Call Frames and Optimized Code

When the compiler generates optimized machine code, the optimized code maintains the call stack, but sometimes the function boundaries are changed in either or both of two ways:

Inlining is when the compiler completely eliminates the call by instead generating the instructions for the called function at the call site, usually followed by merging those instructions with the other instructions surrounding the call site.
Outlining is when the compiler creates a function where one did not explicitly exist in the source. For example, the compiler turns a loop body into a function, so that it can generate code that uses threads to execute the different iterations in parallel. Or the compiler creates a single shared function to replace several sections of the source that are similar.

Depending on the information the compiler makes available to the debugger, inlined calls may or may not show up in the call stack display. Outlined calls will show up and will be correlated to the code they came from. The compiler will probably have supplied the debugger with an invented name for the function.