Add portable SIMD optimization for find_max_char (STRINGLIB_SIZEOF_CHAR==1)

Use GCC/Clang vector extensions to check 64 bytes at a time for
non-ASCII characters, similar to the approach in pythongh-143991 (pystrhex).
The compiler lowers the 64-byte vector to the widest native SIMD
available (4x NEON on AArch64, 2x AVX-256 or 1x AVX-512 on x86-64).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: kumaraditya303

Objects/stringlib/find_max_char.h

@@ -17,6 +17,57 @@
 
 #if STRINGLIB_SIZEOF_CHAR == 1
 
+#ifdef HAVE_EFFICIENT_BUILTIN_SHUFFLEVECTOR
+
+/* 512-bit vector of 64 unsigned bytes.  GCC/Clang will lower this to
+   the widest SIMD the target supports (4x NEON on AArch64, 2x AVX-256
+   or 1x AVX-512 on x86-64) and unroll automatically. */
+typedef unsigned char v64u8 __attribute__((vector_size(64)));
+
+Py_LOCAL_INLINE(Py_UCS4)
+STRINGLIB(find_max_char)(const STRINGLIB_CHAR *begin, const STRINGLIB_CHAR *end)
+{
+    const unsigned char *p = (const unsigned char *)begin;
+    const unsigned char *_end = (const unsigned char *)end;
+
+    /* SIMD path: OR 64 bytes at a time into an accumulator, then check
+       the high bits once at the end.  The branchless inner loop lets
+       the compiler schedule multiple independent vector loads.
+       Uses memcpy for safe unaligned loads; the compiler optimizes
+       these to native vector load instructions. */
+    v64u8 accum = {0};
+    while (p + 64 <= _end) {
+        v64u8 data;
+        memcpy(&data, p, 64);
+        accum |= data;
+        p += 64;
+    }
+
+    /* Reduce: OR 64-byte accumulator down to a scalar high-bit check */
+    uint64_t a, b, c, d, e, f, g, h;
+    memcpy(&a, (const char *)&accum +  0, 8);
+    memcpy(&b, (const char *)&accum +  8, 8);
+    memcpy(&c, (const char *)&accum + 16, 8);
+    memcpy(&d, (const char *)&accum + 24, 8);
+    memcpy(&e, (const char *)&accum + 32, 8);
+    memcpy(&f, (const char *)&accum + 40, 8);
+    memcpy(&g, (const char *)&accum + 48, 8);
+    memcpy(&h, (const char *)&accum + 56, 8);
+    if ((a | b | c | d | e | f | g | h) & UCS1_ASCII_CHAR_MASK) {
+        return 255;
+    }
+
+    /* Scalar tail for remaining bytes */
+    while (p < _end) {
+        if (*p++ & 0x80) {
+            return 255;
+        }
+    }
+    return 127;
+}
+
+#else /* !HAVE_EFFICIENT_BUILTIN_SHUFFLEVECTOR */
+
 Py_LOCAL_INLINE(Py_UCS4)
 STRINGLIB(find_max_char)(const STRINGLIB_CHAR *begin, const STRINGLIB_CHAR *end)
 {
@@ -43,6 +94,8 @@ STRINGLIB(find_max_char)(const STRINGLIB_CHAR *begin, const STRINGLIB_CHAR *end)
     return 127;
 }
 
+#endif /* HAVE_EFFICIENT_BUILTIN_SHUFFLEVECTOR */
+
 #undef ASCII_CHAR_MASK
 
 #else /* STRINGLIB_SIZEOF_CHAR == 1 */