“inline” Is Yet Another Word For “Premature Optimization”

The fact that some C++ developers use the ‘inline’ keyword so much has always been a conundrum to me — I’ve never liked it. Why? First and foremost because it clutters up header files and exposes implementation details to the users of a class.

Most likely, inline aficionados believe that these disadvantages are more than compensated for by the fact that inlining gives them faster code, but this is not necessarily the case: according to the C++ standard (ISO/IEC 14882:2014), the compiler is allowed to silently ignore the ‘inline’ keyword:

“An implementation is not required to perform this inline substitution at the point of call”

Believing is not knowing, as the old saying goes. This is another reason why I don’t like the ‘inline’ keyword: it doesn’t guarantee you anything.

But let’s attack the ‘inline’ keyword from another angle. Even if we knew that declaring a method inline made it faster, shouldn’t we have to ask ourselves first if there is actually a performance case? Without profiling, without a proven need, any optimization is premature optimization, which — according to Donald Knuth — is the root of all evil. The fact that an optimization gives a local improvement doesn’t justify it sufficiently — it’s the overall improvement of the major use cases that matters. Otherwise we would implement all of our functions with inline assembly, wouldn’t we?

In the old days of C programming, developers used the ‘register’ keyword as a hint to tell the compiler what variables should be kept in registers for performance reasons. Nowadays, every C compiler is much better at allocating variables to registers than any human being. Consequently, the ‘register’ keyword has been deprecated in C11.

By the same token, today’s C++ compilers do a much better job of figuring out which functions should be inlined than we are able to do. Therefore, instead of giving hints to the compiler we should rather rely on automated, transparent inlinining that doesn’t clutter up class interfaces.

As an example, at optimization level -O2, the g++ compiler automatically inlines all functions that are small or called only once. Specifying -finline-functions (enabled by default at -O3) uses a heuristic to determine if its worthwhile to inline a function or not — without the need for any developer intervention.

To me, it’s about time that ‘inline’ goes the way of the ‘register’ keyword.

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