_top(self):
+        source = """
+def foo():
+    _ = 123
+    return 456
+"""
+        func = compile_function(source, "foo")
+        self.assertEqual(
+            dis(func.__code__),
+            """\
+LOAD_CONST 123
+POP_TOP
+LOAD_CONST 456
+RETURN_VALUE
+""",
+        )
+        self.assertEqual(func(), 456)
+
+    def test_store_in_loop_replaced_with_pop_top(self):
+        source = """
+def foo(x):
+    for _ in x:
+        pass
+"""
+        func = compile_function(source, "foo")
+        self.assertEqual(
+            dis(func.__code__),
+            """\
+LOAD_FAST_REVERSE_UNCHECKED x
+GET_ITER
+FOR_ITER 4
+POP_TOP
+JUMP_ABSOLUTE 4
+LOAD_CONST None
+RETURN_VALUE
+""",
+        )
+        self.assertEqual(func(()), None)
+
+    def test_multiple_store_no_read_replaced_with_pop_top(self):
+        source = """
+def foo():
+    x = 123
+    y = 456
+    z = 789
+    return x
+"""
+        func = compile_function(source, "foo")
+        self.assertEqual(
+            dis(func.__code__),
+            """\
+LOAD_CONST 123
+STORE_FAST_REVERSE x
+LOAD_CONST 456
+POP_TOP
+LOAD_CONST 789
+POP_TOP
+LOAD_FAST_REVERSE_UNCHECKED x
+RETURN_VALUE
+""",
+        )
+        self.assertEqual(func(), 123)
+
+
 if __name__ == "__main__":
     unittest.main()

library/_compiler.py

@@ -211,6 +211,25 @@ def visitBinOp(self, node: ast.BinOp) -> ast.expr:
 class PyroFlowGraph(PyFlowGraph38):
     opcode = opcodepyro.opcode
 
+    def optimizeStoreFast(self):
+        if "locals" in self.varnames or "locals" in self.names:
+            # A bit of a hack: if someone is using locals(), we shouldn't mess
+            # with them.
+            return
+        used = set()
+        for block in self.getBlocksInOrder():
+            for instr in block.getInstructions():
+                if instr.opname == "LOAD_FAST" or instr.opname == "DELETE_FAST":
+                    used.add(instr.oparg)
+        # We never read from or delete the local, so we can replace all stores
+        # to it with POP_TOP.
+        for block in self.getBlocksInOrder():
+            for instr in block.getInstructions():
+                if instr.opname == "STORE_FAST" and instr.oparg not in used:
+                    instr.opname = "POP_TOP"
+                    instr.oparg = 0
+                    instr.ioparg = 0
+
     def optimizeLoadFast(self):
         blocks = self.getBlocksInOrder()
         preds = tuple(set() for i in range(self.block_count))
@@ -297,6 +316,7 @@ def process_one_block(block, modify=False):
             self.entry.insts = deletes + self.entry.insts
 
     def getCode(self):
+        self.optimizeStoreFast()
         self.optimizeLoadFast()
         return super().getCode()