python · ericsnowcurrently · Feb 28, 2022 · Feb 14, 2022 · Feb 14, 2022 · Feb 15, 2022
diff --git a/Include/internal/pycore_gc.h b/Include/internal/pycore_gc.h
@@ -20,6 +20,7 @@ typedef struct {
 } PyGC_Head;
 
 #define _Py_AS_GC(o) ((PyGC_Head *)(o)-1)
+#define _PyGC_Head_UNUSED PyGC_Head
 
 /* True if the object is currently tracked by the GC. */
 #define _PyObject_GC_IS_TRACKED(o) (_Py_AS_GC(o)->_gc_next != 0)

@@ -8,6 +8,7 @@ extern "C" {
 #  error "this header requires Py_BUILD_CORE define"
 #endif
 
+#include "pycore_gc.h"              // PyGC_Head
 #include "pycore_global_strings.h"  // struct _Py_global_strings
 
 
@@ -40,6 +41,9 @@ struct _Py_global_objects {
         } bytes_characters[256];
 
         struct _Py_global_strings strings;
+
+        _PyGC_Head_UNUSED _tuple_empty_gc_not_used;
+        PyTupleObject tuple_empty;
     } singletons;
 };
 

@@ -964,6 +964,10 @@ extern "C" {
                 INIT_ID(zipimporter), \
             }, \
         }, \
+        \
+        .tuple_empty = { \
+            .ob_base = _PyVarObject_IMMORTAL_INIT(&PyTuple_Type, 0) \
+        }, \
     }, \
 }
 /* End auto-generated code */

diff --git a/Include/internal/pycore_tuple.h b/Include/internal/pycore_tuple.h
@@ -20,30 +20,45 @@ extern void _PyTuple_Fini(PyInterpreterState *);
 
 /* other API */
 
-#ifndef WITH_FREELISTS
-// without freelists
-// for tuples only store empty tuple singleton
-#  define PyTuple_MAXSAVESIZE 1
-#  define PyTuple_MAXFREELIST 1
-#endif
+// PyTuple_MAXSAVESIZE - largest tuple to save on free list
+// PyTuple_MAXFREELIST - maximum number of tuples of each size to save
 
-/* Speed optimization to avoid frequent malloc/free of small tuples */
-#ifndef PyTuple_MAXSAVESIZE
-   // Largest tuple to save on free list
-#  define PyTuple_MAXSAVESIZE 20
-#endif
-#ifndef PyTuple_MAXFREELIST
-   // Maximum number of tuples of each size to save
-#  define PyTuple_MAXFREELIST 2000
+#if defined(PyTuple_MAXSAVESIZE) && PyTuple_MAXSAVESIZE <= 0
+   // A build indicated that tuple freelists should not be used.
+#  define PyTuple_NFREELISTS 0
+#  undef PyTuple_MAXSAVESIZE
+#  undef PyTuple_MAXFREELIST
+
+#elif !defined(WITH_FREELISTS)
+#  define PyTuple_NFREELISTS 0
+#  undef PyTuple_MAXSAVESIZE
+#  undef PyTuple_MAXFREELIST
+
+#else
+   // We are using a freelist for tuples.
+#  ifndef PyTuple_MAXSAVESIZE
+#    define PyTuple_MAXSAVESIZE 20
+#  endif
+#  define PyTuple_NFREELISTS PyTuple_MAXSAVESIZE
+#  ifndef PyTuple_MAXFREELIST
+#    define PyTuple_MAXFREELIST 2000
+#  endif
 #endif
 
 struct _Py_tuple_state {
-#if PyTuple_MAXSAVESIZE > 0
-    /* Entries 1 up to PyTuple_MAXSAVESIZE are free lists,
-       entry 0 is the empty tuple () of which at most one instance
-       will be allocated. */
-    PyTupleObject *free_list[PyTuple_MAXSAVESIZE];
-    int numfree[PyTuple_MAXSAVESIZE];
+#if PyTuple_NFREELISTS > 0
+    /* There is one freelist for each size from 1 to PyTuple_MAXSAVESIZE.
+       The empty tuple is handled separately.
+
+       Each tuple stored in the array is the head of the linked list
+       (and the next available tuple) for that size.  The actual tuple
+       object is used as the linked list node, with its first item
+       (ob_item[0]) pointing to the next node (i.e. the previous head).
+       Each linked list is initially NULL. */
+    PyTupleObject *free_list[PyTuple_NFREELISTS];
+    int numfree[PyTuple_NFREELISTS];
+#else
+    char _unused;  // Empty structs are not allowed.
 #endif
 };