8.2.4 : The performances

Ok, let's be realistic : there is a problem with the performances.

• Performances -O0 : slow but reasonable
• Other performances (-O1, -O2, -O3, -Ofast) are too fast (non sence)

GCC is smart of guileful depending on the points of view.

• GCC noticed you do not use the result of the reduction function.
• The call to reduction is considered as dead code (or never called code).

To avoid that, you have to compile the reduction function in an other file.