gh-144995: Optimize memoryview == memoryview

Lib/test/test_memoryview.py

@@ -575,6 +575,55 @@ def test_array_assign(self):
         m[:] = new_a
         self.assertEqual(a, new_a)
 
+    def test_compare_equal(self):
+        # A memoryview is equal to itself: there is no need to compare
+        # individual values. This is not true for float values since they can
+        # be NaN, and NaN is not equal to itself.
+
+        # Test integer formats
+        for int_format in 'bBhHiIlLqQ':
+            with self.subTest(format=int_format):
+                a = array.array(int_format, [1, 2, 3])
+                m = memoryview(a)
+                self.assertTrue(m == m)
+                self.assertFalse(m != m)
+
+                if int_format in 'bB':
+                    m2 = m.cast('@' + m.format)
+                    self.assertTrue(m2 == m2)
+                    self.assertFalse(m2 != m2)
+
+        # Test '?' format
+        m = memoryview(b'\0\1\2').cast('?')
+        self.assertTrue(m == m)
+        self.assertFalse(m != m)
+
+        # Test 'P' format
+        if struct.calcsize('L') == struct.calcsize('P'):
+            int_format = 'L'
+        elif struct.calcsize('Q') == struct.calcsize('P'):
+            int_format = 'Q'
+        else:
+            raise ValueError('unable to get void* format in struct')
+        a = array.array(int_format, [1, 2, 3])
+        m = memoryview(a.tobytes()).cast('P')
+        self.assertTrue(m == m)
+        self.assertFalse(m != m)
+
+        # Test float formats
+        for float_format in 'fd':
+            with self.subTest(format=float_format):
+                a = array.array(float_format, [1.0, 2.0, float('nan')])
+                m = memoryview(a)
+                # nan is not equal to nan
+                self.assertFalse(m == m)
+                self.assertTrue(m != m)
+
+                a = array.array(float_format, [1.0, 2.0, 3.0])
+                m = memoryview(a)
+                self.assertTrue(m == m)
+                self.assertFalse(m != m)
+
 
 class BytesMemorySliceTest(unittest.TestCase,
     BaseMemorySliceTests, BaseBytesMemoryTests):

Misc/NEWS.d/next/Core_and_Builtins/2026-02-19-12-49-15.gh-issue-144995.Ob2oYJ.rst

@@ -0,0 +1,2 @@
+Optimize :class:`memoryview` comparison: a :class:`memoryview` is equal to
+itself, there is no need to compare values. Patch by Victor Stinner.

Objects/memoryobject.c

@@ -3122,6 +3122,32 @@ memory_richcompare(PyObject *v, PyObject *w, int op)
     }
     vv = VIEW_ADDR(v);
 
+    // For formats supported by the struct module a memoryview is equal to
+    // itself: there is no need to compare individual values.
+    // This is not true for float values since they can be NaN, and NaN
+    // is not equal to itself.  So only use this optimization on format known to
+    // not use floats.
+    if (v == w) {
+        int can_compare_ptr;
+        const char *format = vv->format;
+        if (format != NULL) {
+            if (*format == '@') {
+                format++;
+            }
+            // Include only formats known by struct, exclude formats "d" (double),
+            // "f" (float), "e" (16-bit float)
+            can_compare_ptr = (strchr("bBchHiIlLnNPqQ?", format[0]) != NULL
+                               && format[1] == 0);
+        }
+        else {
+            can_compare_ptr = 1;
+        }
+        if (can_compare_ptr) {
+            equal = 1;
+            goto result;
+        }
+    }
+
     if (PyMemoryView_Check(w)) {
         if (BASE_INACCESSIBLE(w)) {
             equal = (v == w);