> For the complete documentation index, see [llms.txt](https://blog.ruipan.xyz/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://blog.ruipan.xyz/earlier-readings-and-notes/cs759-hpc-course-notes/lecture-24-critical-thinking.-code-optimizatino-aspects..md).

# Lecture 24: Critical Thinking. Code Optimization Aspects.

## Lecture Summary

* Last time
  * Parallel computing, multi-core: how caches come into play
  * Critical thinking, and similar tricks for speeding up your code
* Today
  * Critical thinking, and other similar tricks for speeding up your code

## Know your hardware

![Know your bandwidth/latency](/files/-MXzDYa0tV1dJtz9aCO7)

## Choose the right algorithm

* When working on a problem, there's more to performance than asymptotic complexity
  * Because asymptotic complexity is most often defined by the number of operations
  * Memory transactions are rarely considered: they are specific to the hardware
* Assess the arithmetic intensity associated with your problem

![](/files/-MXzESdkvKR6BfxCfv61)

* Simple optimization: Fusing transformations and do not bring data into the cache twice

## Compiler

* Aggressive optimizations done by the compiler might change the behavior of your code
* To help the compiler:
  * Allow it to see as much code as possible
  * Provide flags to convey information (e.g., the target architecture)
* There are a lot of amazing things covered in this lecture. The takeaways are:
  * Compilers are fantastic
  * Know them better to use them wisely
* A quick example is down below. Refer to the [slides](https://uwmadison.app.box.com/s/kapdp4qt18c6869dnaenlo35iytnlp2r) for a lot more fun facts

![](/files/-MXzI3IUdr6jxw-hCJ01)

![](/files/-MXzI64jS_qzpu4BFr0-)