gh-115999: Add free-threaded specialization for `UNPACK_SEQUENCE` (#126600)

Add free-threaded specialization for `UNPACK_SEQUENCE` opcode.
`UNPACK_SEQUENCE_TUPLE/UNPACK_SEQUENCE_TWO_TUPLE` are already thread safe since tuples are immutable.
`UNPACK_SEQUENCE_LIST` is not thread safe because of nature of lists (there is nothing preventing another thread from adding items to or removing them the list while the instruction is executing). To achieve thread safety we add a critical section to the implementation of `UNPACK_SEQUENCE_LIST`, especially around the parts where we check the size of the list and push items onto the stack.


---------

Co-authored-by: Matt Page <mpage@meta.com>
Co-authored-by: mpage <mpage@cs.stanford.edu>

1 files changed, 12 insertions, 18 deletions

diff --git a/Python/specialize.c b/Python/specialize.c
index c69f61c..716d53a 100644
--- a/Python/specialize.c
+++ b/Python/specialize.c
@@ -2487,39 +2487,33 @@ _Py_Specialize_UnpackSequence(_PyStackRef seq_st, _Py_CODEUNIT *instr, int oparg
 {
     PyObject *seq = PyStackRef_AsPyObjectBorrow(seq_st);
 
-    assert(ENABLE_SPECIALIZATION);
+    assert(ENABLE_SPECIALIZATION_FT);
     assert(_PyOpcode_Caches[UNPACK_SEQUENCE] ==
            INLINE_CACHE_ENTRIES_UNPACK_SEQUENCE);
-    _PyUnpackSequenceCache *cache = (_PyUnpackSequenceCache *)(instr + 1);
     if (PyTuple_CheckExact(seq)) {
         if (PyTuple_GET_SIZE(seq) != oparg) {
             SPECIALIZATION_FAIL(UNPACK_SEQUENCE, SPEC_FAIL_EXPECTED_ERROR);
-            goto failure;
+            unspecialize(instr);
+            return;
         }
         if (PyTuple_GET_SIZE(seq) == 2) {
-            instr->op.code = UNPACK_SEQUENCE_TWO_TUPLE;
-            goto success;
+            specialize(instr, UNPACK_SEQUENCE_TWO_TUPLE);
+            return;
         }
-        instr->op.code = UNPACK_SEQUENCE_TUPLE;
-        goto success;
+        specialize(instr, UNPACK_SEQUENCE_TUPLE);
+        return;
     }
     if (PyList_CheckExact(seq)) {
         if (PyList_GET_SIZE(seq) != oparg) {
             SPECIALIZATION_FAIL(UNPACK_SEQUENCE, SPEC_FAIL_EXPECTED_ERROR);
-            goto failure;
+            unspecialize(instr);
+            return;
         }
-        instr->op.code = UNPACK_SEQUENCE_LIST;
-        goto success;
+        specialize(instr, UNPACK_SEQUENCE_LIST);
+        return;
     }
     SPECIALIZATION_FAIL(UNPACK_SEQUENCE, unpack_sequence_fail_kind(seq));
-failure:
-    STAT_INC(UNPACK_SEQUENCE, failure);
-    instr->op.code = UNPACK_SEQUENCE;
-    cache->counter = adaptive_counter_backoff(cache->counter);
-    return;
-success:
-    STAT_INC(UNPACK_SEQUENCE, success);
-    cache->counter = adaptive_counter_cooldown();
+    unspecialize(instr);
 }
 
 #ifdef Py_STATS