Inlined Functions

An inlined function is a function for which the instructions generated by the compiler are inserted at the call site of the function instead of an actual call. There are two kinds of inlining, both of which are done to improve performance, and both of which affect the Analyzer.

• C++ inline function definitions. The rationale for inlining in this case is that the cost of calling a function is much greater than the work done by the inlined function, so it is better to simply insert the code for the function at the call site, instead of setting up a call. Typically, access functions are defined to be inlined, because they often only require one instruction. When you compile with the -g option, inlining of functions is disabled; compilation with -g0 permits inlining of functions, and is recommended.

• Explicit or automatic inlining performed by the compiler at high optimization levels (4 and 5). Explicit and automatic inlining is performed even when -g is turned on. The rationale for this type of inlining can be to save the cost of a function call, but more often it is to provide more instructions for which register usage and instruction scheduling can be optimized.

Both kinds of inlining have the same effect on the display of metrics. Functions that appear in the source code but have been inlined do not show up in the function list, nor do they appear as callees of the functions into which they have been inlined. Metrics that would otherwise appear as inclusive metrics at the call site of the inlined function, representing time spent in the called function, are actually shown as exclusive metrics attributed to the call site, representing the instructions of the inlined function.

Inlining can make data difficult to interpret, so you might want to disable inlining when you compile your program for performance analysis.

In some cases, even when a function is inlined, a so-called out-of-line function is left. Some call sites call the out-of-line function, but others have the instructions inlined. In such cases, the function appears in the function list but the metrics attributed to it represent only the out-of-line calls.