Question

I am wondering if anyone knows the logic which C++ compilers generally use to decide whether or not to inline a function at compilation (assuming inline has been requested).

Is this type of thing public knowledge?

Answer 1

I gave a more thorough answer in this other question.

Basically compilers have heurstics based on cost-analysis, quoting myself (does this mean I am going senile ?)

If you think about inlining, and its consequences, you’ll realise it:

• you avoid a function call (with all the register saving/frame adjustment)
• you expose more context to the optimizer (dead stores, dead code, common sub-expression elimintation…)
• at the cost of duplicating code (bloating the instruction cache and the executable size, among other things)

And of course, there is also partial inlining in which only part of the function is inline, typically a leading if guard like foo(T* t) { if (!t) { return; } <many many things> }.