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.
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.