What are some tricks that a processor does to optimize code?

Question

I am looking for things like reordering of code that could even break the code in the case of a multiple processor.

Answer 1

The most important one would be memory access reordering.

Absent memory fences or serializing instructions, the processor is free to reorder memory accesses. Some processor architectures have restrictions on how much they can reorder; Alpha is known for being the weakest (i.e., the one which can reorder the most).

A very good treatment of the subject can be found in the Linux kernel source documentation, at Documentation/memory-barriers.txt.

Most of the time, it's best to use locking primitives from your compiler or standard library; these are well tested, should have all the necessary memory barriers in place, and are probably quite optimized (optimizing locking primitives is tricky; even the experts can get them wrong sometimes).