Perform build and magnum tuning

Building o//third_party/python now takes 5 seconds on my PC

This change works towards modifying Python to use runtime dispatching
when appropriate. For example, when loading the magnums in the socket
module, it's a good idea to check if the magnum is zero, because that
means the local system platform doesn't support it.

third_party/python/Objects/stringlib/find_max_char.inc

@@ -0,0 +1,135 @@
+/* clang-format off */
+/* Finding the optimal width of unicode characters in a buffer */
+
+#if !STRINGLIB_IS_UNICODE
+# error "find_max_char.h is specific to Unicode"
+#endif
+
+/* Mask to quickly check whether a C 'long' contains a
+   non-ASCII, UTF8-encoded char. */
+#if (SIZEOF_LONG == 8)
+# define UCS1_ASCII_CHAR_MASK 0x8080808080808080UL
+#elif (SIZEOF_LONG == 4)
+# define UCS1_ASCII_CHAR_MASK 0x80808080UL
+#else
+# error C 'long' size should be either 4 or 8!
+#endif
+
+#if STRINGLIB_SIZEOF_CHAR == 1
+
+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 *aligned_end =
+            (const unsigned char *) _Py_ALIGN_DOWN(end, SIZEOF_LONG);
+
+    while (p < end) {
+        if (_Py_IS_ALIGNED(p, SIZEOF_LONG)) {
+            /* Help register allocation */
+            const unsigned char *_p = p;
+            while (_p < aligned_end) {
+                unsigned long value = *(unsigned long *) _p;
+                if (value & UCS1_ASCII_CHAR_MASK)
+                    return 255;
+                _p += SIZEOF_LONG;
+            }
+            p = _p;
+            if (p == end)
+                break;
+        }
+        if (*p++ & 0x80)
+            return 255;
+    }
+    return 127;
+}
+
+#undef ASCII_CHAR_MASK
+
+#else /* STRINGLIB_SIZEOF_CHAR == 1 */
+
+#define MASK_ASCII 0xFFFFFF80
+#define MASK_UCS1 0xFFFFFF00
+#define MASK_UCS2 0xFFFF0000
+
+#define MAX_CHAR_ASCII 0x7f
+#define MAX_CHAR_UCS1  0xff
+#define MAX_CHAR_UCS2  0xffff
+#define MAX_CHAR_UCS4  0x10ffff
+
+Py_LOCAL_INLINE(Py_UCS4)
+STRINGLIB(find_max_char)(const STRINGLIB_CHAR *begin, const STRINGLIB_CHAR *end)
+{
+#if STRINGLIB_SIZEOF_CHAR == 2
+    const Py_UCS4 mask_limit = MASK_UCS1;
+    const Py_UCS4 max_char_limit = MAX_CHAR_UCS2;
+#elif STRINGLIB_SIZEOF_CHAR == 4
+    const Py_UCS4 mask_limit = MASK_UCS2;
+    const Py_UCS4 max_char_limit = MAX_CHAR_UCS4;
+#else
+#error Invalid STRINGLIB_SIZEOF_CHAR (must be 1, 2 or 4)
+#endif
+    Py_UCS4 mask;
+    Py_ssize_t n = end - begin;
+    const STRINGLIB_CHAR *p = begin;
+    const STRINGLIB_CHAR *unrolled_end = begin + _Py_SIZE_ROUND_DOWN(n, 4);
+    Py_UCS4 max_char;
+
+    max_char = MAX_CHAR_ASCII;
+    mask = MASK_ASCII;
+    while (p < unrolled_end) {
+        STRINGLIB_CHAR bits = p[0] | p[1] | p[2] | p[3];
+        if (bits & mask) {
+            if (mask == mask_limit) {
+                /* Limit reached */
+                return max_char_limit;
+            }
+            if (mask == MASK_ASCII) {
+                max_char = MAX_CHAR_UCS1;
+                mask = MASK_UCS1;
+            }
+            else {
+                /* mask can't be MASK_UCS2 because of mask_limit above */
+                assert(mask == MASK_UCS1);
+                max_char = MAX_CHAR_UCS2;
+                mask = MASK_UCS2;
+            }
+            /* We check the new mask on the same chars in the next iteration */
+            continue;
+        }
+        p += 4;
+    }
+    while (p < end) {
+        if (p[0] & mask) {
+            if (mask == mask_limit) {
+                /* Limit reached */
+                return max_char_limit;
+            }
+            if (mask == MASK_ASCII) {
+                max_char = MAX_CHAR_UCS1;
+                mask = MASK_UCS1;
+            }
+            else {
+                /* mask can't be MASK_UCS2 because of mask_limit above */
+                assert(mask == MASK_UCS1);
+                max_char = MAX_CHAR_UCS2;
+                mask = MASK_UCS2;
+            }
+            /* We check the new mask on the same chars in the next iteration */
+            continue;
+        }
+        p++;
+    }
+    return max_char;
+}
+
+#undef MASK_ASCII
+#undef MASK_UCS1
+#undef MASK_UCS2
+#undef MAX_CHAR_ASCII
+#undef MAX_CHAR_UCS1
+#undef MAX_CHAR_UCS2
+#undef MAX_CHAR_UCS4
+
+#endif /* STRINGLIB_SIZEOF_CHAR == 1 */
+