Inlining of vararg functions

Question

While playing about with optimisation settings, I noticed an interesting phenomenon: functions taking a variable number of arguments (...) never seemed to get i

Answer 1

The variable arguments implementation generally have the following algorithm: Take the first address from the stack which is after the format string, and while parsing the input format string use the value at the given position as the required datatype. Now increment the stack parsing pointer with the size of the required datatype, advance in the format string and use the value at the new position as the required datatype ... and so on.

Some values automatically get converted (ie: promoted) to "larger" types (and this is more or less implementation dependant) such as char or short gets promoted to int and float to double.

Certainly, you do not need a format string, but in this case you need to know the type of the arguments passed in (such as: all ints, or all doubles, or the first 3 ints, then 3 more doubles ..).

So this is the short theory.

Now, to the practice, as the comment from n.m. above shows, gcc does not inline functions which have variable argument handling. Possibly there are pretty complex operations going on while handling the variable arguments which would increase the size of the code to an un-optimal size so it is simply not worth inlining these functions.

EDIT:

After doing a quick test with VS2012 I don't seem to be able to convince the compiler to inline the function with the variable arguments. Regardless of the combination of flags in the "Optimization" tab of the project there is always a call totest and there is always a test method. And indeed:

http://msdn.microsoft.com/en-us/library/z8y1yy88.aspx

says that

Even with __forceinline, the compiler cannot inline code in all circumstances. The compiler cannot inline a function if: ...

• The function has a variable argument list.