Code can be optimized for Speed or memory.
Here I am discussing the methods for optimization for speed
Optimization can can be done at different levels
Design level
At the highest level, the design may be optimized to make best use of the available resources. The implementation of this design will benefit from a good choice of efficient algorithms and the implementation of these algorithms will benefit from being written well. The architectural design of a system overwhelmingly affects its performance. The choice of algorithm affects efficiency more than any other item of the design and, since the choice of algorithm usually is the first thing that must be decided, arguments against early or "premature optimization" may be hard to justify.
In some cases, however, optimization relies on using more elaborate algorithms, making use of "special cases" and special "tricks" and performing complex trade-offs. A "fully optimized" program might be more difficult to comprehend and hence may contain more faults than unoptimized versions.
Source code level
Avoiding poor quality coding can also improve performance, by avoiding obvious "slowdowns". After that, however, some optimizations are possible that actually decrease maintainability. Some, but not all, optimizations can nowadays be performed by optimizing compilers.
Build level
Between the source and compile level, directives and build flags can be used to tune performance options in the source code and compiler respectively, such as using preprocessor defines to disable unneeded software features, or optimizing for specific processor models or hardware capabilities. Source-based software distribution systems such as BSD's Ports and Gentoo's Portage can take advantage of this form of optimization.
Compile level
Use of an optimizing compiler tends to ensure that the executable program is optimized at least as much as the compiler can predict.
Using Intrisics
Intrisics are functions equivalent to assembly instructions - one to one or one to many mapping.
These can be used in the code like a function.
Assembly level
At the lowest level, writing code using an assembly language, designed for a particular hardware platform can produce the most efficient and compact code if the programmer takes advantage of the full repertoire of machine instructions. Many operating systems used on embedded systems have been traditionally written in assembler code for this reason. Programs (other than very small programs) are seldom written from start to finish in assembly due to the time and cost involved. Most are compiled down from a high level language to assembly and hand optimized from there. When efficiency and size are less important large parts may be written in a high-level language.
With more modern optimizing compilers and the greater complexity of recent CPUs, it is harder to write more efficient code than what the compiler generates, and few projects need this "ultimate" optimization step.
Much code written today is intended to run on as many machines as possible. As a consequence, programmers and compilers don't always take advantage of the more efficient instructions provided by newer CPUs or quirks of older models. Additionally, assembly code tuned for a particular processor without using such instructions might still be suboptimal on a different processor, expecting a different tuning of the code.
Run time
Just-in-time compilers and assembler programmers may be able to perform run time optimization exceeding the capability of static compilers by dynamically adjusting parameters according to the actual input or other factors.
Self-modifying code can alter itself in response to run time conditions in order to optimize code.
Some CPU designs can perform some optimizations at runtime. Some examples include Out-of-order execution, Instruction pipelines, and Branch predictors. Compilers can help the program take advantage of these CPU features, for example through instruction scheduling.
