gh-153563 feat: add unicode_memeq() to compare either directly or SSE2 vector

Misc/NEWS.d/next/Core_and_Builtins/2026-07-11-17-15-55.gh-issue-153198.dmBbIm.rst

@@ -0,0 +1,2 @@
+Optimize unicode_eq string equality comparisons for short strings by using
+inline integer casts and SSE2 vector instructions.

Objects/stringlib/eq.h

@@ -1,5 +1,61 @@
 /* Fast unicode equal function optimized for dictobject.c and setobject.c */
 
+/* Return 1 if two unicode objects are equal, 0 if not.
+ * unicode_eq() is called when the hash of two unicode objects is equal.
+ */
+#if defined(__x86_64__) || defined(_M_X64) || defined(__i386__) || defined(_M_IX86)
+#  include <emmintrin.h>
+#  define USE_SSE2 1
+#endif
+
+static inline int
+unicode_memeq(const void *p1, const void *p2, size_t len) {
+    if (len == 0) return 1;
+
+    const uint8_t *u1 = (const uint8_t *)p1;
+    const uint8_t *u2 = (const uint8_t *)p2;
+
+    switch (len) {
+        case 1:
+            return *u1 == *u2;
+        case 2:
+            return *(const uint16_t *)u1 == *(const uint16_t *)u2;
+        case 3:
+            return (*(const uint16_t *)u1 == *(const uint16_t *)u2) &&
+                   (*(const uint16_t *)(u1 + 1) == *(const uint16_t *)(u2 + 1));
+        case 4:
+            return *(const uint32_t *)u1 == *(const uint32_t *)u2;
+        case 5:
+        case 6:
+        case 7:
+            return (*(const uint32_t *)u1 == *(const uint32_t *)u2) &&
+                   (*(const uint32_t *)(u1 + len - 4) == *(const uint32_t *)(u2 + len - 4));
+        case 8:
+            return *(const uint64_t *)u1 == *(const uint64_t *)u2;
+        default:
+            if (len <= 16) {
+                return (*(const uint64_t *)u1 == *(const uint64_t *)u2) &&
+                       (*(const uint64_t *)(u1 + len - 8) == *(const uint64_t *)(u2 + len - 8));
+            }
+#ifdef USE_SSE2
+            if (len <= 32) {
+                __m128i v1_a = _mm_loadu_si128((const __m128i *)u1);
+                __m128i v2_a = _mm_loadu_si128((const __m128i *)u2);
+                __m128i cmp_a = _mm_cmpeq_epi8(v1_a, v2_a);
+                int mask_a = _mm_movemask_epi8(cmp_a);
+
+                __m128i v1_b = _mm_loadu_si128((const __m128i *)(u1 + len - 16));
+                __m128i v2_b = _mm_loadu_si128((const __m128i *)(u2 + len - 16));
+                __m128i cmp_b = _mm_cmpeq_epi8(v1_b, v2_b);
+                int mask_b = _mm_movemask_epi8(cmp_b);
+
+                return (mask_a == 0xFFFF) && (mask_b == 0xFFFF);
+            }
+#endif
+            return memcmp(p1, p2, len) == 0;
+    }
+}
+
 /* Return 1 if two unicode objects are equal, 0 if not.
  * unicode_eq() is called when the hash of two unicode objects is equal.
  */
@@ -18,5 +74,5 @@ unicode_eq(PyObject *str1, PyObject *str2)
 
     const void *data1 = PyUnicode_DATA(str1);
     const void *data2 = PyUnicode_DATA(str2);
-    return (memcmp(data1, data2, len * kind) == 0);
+    return unicode_memeq(data1, data2, len * kind);
 }