How is ccache useful when we use make in the first place?

[Alex Mayle]

I haven't used 'make' extensively until I started tinkering with the AOSP, but isn't the whole point of it to avoid compiling things that don't need to be recompiled? If a file and its dependencies haven't changed since the last compilation, then there is no point in compiling it again. 

If the file doesn't need to be recompiled, we don't need ccache. If the file does need to be recompiled, then ccache only has an outdated version.

Does it take advantage of the fact that a large, edited source file may have only been changed slightly?

[Xin Li]

 You can also benefit from it if you are building multiple branches or directories (assuming the C/C++ files they share, after pre-processing were the same).  If you work only on one tree it's probably not very helpful.

[Peter Kalauskas]

It's a cache of the compiled objects. Compiling the source files takes some time, and it's faster to pull the results out of a cache.

For instance, if you 'touch main.c' and then run make (without ccache) it will compile the file again. Even if you didn't change the file, 'make' doesn't know better; it only cares about the timestamp. But if you were to use ccache, it would see that it's already compiled main.c and pull the result out of the cache.

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[Alex Mayle]

I see. Thank you for the clarification.