Page "Dhrystone" Paragraph 13

* it is susceptible to compiler optimizations.

For example, it does a lot of string copying in an attempt to measure string copying performance.

However, the strings in Dhrystone are of known constant length and their starts are aligned on natural boundaries, two characteristics usually absent from real programs.

Therefore an optimizer can replace a string copy with a sequence of word moves without any loops, which will be much faster.

This optimization consequently overstates system performance, sometimes by more than 30 %.