The folding of statements helps GCC elimate some of the intermediate stores it was performing. The elimination saved about 1.0 cpb. SIMON-128 is now running around 10 cpb, but it is still off the Simon and Speck team's numbers of 3.5 cpb
Louis Wingers and Bryan Weeks from the Simon and Speck team offered the suggestion. The change save 0.7 cpb for Speck, and 5 cpb for Simon on x86_64.
Speck is now running very close to the Team's time sor SSE4. Simon is still off, but we know the root cause. For Simon, the Team used a fast bit-sliced implementation
Jeffrey Walton