Fix subtle bug for SIMD finding new line in chapter 10-4

Author: YukunJ

chapters/10-Optimizing-Branch-Prediction/10-4 Multiple Compares Single Branch.md

@@ -39,16 +39,22 @@ uint32_t longestLine(const std::string &str) {
     uint64_t vect = *((const uint64_t*)(buf + pos));
     // Check all characters in this chunk.
     uint8_t mask = compareBytes(vect, eol);
+    uint8_t offset = 0;
     while (mask) {
-      uint16_t eolPos = tzcnt(mask);
+      uint16_t localEolPos = tzcnt(mask);
+      uint16_t eolPos = offset + localEolPos;
       // Compute the length of the current string.
       uint32_t curLen = (pos - lineBeginPos) + eolPos;
       // New line starts with the character after '\n'
       lineBeginPos += curLen + 1;
+      offset = eolPos + 1;
       // Is this line the longest?
       maxLen = std::max(curLen, maxLen);
       // Shift the mask to check if we have more '\n'
-      mask >>= eolPos + 1;
+      if (localEolPol + 1 >= 8) {
+        break; // UB if shift 8 bytes or more on uint8_t type
+      }
+      mask >>= localEolPos + 1;
     }
   }
   // process remainder (not shown)