yes, the short answer is LLVM uses RegPressureTracker (https://llvm.org/doxygen/classllvm_1_1RegPressureTracker.htm...) to do all those calculations. Slightly longer answer: I should probably be a little more specific that in most cases, Machine Scheduler cares more about register pressure _delta_ caused by a single instruction, either traverses from bottom-up or top-down. In which case it's easier to make an estimation when some of other instructions are not scheduled yet.