Register allocation considerations in a compiler. There is more than one way to do register allocation in a compiler, but the simplest way is to lazily leave a variable allocated to a register until there are not enough available, then do a least-recently-used (LRU) eviction, using the number of instructions since allocation as a reference for the LRU clock. This provides pretty good performance for the common case: You have a section of code that needs to operate on a variable a few times in rapid succession, then once you are past that section, you don’t need to use the variable for a long while. The idea of “pinning” a variable to a register and leaving it there until it goes out of scope is a more obscure one, and it also subtracts from your supply of registers that can be used for LRU management.
The extreme opposite end, loading a variable into a register for one use, computing, and storing it back to memory, is the easiest and most orthogonal to compile, but also the least efficient computation technique.