3.7. ARM Profiling Implementation

3.7. ARM Profiling Implementation
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Profiling is enabled by means of the -pg compiler option. In this mode, the compiler inserts a call to __gnu_mcount_nc into every function prologue. However, no implementation of __gnu_mcount_nc is provided (to do so would be impossible without knowledge of the execution environment).

You must provide your own implementation of __gnu_mcount_nc . Here are the requirements:

On exit, pop the top value from the stack, and place it in the lr register. The sp register should be adjusted accordingly. For example, this is how to write it as a stub function:
```
        .globl __gnu_mcount_nc
        .type __gnu_mcount_nc, %function
__gnu_mcount_nc:
        mov    ip, lr
        pop    { lr }
        bx     ip
```
Preserve all other register state except for r12 and the CPSR condition code bits. In particular all coprocessor state and registers r0-r3 must be preserved.
Record and count all occurrences of the function calls in the program. The caller can be determined from the lr value stored on the top of the stack (on entry to __gnu_mcount_nc), and the callee can be determined from the current value of the lr register (i.e. the caller of this function).
Arrange for the data to be saved to a file named gmon.out when the program exits (via atexit). Refer to the gprof profiler manual for more information.

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