[An Introduction to GCC 学习笔记] 12 优化问题

Posted 2021-07-22 漫小牛

Compiling with Optimization

• GCC is an optimizing compiler. It provides a wide range of options which aim to increase the speed, or reduce the size, of the executable files it generates.
• Optimization is a complex process. For each high-level command in the source code there are usually many possible combinations of machine instructions that can be used to achieve the appropriate final result. The compiler must consider these possibilities and choose among them.
• In general, different code must be generated for different processors, as they use incompatible assembly and machine language.
• Different amouts of time are needed for different instrucitons, depending on how they are ordered. GCC takes all these factors into account and tries to produce the fastest executable for a given system when compiling with optimization.

Source-level Optimization

The source-level optimizaiton occurs at the source-code level, and does not requires any knowledge of the machine instructions. There are many source-level optimization techniques. We just discuss two common types here:

• Common Subexpression Elimination(CSE) ~ computing an expression in the source code with fewer instructions by reusing already-computed results. CSE is performed automatically when optimization is turned on. It is powerful, because it simultaneously increases the speed and reduces the size of the code.
• Function Inlining (FL) ~ eliminates the function-call overhead by replacing calls to a funciton by the code of the function itself(known as placing the code in-line)

Speed-space Tradeoffs

• While some forms of optimization, such as CSE, are able to increase the speed and reduce the size of a program simultaneously, other types of optimizaiton produce faster code at the expense of increasing the size of the executable.
• This choice between speed of memory is referred to as a speed-space tradeoff
• Optimizations with a speed-space tradeoff can also be used to make an executable smaller, at the expense of making it run slower.