编译器处理包含保护的标头有多昂贵? [英] How expensive it is for the compiler to process an include-guarded header?

问题描述

To speed up the compilation of a large source file does it make more sense to prune back the sheer number of headers used in a translation unit, or does the cost of compiling code far outweigh the time it takes to process-out an include-guarded header?

If the latter is true an engineering effort would be better spent creating more, lightweight headers instead of less.

So how long does it take for a modern compiler to handle a header that is effectively include-guarded out? At what point would the inclusion of such headers become a hit on compilation performance?

(related to this question)

解决方案

The answer:

It can be very expensive!

I found an article where someone had done some testing of the very issue addressed here, and am astonished to find you can increase your compilation time under MSVC by at least an order of magnitude if you write your include guards properly:

http://www.bobarcher.org/software/include/index.html

The most astonishing line from the results is that a test file compiled under MSVC 2008 goes from 5.48s to 0.13s given the right include guard methodology.

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