In order to know for sure how much stack is needed (or to replace the stack with a static allocation, which used to be common on older machines and still today in deep embedded code, and even on GPU!), you must ensure that any functions you call within your thread are non-reentrant, or else that they resort to an auxiliary stack-like allocation if reentrancy is required. This is a fundamental constraint (not something limited to current LLVM) which in practice leads you right back into the "what color are your functions?" world.