Out-of-order execution
1、简单而言，OoOE是CPU为了performance，不按照In-order execution，即instruction cycle，而是采取特殊的执行方式。
2、最最典型的Out-of-order execution是memory reordering，在 CPU-memory-access\Memory-ordering 章节进行了总结。
Wikipedia Out-of-order execution
In computer engineering, out-of-order execution (or more formally dynamic execution), is a paradigm used in most high-performance microprocessors to make use of instruction cycles that would otherwise be wasted by a certain type of costly delay. In this paradigm, a processor executes instructions in an order governed by the availability of input data, rather than by their original order in a program.[1] In doing so, the processor can avoid being idle while waiting for the preceding instruction to complete to retrieve data for the next instruction in a program, processing instead the next instructions which are able to run immediately and independently.[2] It can be viewed as a hardware based dynamic recompilation or just-in-time compilation (JIT) to improve instruction scheduling.
NOTE: CPU的out-of-order execution是处于performance考虑的
Basic concept
In-order processors
Main article: Instruction cycle
Out-of-order processors
NOTE: 比较复杂
The key concept of OoOE processing is to allow the processor to avoid a class of stalls that occur when the data needed to perform an operation are unavailable.
The benefit of OoOE processing grows as the instruction pipeline deepens and the speed difference between main memory (or cache memory) and the processor widens. On modern machines, the processor runs many times faster than the memory, so during the time an in-order processor spends waiting for data to arrive, it could have processed a large number of instructions.
素材
zhihu 如何理解 C++11 的六种 memory order？ # A
啥？...还真的是这样。原因在于当代CPU内部也有指令重排。也就是说，CPU执行指令的顺序，也不见得是完全严格按照机器码的顺序。特别是，当代CPU的IPC（每时钟执行指令数）一般都远大于1，也就是所谓的**多发射**，很多命令都是同时执行的。比如，当代CPU当中（一个核心）一般会有2套以上的整数ALU（加法器），2套以上的浮点ALU（加法器），往往还有独立的乘法器，以及，独立的Load和Store执行器。Load和Store模块往往还有8个以上的队列，也就是可以同时进行8个以上内存地址（cache line）的读写交换。
是不是有些晕？简单来说，你可以理解当代CPU不仅是多核心，而且每个核心还是多任务（多指令）并行的。计算机课本上的那种一个时钟一条指令的，早就是老黄历了 （当然，宏观来看基本原理并没有改变）
Processor technologies # Out-of-order
其中总结了一些out of order的内容。
"wide issue"应该就是"multiple issue"，即多发射。
umd.edu Multiple Issue Processors I
NOTE:
1、介绍地比较详细
Types of Multiple Issue Processors: There are basically two variations in multiple issue processors – Superscalar processors and VLIW (Very Long Instruction Word) processors. There are two types of superscalar processors that issue varying numbers of instructions per clock. They are
- statically scheduled superscalars that use in-order execution
- dynamically scheduled superscalars that use out-of-order execution