diff --git a/bolt/utils/llvm-bolt-wrapper.py b/bolt/utils/llvm-bolt-wrapper.py
--- a/bolt/utils/llvm-bolt-wrapper.py
+++ b/bolt/utils/llvm-bolt-wrapper.py
@@ -206,7 +206,8 @@
     Extracts byte number from cmp output:
     file1 file2 differ: byte X, line Y
     '''
-    return int(re.search(r'byte (\d+),', cmp_out).groups()[0])
+    # NOTE: cmp counts bytes starting from 1!
+    return int(re.search(r'byte (\d+),', cmp_out).groups()[0]) - 1
 
 def report_real_time(binary, main_err, cmp_err, cfg):
     '''