Everything is in the title:

What Every Programmer Should Know About Compiler Optimizations

by Hadi Brais

From the article:

High-level programming languages offer many abstract programming constructs such as functions, conditional statements and loops that make us amazingly productive. However, one disadvantage of writing code in a high-level programming language is the potentially significant decrease in performance. Ideally, you should write understandable, maintainable code—without compromising performance. For this reason, compilers attempt to automatically optimize the code to improve its performance, and they’ve become quite sophisticated in doing so nowadays. They can transform loops, conditional statements, and recursive functions; eliminate whole blocks of code; and take advantage of the target instruction set architecture (ISA) to make the code fast and compact. It’s much better to focus on writing understandable code, than making manual optimizations that result in cryptic, hard-to-maintain code. In fact, manually optimizing the code might prevent the compiler from performing additional or more efficient optimizations...