gh-152600: Remove LOAD_FAST/LOAD_FAST_BORROW; POP_TOP pairs

Lib/test/test_peepholer.py

@@ -868,7 +868,7 @@
         for func in (f, g):
             with self.subTest(func=func.__name__):
                 self.assertNotInBytecode(func, 'STORE_FAST_LOAD_FAST')
                 self.assertInBytecode(func, 'LOAD_FAST_BORROW', 'shared')
         self.assertEqual([i.opname for i in dis.get_instructions(f)],
                          [i.opname for i in dis.get_instructions(g)])
 
@@ -2712,8 +2712,8 @@
         ]
         expected = [
             ("LOAD_FAST", 0, 1),
-            ("LOAD_FAST_BORROW", 1, 2),
-            ("POP_TOP", None, 3),
+            ("NOP", None, 2),
+            ("NOP", None, 3),
         ]
         self.check(insts, expected)
 

Misc/NEWS.d/next/Core_and_Builtins/2026-07-25-18-05-19.gh-issue-152600._TEXv2.rst

@@ -0,0 +1,2 @@
+Now, :opcode:`LOAD_FAST`; :opcode:`POP_TOP`, and :opcode:`LOAD_FAST_BORROW`;
+:opcode:`POP_TOP` pairs become :opcode:`NOPs <NOP>`.

Python/flowgraph.c

@@ -1161,6 +1161,9 @@ remove_redundant_nops_and_pairs(basicblock *entryblock)
                    else if (prev_opcode == COPY && prev_oparg == 1) {
                        is_redundant_pair = true;
                    }
+                   else if (prev_opcode == LOAD_FAST) {
+                       is_redundant_pair = true;
+                   }
                 }
                 if (is_redundant_pair) {
                     INSTR_SET_OP0(prev_instr, NOP);
@@ -2669,9 +2672,14 @@ remove_redundant_nops_and_jumps(cfg_builder *g)
    Code trasnformations that reduce code size initially fill the gaps with
    NOPs.  Later those NOPs are removed.
 */
+
+static int
+add_checks_for_loads_of_uninitialized_variables(basicblock *entryblock,
+                                                int nlocals, int nparams);
+
 static int
 optimize_cfg(cfg_builder *g, PyObject *consts, PyObject *const_cache,
-             _Py_hashtable_t *consts_index, int firstlineno)
+             _Py_hashtable_t *consts_index, int firstlineno, int nlocals, int nparams)
 {
     assert(PyDict_CheckExact(const_cache));
     RETURN_IF_ERROR(check_cfg(g));
@@ -2682,6 +2690,9 @@ optimize_cfg(cfg_builder *g, PyObject *consts, PyObject *const_cache,
     for (basicblock *b = g->g_entryblock; b != NULL; b = b->b_next) {
         RETURN_IF_ERROR(optimize_basic_block(const_cache, b, consts, consts_index));
     }
+    RETURN_IF_ERROR(
+        add_checks_for_loads_of_uninitialized_variables(
+            g->g_entryblock, nlocals, nparams));
     RETURN_IF_ERROR(remove_redundant_nops_and_pairs(g->g_entryblock));
     RETURN_IF_ERROR(remove_unreachable(g->g_entryblock));
     RETURN_IF_ERROR(remove_redundant_nops_and_jumps(g));
@@ -3830,16 +3841,13 @@ _PyCfg_OptimizeCodeUnit(cfg_builder *g, PyObject *consts, PyObject *const_cache,
         }
     }
 
-    int ret = optimize_cfg(g, consts, const_cache, consts_index, firstlineno);
+    int ret = optimize_cfg(g, consts, const_cache, consts_index, firstlineno, nlocals, nparams);
 
     _Py_hashtable_destroy(consts_index);
 
     RETURN_IF_ERROR(ret);
 
     RETURN_IF_ERROR(remove_unused_consts(g->g_entryblock, consts));
-    RETURN_IF_ERROR(
-        add_checks_for_loads_of_uninitialized_variables(
-            g->g_entryblock, nlocals, nparams));
     RETURN_IF_ERROR(push_cold_blocks_to_end(g));
     RETURN_IF_ERROR(resolve_line_numbers(g, firstlineno));
     // temporarily remove assert. See https://github.com/python/cpython/issues/125845