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When is "inline" ineffective? (in C)

Asked 2009-02-24T18:21:50.453
15

Some people love using inline keyword in C, and put big functions in headers. When do you consider this to be ineffective? I consider it sometime even annoying, because it is unusual.

My principle is that inline should be used for small functions accessed very frequently, or in order to have real type checking. Anyhow, my taste guide me, but I am not sure how to explain best the reasons why inline is not so useful for big functions.

In this question people suggest that the compiler can do a better job at guessing the right thing to do. That was also my assumption. When I try to use this argument, people reply it does not work with functions coming from different objects. Well, I don't know (for example, using GCC).

Thanks for your answers!

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Personally I don't think you should ever inline, unless you have first run a profiler on your code and have proven that there is a significant bottleneck on that routine that can be partially alleviated by inlining.

This is yet another case of the Premature Optimization Knuth warned about.

answered 2009-02-24T19:20:23.713
1

Inline functions should be approximately 10 lines or less, give or take, depending on your compiler of choice.

You can tell your compiler that you want something inlined .. its up to the compiler to do so. There is no -force-inline option that I know of which the compiler can't ignore. That is why you should look at the assembler output and see if your compiler actually did inline the function, if not, why not? Many compilers just silently say 'screw you!' in that respect.

so if:

static inline unsigned int foo(const char *bar)

.. does not improve things over static int foo() its time to revisit your optimizations (and likely loops) or argue with your compiler. Take special care to argue with your compiler first, not the people who develop it.. or your just in store for lots of unpleasant reading when you open your inbox the next day.

Meanwhile, when making something (or attempting to make something) inline, does doing so actually justify the bloat? Do you really want that function expanded every time its called? Is the jump so costly?, your compiler is usually correct 9/10 times, check the intermediate output (or asm dumps).

answered 2009-02-25T12:28:05.897

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