numpy
diff --git a/doc/release/upcoming_changes/27896.performance.rst b/doc/release/upcoming_changes/27896.performance.rst
@@ -0,0 +1,2 @@
+* Improved multithreaded scaling on the free-threaded build when many threads
+  simultaneously call the same ufunc operations.
diff --git a/numpy/_core/code_generators/genapi.py b/numpy/_core/code_generators/genapi.py
@@ -85,7 +85,7 @@ def get_processor():
              join('multiarray', 'stringdtype', 'static_string.c'),
              join('multiarray', 'strfuncs.c'),
              join('multiarray', 'usertypes.c'),
-             join('umath', 'dispatching.c'),
+             join('umath', 'dispatching.cpp'),
              join('umath', 'extobj.c'),
              join('umath', 'loops.c.src'),
              join('umath', 'reduction.c'),

diff --git a/numpy/_core/code_generators/generate_umath.py b/numpy/_core/code_generators/generate_umath.py
@@ -1592,13 +1592,10 @@ def make_code(funcdict, filename):
     #include "matmul.h"
     #include "clip.h"
     #include "dtypemeta.h"
+    #include "dispatching.h"
     #include "_umath_doc_generated.h"
 
     %s
-    /* Returns a borrowed ref of the second value in the matching info tuple */
-    PyObject *
-    get_info_no_cast(PyUFuncObject *ufunc, PyArray_DTypeMeta *op_dtype,
-                     int ndtypes);
 
     static int
     InitOperators(PyObject *dictionary) {

diff --git a/numpy/_core/include/numpy/ndarraytypes.h b/numpy/_core/include/numpy/ndarraytypes.h
@@ -1,6 +1,10 @@
 #ifndef NUMPY_CORE_INCLUDE_NUMPY_NDARRAYTYPES_H_
 #define NUMPY_CORE_INCLUDE_NUMPY_NDARRAYTYPES_H_
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 #include "npy_common.h"
 #include "npy_endian.h"
 #include "npy_cpu.h"
@@ -1922,4 +1926,8 @@ typedef struct {
  */
 #undef NPY_DEPRECATED_INCLUDES
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif  /* NUMPY_CORE_INCLUDE_NUMPY_NDARRAYTYPES_H_ */
diff --git a/numpy/_core/meson.build b/numpy/_core/meson.build
@@ -713,7 +713,7 @@ py.extension_module('_multiarray_tests',
     src_file.process('src/multiarray/_multiarray_tests.c.src'),
     'src/common/mem_overlap.c',
     'src/common/npy_argparse.c',
-    'src/common/npy_hashtable.c',
+    'src/common/npy_hashtable.cpp',
     src_file.process('src/common/templ_common.h.src')
   ],
   c_args: c_args_common,
@@ -1042,7 +1042,7 @@ src_multiarray_umath_common = [
   'src/common/gil_utils.c',
   'src/common/mem_overlap.c',
   'src/common/npy_argparse.c',
-  'src/common/npy_hashtable.c',
+  'src/common/npy_hashtable.cpp',
   'src/common/npy_import.c',
   'src/common/npy_longdouble.c',
   'src/common/ucsnarrow.c',
@@ -1153,7 +1153,7 @@ src_umath = umath_gen_headers + [
   'src/umath/ufunc_type_resolution.c',
   'src/umath/clip.cpp',
   'src/umath/clip.h',
-  'src/umath/dispatching.c',
+  'src/umath/dispatching.cpp',
   'src/umath/extobj.c',
   'src/umath/legacy_array_method.c',
   'src/umath/override.c',

diff --git a/numpy/_core/src/common/npy_hashtable.c → numpy/_core/src/common/npy_hashtable.cpp b/numpy/_core/src/common/npy_hashtable.c → numpy/_core/src/common/npy_hashtable.cpp
@@ -12,6 +12,9 @@
  * case is likely desired.
  */
 
+#include <mutex>
+#include <shared_mutex>
+
 #include "templ_common.h"
 #include "npy_hashtable.h"
 
@@ -89,7 +92,7 @@ find_item(PyArrayIdentityHash const *tb, PyObject *const *key)
 NPY_NO_EXPORT PyArrayIdentityHash *
 PyArrayIdentityHash_New(int key_len)
 {
-    PyArrayIdentityHash *res = PyMem_Malloc(sizeof(PyArrayIdentityHash));
+    PyArrayIdentityHash *res = (PyArrayIdentityHash *)PyMem_Malloc(sizeof(PyArrayIdentityHash));
     if (res == NULL) {
         PyErr_NoMemory();
         return NULL;
@@ -100,12 +103,21 @@ PyArrayIdentityHash_New(int key_len)
     res->size = 4;  /* Start with a size of 4 */
     res->nelem = 0;
 
-    res->buckets = PyMem_Calloc(4 * (key_len + 1), sizeof(PyObject *));
+    res->buckets = (PyObject **)PyMem_Calloc(4 * (key_len + 1), sizeof(PyObject *));
     if (res->buckets == NULL) {
         PyErr_NoMemory();
         PyMem_Free(res);
         return NULL;
     }
+
+#ifdef Py_GIL_DISABLED
+    res->mutex = new(std::nothrow) std::shared_mutex();
+    if (res->mutex == nullptr) {
+        PyErr_NoMemory();
+        PyMem_Free(res);
+        return NULL;
+    }
+#endif
     return res;
 }
 
@@ -115,6 +127,9 @@ PyArrayIdentityHash_Dealloc(PyArrayIdentityHash *tb)
 {
     PyMem_Free(tb->buckets);
     PyMem_Free(tb);
+#ifdef Py_GIL_DISABLED
+    delete (std::shared_mutex *)tb->mutex;
+#endif
 }
 
 
@@ -149,7 +164,7 @@ _resize_if_necessary(PyArrayIdentityHash *tb)
     if (npy_mul_sizes_with_overflow(&alloc_size, new_size, tb->key_len + 1)) {
         return -1;
     }
-    tb->buckets = PyMem_Calloc(alloc_size, sizeof(PyObject *));
+    tb->buckets = (PyObject **)PyMem_Calloc(alloc_size, sizeof(PyObject *));
     if (tb->buckets == NULL) {
         tb->buckets = old_table;
         PyErr_NoMemory();

diff --git a/numpy/_core/src/common/npy_hashtable.h b/numpy/_core/src/common/npy_hashtable.h
@@ -7,12 +7,19 @@
 #include "numpy/ndarraytypes.h"
 
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 typedef struct {
     int key_len;  /* number of identities used */
     /* Buckets stores: val1, key1[0], key1[1], ..., val2, key2[0], ... */
     PyObject **buckets;
     npy_intp size;  /* current size */
     npy_intp nelem;  /* number of elements */
+#ifdef Py_GIL_DISABLED
+    void *mutex;
+#endif
 } PyArrayIdentityHash;
 
 
@@ -29,4 +36,8 @@ PyArrayIdentityHash_New(int key_len);
 NPY_NO_EXPORT void
 PyArrayIdentityHash_Dealloc(PyArrayIdentityHash *tb);
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif  /* NUMPY_CORE_SRC_COMMON_NPY_NPY_HASHTABLE_H_ */
diff --git a/numpy/_core/src/multiarray/common.h b/numpy/_core/src/multiarray/common.h
@@ -12,6 +12,10 @@
 #include "npy_import.h"
 #include <limits.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 #define error_converting(x)  (((x) == -1) && PyErr_Occurred())
 
 #ifdef NPY_ALLOW_THREADS
@@ -104,13 +108,13 @@ check_and_adjust_index(npy_intp *index, npy_intp max_item, int axis,
         /* Try to be as clear as possible about what went wrong. */
         if (axis >= 0) {
             PyErr_Format(PyExc_IndexError,
-                         "index %"NPY_INTP_FMT" is out of bounds "
-                         "for axis %d with size %"NPY_INTP_FMT,
+                         "index %" NPY_INTP_FMT" is out of bounds "
+                         "for axis %d with size %" NPY_INTP_FMT,
                          *index, axis, max_item);
         } else {
             PyErr_Format(PyExc_IndexError,
-                         "index %"NPY_INTP_FMT" is out of bounds "
-                         "for size %"NPY_INTP_FMT, *index, max_item);
+                         "index %" NPY_INTP_FMT " is out of bounds "
+                         "for size %" NPY_INTP_FMT, *index, max_item);
         }
         return -1;
     }
@@ -163,7 +167,9 @@ check_and_adjust_axis(int *axis, int ndim)
  * <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52023>.
  * clang versions < 8.0.0 have the same bug.
  */
-#if (!defined __STDC_VERSION__ || __STDC_VERSION__ < 201112 \
+#ifdef __cplusplus
+#define NPY_ALIGNOF(type) alignof(type)
+#elif (!defined __STDC_VERSION__ || __STDC_VERSION__ < 201112 \
      || (defined __GNUC__ && __GNUC__ < 4 + (__GNUC_MINOR__ < 9) \
   && !defined __clang__) \
      || (defined __clang__ && __clang_major__ < 8))
@@ -347,4 +353,8 @@ new_array_for_sum(PyArrayObject *ap1, PyArrayObject *ap2, PyArrayObject* out,
  */
 #define NPY_ITER_REDUCTION_AXIS(axis) (axis + (1 << (NPY_BITSOF_INT - 2)))
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif  /* NUMPY_CORE_SRC_MULTIARRAY_COMMON_H_ */
diff --git a/numpy/_core/src/multiarray/npy_static_data.h b/numpy/_core/src/multiarray/npy_static_data.h
@@ -1,6 +1,10 @@
 #ifndef NUMPY_CORE_SRC_MULTIARRAY_STATIC_DATA_H_
 #define NUMPY_CORE_SRC_MULTIARRAY_STATIC_DATA_H_
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 NPY_NO_EXPORT int
 initialize_static_globals(void);
 
@@ -168,4 +172,8 @@ NPY_VISIBILITY_HIDDEN extern npy_interned_str_struct npy_interned_str;
 NPY_VISIBILITY_HIDDEN extern npy_static_pydata_struct npy_static_pydata;
 NPY_VISIBILITY_HIDDEN extern npy_static_cdata_struct npy_static_cdata;
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif  // NUMPY_CORE_SRC_MULTIARRAY_STATIC_DATA_H_
diff --git a/numpy/_core/src/umath/dispatching.c → numpy/_core/src/umath/dispatching.cpp b/numpy/_core/src/umath/dispatching.c → numpy/_core/src/umath/dispatching.cpp
@@ -38,6 +38,9 @@
 #define _MULTIARRAYMODULE
 #define _UMATHMODULE
 
+#include <mutex>
+#include <shared_mutex>
+
 #define PY_SSIZE_T_CLEAN
 #include <Python.h>
 #include <convert_datatype.h>
@@ -504,8 +507,9 @@ call_promoter_and_recurse(PyUFuncObject *ufunc, PyObject *info,
         PyObject *promoter = PyTuple_GET_ITEM(info, 1);
         if (PyCapsule_CheckExact(promoter)) {
             /* We could also go the other way and wrap up the python function... */
-            PyArrayMethod_PromoterFunction *promoter_function = PyCapsule_GetPointer(
-                    promoter, "numpy._ufunc_promoter");
+            PyArrayMethod_PromoterFunction *promoter_function =
+                    (PyArrayMethod_PromoterFunction *)PyCapsule_GetPointer(
+                            promoter, "numpy._ufunc_promoter");
             if (promoter_function == NULL) {
                 return NULL;
             }
@@ -770,8 +774,9 @@ promote_and_get_info_and_ufuncimpl(PyUFuncObject *ufunc,
      * 2. Check all registered loops/promoters to find the best match.
      * 3. Fall back to the legacy implementation if no match was found.
      */
-    PyObject *info = PyArrayIdentityHash_GetItem(ufunc->_dispatch_cache,
-                (PyObject **)op_dtypes);
+    PyObject *info = PyArrayIdentityHash_GetItem(
+            (PyArrayIdentityHash *)ufunc->_dispatch_cache,
+            (PyObject **)op_dtypes);
     if (info != NULL && PyObject_TypeCheck(
             PyTuple_GET_ITEM(info, 1), &PyArrayMethod_Type)) {
         /* Found the ArrayMethod and NOT a promoter: return it */
@@ -793,8 +798,9 @@ promote_and_get_info_and_ufuncimpl(PyUFuncObject *ufunc,
              * Found the ArrayMethod and NOT promoter.  Before returning it
              * add it to the cache for faster lookup in the future.
              */
-            if (PyArrayIdentityHash_SetItem(ufunc->_dispatch_cache,
-                    (PyObject **)op_dtypes, info, 0) < 0) {
+            if (PyArrayIdentityHash_SetItem(
+                        (PyArrayIdentityHash *)ufunc->_dispatch_cache,
+                        (PyObject **)op_dtypes, info, 0) < 0) {
                 return NULL;
             }
             return info;
@@ -815,8 +821,9 @@ promote_and_get_info_and_ufuncimpl(PyUFuncObject *ufunc,
         }
         else if (info != NULL) {
             /* Add result to the cache using the original types: */
-            if (PyArrayIdentityHash_SetItem(ufunc->_dispatch_cache,
-                    (PyObject **)op_dtypes, info, 0) < 0) {
+            if (PyArrayIdentityHash_SetItem(
+                        (PyArrayIdentityHash *)ufunc->_dispatch_cache,
+                        (PyObject **)op_dtypes, info, 0) < 0) {
                 return NULL;
             }
             return info;
@@ -882,13 +889,51 @@ promote_and_get_info_and_ufuncimpl(PyUFuncObject *ufunc,
     }
 
     /* Add this to the cache using the original types: */
-    if (cacheable && PyArrayIdentityHash_SetItem(ufunc->_dispatch_cache,
-            (PyObject **)op_dtypes, info, 0) < 0) {
+    if (cacheable && PyArrayIdentityHash_SetItem(
+                (PyArrayIdentityHash *)ufunc->_dispatch_cache,
+                (PyObject **)op_dtypes, info, 0) < 0) {
         return NULL;
     }
     return info;
 }
 
+#ifdef Py_GIL_DISABLED
+/*
+ * Fast path for promote_and_get_info_and_ufuncimpl.
+ * Acquires a read lock to check for a cache hit and then
+ * only acquires a write lock on a cache miss to fill the cache
+ */
+static inline PyObject *
+promote_and_get_info_and_ufuncimpl_with_locking(
+        PyUFuncObject *ufunc,
+        PyArrayObject *const ops[],
+        PyArray_DTypeMeta *signature[],
+        PyArray_DTypeMeta *op_dtypes[],
+        npy_bool legacy_promotion_is_possible)
+{
+    std::shared_mutex *mutex = ((std::shared_mutex *)((PyArrayIdentityHash *)ufunc->_dispatch_cache)->mutex);
+    mutex->lock_shared();
+    PyObject *info = PyArrayIdentityHash_GetItem(
+            (PyArrayIdentityHash *)ufunc->_dispatch_cache,
+            (PyObject **)op_dtypes);
+    mutex->unlock_shared();
+
+    if (info != NULL && PyObject_TypeCheck(
+                    PyTuple_GET_ITEM(info, 1), &PyArrayMethod_Type)) {
+        /* Found the ArrayMethod and NOT a promoter: return it */
+        return info;
+    }
+
+    // cache miss, need to acquire a write lock and recursively calculate the
+    // correct dispatch resolution
+    mutex->lock();
+    info = promote_and_get_info_and_ufuncimpl(ufunc,
+            ops, signature, op_dtypes, legacy_promotion_is_possible);
+    mutex->unlock();
+
+    return info;
+}
+#endif
 
 /**
  * The central entry-point for the promotion and dispatching machinery.
@@ -941,6 +986,8 @@ promote_and_get_ufuncimpl(PyUFuncObject *ufunc,
 {
     int nin = ufunc->nin, nargs = ufunc->nargs;
     npy_bool legacy_promotion_is_possible = NPY_TRUE;
+    PyObject *all_dtypes = NULL;
+    PyArrayMethodObject *method = NULL;
 
     /*
      * Get the actual DTypes we operate with by setting op_dtypes[i] from
@@ -976,18 +1023,20 @@ promote_and_get_ufuncimpl(PyUFuncObject *ufunc,
         }
     }
 
-    PyObject *info;
-    Py_BEGIN_CRITICAL_SECTION((PyObject *)ufunc);
-    info = promote_and_get_info_and_ufuncimpl(ufunc,
+#ifdef Py_GIL_DISABLED
+    PyObject *info = promote_and_get_info_and_ufuncimpl_with_locking(ufunc,
+            ops, signature, op_dtypes, legacy_promotion_is_possible);
+#else
+    PyObject *info = promote_and_get_info_and_ufuncimpl(ufunc,
             ops, signature, op_dtypes, legacy_promotion_is_possible);
-    Py_END_CRITICAL_SECTION();
+#endif
 
     if (info == NULL) {
         goto handle_error;
     }
 
-    PyArrayMethodObject *method = (PyArrayMethodObject *)PyTuple_GET_ITEM(info, 1);
-    PyObject *all_dtypes = PyTuple_GET_ITEM(info, 0);
+    method = (PyArrayMethodObject *)PyTuple_GET_ITEM(info, 1);
+    all_dtypes = PyTuple_GET_ITEM(info, 0);
 
     /*
      * In certain cases (only the logical ufuncs really), the loop we found may
@@ -1218,7 +1267,7 @@ install_logical_ufunc_promoter(PyObject *ufunc)
     if (dtype_tuple == NULL) {
         return -1;
     }
-    PyObject *promoter = PyCapsule_New(&logical_ufunc_promoter,
+    PyObject *promoter = PyCapsule_New((void *)&logical_ufunc_promoter,
             "numpy._ufunc_promoter", NULL);
     if (promoter == NULL) {
         Py_DECREF(dtype_tuple);
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,2 @@
		* Improved multithreaded scaling on the free-threaded build when many threads
		simultaneously call the same ufunc operations.