Justification

There can be cases where you want a variable that is replicated for every matched event, but is not initialized every time the probe body is executed. This requires that you have global state that is only reference-able locally for a probe body.

Enter the unshared variable.

Use Case

Consider the following use case for this variable:

wasm:opcode:memory_grow {
    unshared int count;
    if count <= 0 {
        ...
        count = rand() % 1000; // some random number between 0 and 1000
    }
    count--;
}

This effectively samples the execution of each memory_grow operation using random sampling. Continuing on this idea, you could use a global count to sample memory.grow at a random interval for the entire application, e.g.:

int count;
wasm:opcode:memory_grow {
    if count <= 0 {
        ...
        count = rand() % 1000; // some random number between 0 and 1000
    }
    count--;
}

Implementation Details

Basically implement a report variable that is not reported at the end of program execution.

Make It Fast (Wizard)

1. The JIT could put the if count <= 0 check inline (~3 opcodes) and then call out to the probe body callback if the predicate is true
2. See Issue Really cool wizard integration features #137, the sampling logic is just a check on global state, factor this out to a dynamic check matches function.