F29H859TU-Q1: TI F29 / C29 CLANG compiler: how to create a barrier in the code

We do make a guarantee with both asm volatile and __builtin_instrumentation label. No volatile or memory operations will move around the given statement.

However, non-memory operations such as common arithmetic, comparisons, bitwise operations, etc. Will still possibly move when optimization is turned on.

You can achieve the most consistent numbers by placing the code you want to profile into a function marked with __attribute__((noinline)), and then call this function from a function which is marked with __attribute__((optnone)). The call to the profiled code should be surrounded by your timing functions of choice and instrumentation labels. The 'optnone' attribute on the profiling function ensures that your numbers will not vary due to changes in the profiling function itself when the optimization level (-On option) changes.

__attribute__((noinline)) void profiled_code(int *out, int *in) {
    /* Profiled code goes here */
}

extern unsigned int read_timer();
extern unsigned int calc_time(unsigned int start, unsigned int stop);
__attribute((optnone)) void do_profile() {
    int *in_array; /*= ...*/;
    int *out_array; /* = ...*/;
    unsigned int start = 0, stop = 0;


    start = read_timer();
    __builtin_instrumentation_label("profile_start");
    profiled_code(out_array, in_array);
    __builtin_instrumentation_label("profile_stop");
    stop = read_timer();

    unsigned int total_time = calc_time(start, stop);
    /* Do something with total_time */
}

For all intents and purposes, __builtin_instrumentation_label("name") is equivalent to asm volatile("name:\n").

However, the compiler treats asm volatile("...") as potentially having any untraceable (aka non-register) effect in addition to being a barrier. The instrumentation label is known to just be a label outside of its barrier-like behavior, so it contributes nothing to size, pipeline behaviors, etc.

For our ARM customers, I'd suggest asm volatile if writing portable code, and the instrumentation label otherwise. For C29, I would use the label, unless you have something to put into the assembly other than a label.