MDEV-17534 Implement fast path for ASCII range in strnxfrm_onelevel_internal()

strings/ctype-uca.ic

@@ -559,6 +559,45 @@ MY_FUNCTION_NAME(strnxfrm_onelevel_internal)(CHARSET_INFO *cs,
 
   DBUG_ASSERT(src || !srclen);
 
+#if MY_UCA_ASCII_OPTIMIZE && !MY_UCA_COMPILE_CONTRACTIONS
+ /*
+    Fast path for the ASCII range with no contractions.
+  */
+  {
+    const uchar *de2= de - 1; /* Last position where 2 bytes fit */
+    const uint16 *weights0= level->weights[0];
+    uint lengths0= level->lengths[0];
+    for ( ; ; src++, srclen--)
+    {
+      const uint16 *weight;
+      if (!srclen || !*nweights)
+        return dst;         /* Done */
+      if (*src > 0x7F)
+        break;              /* Non-ASCII */
+
+      weight= weights0 + (((uint) *src) * lengths0);
+      if (!(s_res= *weight))
+        continue;           /* Ignorable */
+      if (weight[1])        /* Expansion (e.g. in a user defined collation */
+        break;
+
+      /* Here we have a character with extactly one 2-byte UCA weight */
+      if (dst < de2)        /* Most typical case is when both bytes fit */
+      {
+        *dst++= s_res >> 8;
+        *dst++= s_res & 0xFF;
+        (*nweights)--;
+        continue;
+      }
+      if (dst >= de)        /* No space left in "dst" */
+        return dst;
+      *dst++= s_res >> 8;   /* There is space only for one byte */
+      (*nweights)--;
+      return dst;
+    }
+  }
+#endif
+
   my_uca_scanner_init_any(&scanner, cs, level, src, srclen);
   for (; dst < de && *nweights &&
          (s_res= MY_FUNCTION_NAME(scanner_next)(&scanner)) > 0 ; (*nweights)--)