gh-111545: Add Py_HashPointer() function (#112096)

* Implement _Py_HashPointerRaw() as a static inline function.
* Add Py_HashPointer() tests to test_capi.test_hash.
* Keep _Py_HashPointer() function as an alias to Py_HashPointer().

2 files changed, 19 insertions, 3 deletions

diff --git a/Include/cpython/pyhash.h b/Include/cpython/pyhash.h
index 6f7113d..396c208 100644
--- a/Include/cpython/pyhash.h
+++ b/Include/cpython/pyhash.h
@@ -21,7 +21,9 @@
 
 /* Helpers for hash functions */
 PyAPI_FUNC(Py_hash_t) _Py_HashDouble(PyObject *, double);
-PyAPI_FUNC(Py_hash_t) _Py_HashPointer(const void*);
+
+// Kept for backward compatibility
+#define _Py_HashPointer Py_HashPointer
 
 
 /* hash function definition */
@@ -33,3 +35,5 @@ typedef struct {
 } PyHash_FuncDef;
 
 PyAPI_FUNC(PyHash_FuncDef*) PyHash_GetFuncDef(void);
+
+PyAPI_FUNC(Py_hash_t) Py_HashPointer(const void *ptr);
diff --git a/Include/internal/pycore_pyhash.h b/Include/internal/pycore_pyhash.h
index c3b72d9..0ce0890 100644
--- a/Include/internal/pycore_pyhash.h
+++ b/Include/internal/pycore_pyhash.h
@@ -5,8 +5,20 @@
 #  error "this header requires Py_BUILD_CORE define"
 #endif
 
-// Similar to _Py_HashPointer(), but don't replace -1 with -2
-extern Py_hash_t _Py_HashPointerRaw(const void*);
+// Similar to Py_HashPointer(), but don't replace -1 with -2.
+static inline Py_hash_t
+_Py_HashPointerRaw(const void *ptr)
+{
+    uintptr_t x = (uintptr_t)ptr;
+    Py_BUILD_ASSERT(sizeof(x) == sizeof(ptr));
+
+    // Bottom 3 or 4 bits are likely to be 0; rotate x by 4 to the right
+    // to avoid excessive hash collisions for dicts and sets.
+    x = (x >> 4) | (x << (8 * sizeof(uintptr_t) - 4));
+
+    Py_BUILD_ASSERT(sizeof(x) == sizeof(Py_hash_t));
+    return (Py_hash_t)x;
+}
 
 // Export for '_datetime' shared extension
 PyAPI_FUNC(Py_hash_t) _Py_HashBytes(const void*, Py_ssize_t);