When should I use __forceinline instead of inline?

Question

Visual Studio includes support for __forceinline. The Microsoft Visual Studio 2005 documentation states:

The __forceinline keyword overrides the co

Answer 1

For SIMD code.

SIMD code often uses constants/magic numbers. In a regular function, every const __m128 c = _mm_setr_ps(1,2,3,4); becomes a memory reference.

With __forceinline, compiler can load it once and reuse the value, unless your code exhausts registers (usually 16).

CPU caches are great but registers are still faster.

P.S. Just got 12% performance improvement by __forceinline alone.

Answer 2

There are several situations where the compiler is not able to determine categorically whether it is appropriate or beneficial to inline a function. Inlining may involve trade-off's that the compiler is unwilling to make, but you are (e.g,, code bloat).

In general, modern compilers are actually pretty good at making this decision.

Answer 3

Actually, even with the __forceinline keyword. Visual C++ sometimes chooses not to inline the code. (Source: Resulting assembly source code.)

Always look at the resulting assembly code where speed is of importance (such as tight inner loops needed to be run on each frame).

Sometimes using #define instead of inline will do the trick. (of course you loose a lot of checking by using #define, so use it only when and where it really matters).

Answer 4

The one place I am using it is licence verification.

One important factor to protect against easy* cracking is to verify being licenced in multiple places rather than only one, and you don't want these places to be the same function call.

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*) Please don't turn this in a discussion that everything can be cracked - I know. Also, this alone does not help much.

Answer 5

The inline directive will be totally of no use when used for functions which are:

recursive, long, composed of loops,

If you want to force this decision using __forceinline

Answer 6

You know better than the compiler only when your profiling data tells you so.