What would be your opinion of this comment, though?
// Reset the counter for the next run
total_count = 0; // Reset the counter for the next run
total_count = 0;The contention is whether it is necessary or common enough to be assumed. In most places this would be self evident and should be avoided to reduce mental load, however I have seen and done exactly this many times - in distrubuted or embedded systems where the control is neither synchronous or co-located (ie, coming in from interrupts / messages). In such places there may even be call for more description, however presuming the context is appropriate this could be correct.
I would imagine a game with a react redux style state engine might result in a comment like this as well.
// reuse counter for performance, this brings 2% speedup
total_count = 0;
I would need to see a realistic example to believe that something like this would ever be a useful comment.
The story is different, if the variable is captured by a closure/lambda, the current stack frame or part of it may be allocated on heap, leading to perf issues.
Is there somewhere that's documented for LLVM? That's my expectation of what the final compiler output should be, but I read that the IR allocates every variable dynamically, and then they optimize away whatever they can. I haven't been able to figure out how or where it's guaranteed that all allocations will be coalesced.
I wouldn't normally worry about it, but I've talked to folks who don't believe me when I say it doesn't matter whether you declare an integer inside or outside of a loop. It would be nice to be able to explain exactly why it can never matter.
By example, with https://godbolt.org/z/9Kv7oo9oK you can see that values goes into registers (and that thank to 'lea' there is not many registers used).
And if you remove the option -O2, values are spilled on stack.