8.8 Stack Usage Guidance

Recursion occurs when the program directly or indirectly causes a loop where there is a net adjustment of the stack usage. MPLAB XC16 does not limit this identification across function boundaries; indeed (un)carefully constructed assembly language programs could cause this error to be emitted even within a function boundary. The most obvious example would be a traditional, recursive, implementation of factorial. A more subtle version from the standard C library is fflush() - which, if given a NULL parameter, will call fflush() on all the currently open IO streams.

Any recursion that is detected will cause MPLAB XC16 to emit the following messages:

1. Recursion has been detected:
 _function_name (address 0x8bc)
No stack usage predictions can be made.

Each function, where the recursion has been detected, will appear in the list. The output to the Map file will also contain the code address near where recursion has been detected.

Any stack adjustments that cannot be determined at compile time will trigger a message. This may be caused by assembly code writing a value explicitly to the stack pointer (W15) or by a variable allocation using alloca(). In this case, we will emit the following diagnostic:

2. Indeterminate stack adjustment has been detected:
 _function_name (address 0x286c)
No stack usage predictions can be made.

Each function where stack adjustment has been detected will appear in the list. The output to the Map file will also contain the code address near where adjustment has been detected.

Every embedded application will have some interrupt functions. Unfortunately we cannot determine when these will occur during the run-time flow of the application. When interrupt functions are discovered, we will emit the following information:

3. The following labels are interrupt functions:
 __T1Interrupt uses 18 bytes (address 0x2f8)
We cannot determine the stack impact of these events. Please adjust the guidance according to the application flow.

Each interrupt function will appear in the list. It will contain the amount of stack that is used by that interrupt including any functions that may be called. The code address of the interrupt handler will be emitted to the Map file.

The total amount of allocated stack should be increased by the total amount of stack that can be consumed by the largest interrupt flow; this would include any interrupts that may themselves be interrupted. Your application may limit which interrupts are themselves interruptible using various means. As the application developer, please adjust the stack usage based upon your knowledge of how the nested interrupt system will work in your application.

Unconnected executable code cannot be reached by normal flow analysis. Perhaps this code represents functions that are never called, or functions that are called indirectly via function pointers. In any event, we cannot determine when or if these functions are actually called. When detected, the tool will emit the following messages:

4. The following labels cannot be connected to the main call graph.
This is usually caused by some indirection:
 _function_name uses 10 bytes (address 0x598)
We cannot determine the stack impact of these events. Please adjust the guidance according to the application flow.

Each block of code that cannot be reached will be displayed along with the amount of stack consumed. The address of the code block will be presented only in the Map file. These may not be complete functions, though it is unlikely for C code.