MAINT: Align masked with normal ufunc loops

This removes the ability to specialize masked inner loops (for now)
as was already noted in NEP 41 and NEP 43.

The masked array is now passed in as the last argument to use the
identical signature and avoid duplicating the code unnecessary.

This is part of the longer process to refactor ufuncs to NEP 43
and split out, to keep the diff's shorter (or at least easier
to read).

doc/release/upcoming_changes/19259.c_api.rst

@@ -0,0 +1,14 @@
+Masked inner-loops cannot be customized anymore
+-----------------------------------------------
+The masked inner-loop selector is now never used.  A warning
+will be given in the unlikely event that it was customized.
+
+We do not expect that any code uses this.  If you do use it,
+you must unset unset the selector on newer NumPy version.
+Please also contact the NumPy developers, we do anticipate
+providing a new, more specific, mechanism.
+
+This change will not affect the results of operations, since
+the fallback (which is always used internally) will handle
+the operation equivalently, the customization was a planned
+feature to allow for faster masked operation.

numpy/core/include/numpy/ufuncobject.h

@@ -66,27 +66,14 @@ typedef int (PyUFunc_TypeResolutionFunc)(
                                 PyArray_Descr **out_dtypes);
 
 /*
- * Given an array of DTypes as returned by the PyUFunc_TypeResolutionFunc,
- * and an array of fixed strides (the array will contain NPY_MAX_INTP for
- * strides which are not necessarily fixed), returns an inner loop
- * with associated auxiliary data.
- *
- * For backwards compatibility, there is a variant of the inner loop
- * selection which returns an inner loop irrespective of the strides,
- * and with a void* static auxiliary data instead of an NpyAuxData *
- * dynamically allocatable auxiliary data.
+ * Legacy loop selector. (This should NOT normally be used and we can expect
+ * that only the `PyUFunc_DefaultLegacyInnerLoopSelector` is ever set).
+ * However, unlike the masked version, it probably still works.
  *
  * ufunc:             The ufunc object.
  * dtypes:            An array which has been populated with dtypes,
  *                    in most cases by the type resolution function
  *                    for the same ufunc.
- * fixed_strides:     For each input/output, either the stride that
- *                    will be used every time the function is called
- *                    or NPY_MAX_INTP if the stride might change or
- *                    is not known ahead of time. The loop selection
- *                    function may use this stride to pick inner loops
- *                    which are optimized for contiguous or 0-stride
- *                    cases.
  * out_innerloop:     Should be populated with the correct ufunc inner
  *                    loop for the given type.
  * out_innerloopdata: Should be populated with the void* data to
@@ -101,15 +88,7 @@ typedef int (PyUFunc_LegacyInnerLoopSelectionFunc)(
                             PyUFuncGenericFunction *out_innerloop,
                             void **out_innerloopdata,
                             int *out_needs_api);
-typedef int (PyUFunc_MaskedInnerLoopSelectionFunc)(
-                            struct _tagPyUFuncObject *ufunc,
-                            PyArray_Descr **dtypes,
-                            PyArray_Descr *mask_dtype,
-                            npy_intp *fixed_strides,
-                            npy_intp fixed_mask_stride,
-                            PyUFunc_MaskedStridedInnerLoopFunc **out_innerloop,
-                            NpyAuxData **out_innerloopdata,
-                            int *out_needs_api);
+
 
 typedef struct _tagPyUFuncObject {
         PyObject_HEAD
@@ -199,10 +178,8 @@ typedef struct _tagPyUFuncObject {
     #else
         void *reserved2;
     #endif
-        /*
-         * A function which returns a masked inner loop for the ufunc.
-         */
-        PyUFunc_MaskedInnerLoopSelectionFunc *masked_inner_loop_selector;
+        /* Was previously the `PyUFunc_MaskedInnerLoopSelectionFunc` */
+        void *_always_null_previously_masked_innerloop_selector;
 
         /*
          * List of flags for each operand when ufunc is called by nditer object.