Out of Order Execution and Memory Fences

Question

I know that modern CPUs can execute out of order, However they always retire the results in-order, as described by wikipedia.

\"Out of Oder processors fill these \"slots

Answer 1

This tutorial explains the issues: http://www.hpl.hp.com/techreports/Compaq-DEC/WRL-95-7.pdf

FWIW, where memory ordering issues happen on modern x86 processors, the reason is that while the x86 memory consistency model offers quite strong consistency, explicit barriers are needed to handle read-after-write consistency. This is due to something called the "store buffer".

That is, x86 is sequentially consistent (nice and easy to reason about) except that loads may be reordered wrt earlier stores. That is, if the processor executes the sequence

store x
load y

then on the processor bus this may be seen as

load y
store x

The reason for this behavior is the afore-mentioned store buffer, which is a small buffer for writes before they go out on the system bus. Load latency is, OTOH, a critical issue for performance, and hence loads are permitted to "jump the queue".

See Section 8.2 in http://download.intel.com/design/processor/manuals/253668.pdf