Wednesday, April 17, 2013

C and C++ need inline directives at the call site

I don't like the inline keyword, I think it's incredibly broken.

Inlining heuristics make reasonable assumptions about what should be inlined but there's basically no control by the user unless they use something like VC++'s __forceinline directive. Automatic inlining works well with trivial functions, the kinds that populate the cctype/ctype.h header, but fails when the desired inline function grows beyond a few statements.

The ability to inline code can be quite a powerful for developers, especially those who had to develop for consoles like the Xbox 360 and PS3, but because the compiler has incomplete information on where the inlining really needs to go and because there's no way in the language to give it better hints, there is often bloat in areas where you don't want it, and no inline expansion in areas where you do.

I've worked around this in the past by abusing macro expansion, but it's not a very clean solution, and makes debugging difficult.

What I would like to see would be an ability to use the inline declaration at the call site so that I can pick and choose where the inline expansion happens as needed. One example I can think of is multiplying two matrices in a video game. In some cases, say in creating the view/projection matrix, an inline expansion of the multiply would be unnecessary code bloat because this operation only typically happens once per game frame. 

Mat4x4 viewProjection = Mat4x4Multiply(view, projection);

While transforming joint hierarchies for animation there are typically a number of multiplications that happen in a tight loop where inlining would be beneficial:

for (int i = 0; i < numBonesInModel; ++i) {
    Mat4x4 transformedJoint = inline Mat4x4Multiply(jointMatrix[i], jointMatrix[parent[i]]);

I want more control over what gets inlined and what doesn't, without resorting to macros, and I think being able to specify at the call site what I want inlined would go a long way toward that goal.

No comments:

Post a Comment