ENH: Detect Fortran vs C order in array_assign_boolean_subscript (numpy#30201)

This PR fixes numpygh-30156 by detecting when to set KEEPORDER order in array_assign_boolean_subscript and adds a benchmark for it.

Author: hellerve

benchmarks/benchmarks/bench_indexing.py

@@ -84,6 +84,22 @@ def time_assign_cast(self, ndim):
             arr[indx] = val
 
 
+class BooleanAssignmentOrder(Benchmark):
+    params = ['C', 'F']
+    param_names = ['order']
+
+    def setup(self, order):
+        shape = (64, 64, 64)
+        # emulate gh-30156: boolean assignment into a Fortran/C array
+        self.base = np.zeros(shape, dtype=np.uint32, order=order)
+        mask = np.random.RandomState(0).rand(*self.base.shape) > 0.5
+        self.mask = mask.copy(order)
+        self.value = np.uint32(7)
+
+    def time_boolean_assign_scalar(self, order):
+        self.base[self.mask] = self.value
+
+
 class IndexingSeparate(Benchmark):
     def setup(self):
         self.tmp_dir = mkdtemp()

numpy/_core/src/multiarray/mapping.c

@@ -1173,6 +1173,8 @@ array_assign_boolean_subscript(PyArrayObject *self,
     }
     else {
         v_stride = 0;
+        /* If the same value is repeated, iteration order does not matter */
+        order = NPY_KEEPORDER;
     }
 
     v_data = PyArray_DATA(v);